Continuously variable transmission

The continuously variable transmission (CVT) is a transmission in which the ratio of the rotational speeds of two shafts, as the input shaft and output shaft of a vehicle or other machine, can be varied continuously within a given range, providing an infinite number of possible ratios.

The continuously variable transmission should not be confused with the power split transmission (PST), as used in the Toyota Prius and other hybrid vehicles that use two or more inputs with one output, despite some similarities in their function.

A CVT need not be automatic, nor include zero or reverse output. Such features may be adapted to CVTs in certain specific applications.

Other mechanical transmissions only allow a few different discrete gear ratios to be selected, but the continuously variable transmission essentially has an infinite number of ratios available within a finite range, so it enables the relationship between the speed of a vehicle engine and the driven speed of the wheels to be selected within a continuous range. This can provide better fuel economy than other transmissions by enabling the engine to run at its most efficient speeds within a narrow range.

CVT transmissions have been refined over the years and are much improved from their origins.



Types


Infinitely Variable Transmission (IVT)

A specific type of CVT is the infinitely variable transmission (IVT), which has an infinite range of input/output ratios in addition to its infinite number of possible ratios; this qualification for the IVT implies that its range of ratios includes a zero output/input ratio that can be continuously approached from a defined "higher" ratio. A zero output implies an infinite input, which can be continuously approached from a given finite input value with an IVT. Low gears are a reference to low ratios of output/input which have high input/output ratios that are taken to the extreme with IVTs, resulting in a "neutral", or non-driving "low" gear limit. Most continuously variable transmissions are not infinitely variable.

Most (if not all) IVTs result from the combination of a CVT with an epicyclic gear system (which is also known as a planetary gear system) that facilitates the subtraction of one speed from another speed within the set of input and planetary gear rotations. This subtraction only needs to result in an continuous range of values that includes a zero output; the maximum output/input ratio can be arbitrarily chosen from infinite practical possibilities through selection of extraneous input or output gear, pulley or sprocket sizes without affecting the zero output or the continuity of the whole system. Importantly, the IVT is distinguished as being "infinite" in its ratio of high gear to low gear within its range; high gear is infinite times higher than low gear. The IVT is always engaged, even during its zero output adjustment.

The term "Infinitely Variable Transmission" does not imply reverse direction, disengagement, automatic operation, or any other quality except ratio selectabilty within a continuous range of input/output ratios from a defined minimum to an undefined, "infinite" maximum. This means continuous range from a defined output/input to zero output/input ratio.


Ratcheting CVT

The Ratcheting CVT is a transmission that relies on static friction and is based on a set of elements that successively become engaged and then disengaged between the driving system and the driven system, often using oscillating or indexing motion in conjunction with one-way clutches or ratchets that rectify and sum only "forward" motion. The transmission ratio is adjusted by changing linkage geometry within the oscillating elements, so that the summed maximum linkage speed is adjusted, even when the average linkage speed remains constant. Power is transferred from input to output only when the clutch or ratchet is engaged, and therefore when its locked into a static friction mode where the driving & driven rotating surfaces momentarily rotate together without slippage.

These CVT transmissions can transfer substantial torque because their static friction actually increases relative to torque throughput, so slippage is impossible in properly designed systems. Efficiency is generally high because most of the dynamic friction is caused by very slight transitional clutch speed changes. The drawback to ratcheting CVT's is vibration caused by the successive transition in speed required to accelerate the element which must supplant the previously operating & decelerating, power transmitting element. An Infinitely Variable Transmission (IVT) that is based on a Ratcheting CVT and subtraction of one speed from another will greatly amplify the vibration as the IVT output/input ratio approaches zero.

Ratcheting CVT's are distinguished from Variable Diameter Pulleys (VDP's) and Roller-based CVT's by being static friction-based devices, as opposed to being dynamic friction-based devices that waste significant energy through slippage of twisting surfaces.


Variable-diameter pulley (VDP)

This type of CVT uses pulleys, typically connected by a rubber-covered metal or laminated steel belt. A chain may also be used. A large pulley connected to a smaller pulley with a belt or chain will operate in the same manner as a large gear meshing with a smaller gear. Typical CVTs have pulleys formed as pairs of opposing cones. Moving the cones in and out has the effect of changing the pulley diameter since the belt or chain must take a large-diameter path when the conical pulley halves are close together. This motion of the cones can be computer-controlled and driven, for example by a servo motor. However, in the light-weight VDP transmissions used in automatic motorscooters and light motorcycles, the change in pulley diameter is accomplished by a variator, an all-mechanical system that uses weights and springs to change the pulley diameters as a function of belt speed. In higher power types, for example that produced by Van Doorne's Transmissie (part of the Bosch Group), an oil-cooled laminated steel belt is used.

In the case of a chain the links bear on the pulleys via tapered sides on the links. Some such transmissions have been designed to transmit the forces between pulleys using compressive (pushing) rather than traction (pulling) forces. The chain driven transmission designed by LuK and VAG/Audi uses a special lubricant which undergoes a phase change under extreme pressure to form a glassy solid, enabling the chain to transmit considerable torque through small contact surfaces.


Roller-based CVT

(marketed as the Traction CVT, Extroid CVT, Nuvinci CVP, or IVT)

Consider two almost-conical parts, point to point, with the sides dished such that the two parts could fill the central hole of a torus. One part is the input, and the other part is the output (they do not quite touch). Power is transferred from one side to the other by one or more rollers. When the roller's axis is perpendicular to the axis of the almost-conical parts, it contacts the almost-conical parts at same-diameter locations and thus gives a 1:1 gear ratio. The roller can be moved along the axis of the almost-conical parts, changing angle as needed to maintain contact. This will cause the roller to contact the almost-conical parts at varying and distinct diameters, giving a gear ratio of something other than 1:1. Systems may be partial or full toroidal. Full toroidal systems are the most efficient design while partial toroidals may still require a torque converter (e.g., Jatco "Extroid"), and hence lose efficiency.


Hydrostatic CVT

Hydrostatic transmissions use a variable displacement pump and a hydraulic motor. All power is transmitted by hydraulic fluid. These types can generally transmit more torque, but are very sensitive to contamination. Some designs are also very expensive. However, they have the advantage that the hydraulic motor can be mounted directly to the wheel hub, allowing a more flexible suspension system and eliminating efficiency losses from friction in the drive shaft and differential components. This type of transmission has been effectively applied to expensive versions of light duty ridden lawn mowers, garden tractors and some heavy equipment. Agricultral machinery including foragers and comblins butr not anything that works the ground beacause the transmission cannot transmit enough torque.


Hydristor IVT

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The Hydristor torque converter is a true IVT in that the front unit connected to the engine can displace from zero to 27 cubic inches per revolution forward and zero to -10 cubic inches per revolution reverse. The rear unit is capable of zero to 75 cubic inches per revolution. The common "kidney port" plate between the two sections communicates the hydraulic fluid under pressure and suction return in a "serpentine-torodial" flow path between the two Hydristor internal units. The IVT ratio is determined by the ratio of input displacement to output displacement. Therefore, the theoretical range of Hydristor IVT ratios is 1/infinity to +-infinity/1 but real-world ratios are constrained by physics.


Simkins' Ratcheting CVT

This transmission is an example of a Ratcheting CVT, prototyped as a bicycle transmission, protected under U.S. Patent #5516132. The input is the crank with a round hub integrated with it, and an array of twelve arms that are pivotally mounted to pins in the hub circle. Each arm has a pinion gear mounted on a one way clutch that allows only clockwise rotation of the pinion relative to the arm. All of these pinions are engaged with a large ring gear that is integrated with the chainwheel as the output, and the ring gear/chainwheel assembly is mounted to a mechanism that enables it to be adjusted from a position of concentricity with the crank hub to various amounts of eccentricity with the crank hub. Adjustment of this eccentricity variably changes the output/input ratio from 1:1 to 2.6:1 as the ring gear/sprocket assembly is moved from a position concentric with the crank hub to an eccentric position.

The eccentricity control mechanism is connected to a spring that pushes the transmission into its eccentric high gear position shown in the picture. The largest spread of the arms is indicative of the gear ratio because the spreading arms are the only arms whose pinions (and one-way clutches) are locked and driving the ring gear/chainwheel assembly. Strong pedaling torque causes this mechanism to react against the spring, moving the ring gear/chainwheel assembly toward a concentric, lower gear position. When the pedaling torque relaxes to lower levels, the transmission self-adjusts toward higher gears, accompanied by an increase in transmission vibration that produces a foot massage! This transmission behaves according to the definition of a Ratcheting CVT.


Anderson A+CVT

Anderson A+CVT is a technology invented by Larry Anderson, under US patents 6,575,856 and 6,955,620. Two parallel cones have "floating sprocket bars" mounted in longitudinal grooves around the circumference of each cone.

A specially-designed chain meshes with the floating sprocket bars, and is free to slide along the length of cones, changing the gear ratio at each point. The floating sprocket bars make the A+CVT positive-drive, non-friction-dependent. Another advantage of the A+CVT is the simplicity of its design, as it consists of far fewer components than other transmissions. The technology is also adaptable to a variable diameter pulley-type CVT, by mounting the floating sprocket bars on the inner face of the pulley sheaves. A few critics[attribution needed] have speculated that noise could be a problem with the A+CVT. However, Anderson has said that he believes noise will be no more of an issue with the A+CVT than with other transmissions, as the A+CVT will be lubricated and encased in a housing.


Advantages and drawbacks

Compared to hydraulic automatic transmissions:

* CVTs can smoothly compensate for changing vehicle speeds, allowing the engine speed to remain at its level of peak efficiency. They may also avoid torque converter losses. This improves both fuel economy and exhaust emissions. However, some units (e.g., Jatco "Extroid") also employ a torque converter. Fuel efficiency advantages as high as 20% over four-speed automatics can be obtained.

* CVTs have much smoother operation. This can give a perception of low power, because many drivers expect a jerk when they begin to move the vehicle. The satisfying jerk of a non-CVT transmission can be emulated by CVT control software though, eliminating this marketing problem.

* Since the CVT keeps the engine turning at constant RPMs over a wide range of vehicle speeds, pressing on the accelerator pedal will make the car move faster but doesn't change the sound coming from the engine as much as a conventional automatic transmission gear-shift. This confuses some drivers and again, leads to a mistaken impression of a lack of power.

* Most CVTs are simpler to build and repair.

* CVT torque handling capability is limited by the strength of their belt or chain, and by their ability to withstand friction wear between torque source and transmission medium for friction-driven CVTs. CVTs in production prior to 2005 are predominantly belt or chain driven and therefore typically limited to low powered cars and other light duty applications. More advanced IVT units using advanced lubricants, however, have been proven to support any amount of torque in production vehicles, including that used for buses, heavy trucks, and earth moving equipment.

Close-ratio transmission

A close-ratio transmission is a transmission in which there is a relatively little difference between the gear ratios of the gears. Consequently, note that the word close implies "near", not "shut". The gear ratio numbers are in a smaller numeric range, hence closer together.

In the context of close-ratio transmissions, a transmission with large differences between gears is termed "wide-ratio". Close-ratio vs. wide-ratio are relative terms, with no standardization. Therefore, a transmission that a manufacturer terms close-ratio when paired with a V8 engine with a wide power band may be termed wide-ratio when paired with a high-revving straight-4.


Comparison with ordinary transmission

Gear	"Typical" stock 5 speed	"Typical" close-ratio 5 speed for same vehicle
1st	3.25	2.60
2nd	1.90	1.66
3rd	1.20	1.35
4th	1.00	1.15
5th	0.80	1.00

Note how the ordinary 5 speed transmission has a high 5th gear. This is for fuel-efficient cruising with relatively low RPMs at freeway speeds. Such a high gear is not necessarily useful in a race situation. In order for the car's transmission be in that gear and simultaneously for its engine to be in its high-power RPM range, the absolute speed of the vehicle may simply be too great. In other words, the closeness of the ratios of a close-ratio transmission may be obtained by compressing both ends: raising the lower gear ratios, and lowering the higher ones.


Application

A close-ratio type of transmission is designed to allow an engine to remain in a relatively narrow operating speed. Alternately, a wide-ratio transmission requires the engine to operate over a greater speed range, but requires less shifting and allows a wider range of output speeds. Close-ratio transmissions are generally offered in sports cars, in which the engine is tuned for maximum power in a narrow range of operating speeds and the driver can be expected to enjoy shifting often to keep the engine in its power band.

A race is driven at high speed, close to the top speed that is achievable with the car's engine power. The speed has to be reduced for taking turns of various curvatures. Within this range of racing speeds, it may be useful to have many gears to choose from in order to always operate near the maximum engine speed.

Race cars do not have to deal with stop-and-go traffic, intersections, frequent stops, parallel parking, or climbing steep hills at slow speed. Race cars are also not called upon to perform fuel-efficient cruising at low RPM. Consequently, it makes little sense to have gears that support these driving situations at the expense of insufficient gear ratio variety for the intended use, and for this reason, a conventional 5-speed transmission would effectively offer too few useful gears in the race situation. After being used briefly at the very start of the race, the first two gears would never be used again. Moreover, the highest gear could even be too high to get the car into its top speed on a long, straight section of the course. The highest gear should be such that it allows the top speed of the car to coincide with the engine's peak power RPM, where the engine power is just sufficient to fight air drag and other sources of impedance.

The wide gear ratios may also simply be too far apart for fast acceleration, due to each successive gear dropping the engine RPM too low. Suppose that a given engine's power band lies between 7000 and 8000 RPM. Shifting up from a 1.20 gear to a 0.9 gear drops the original RPM by 25% (reducing it to 0.75 of the original rate). That is enough of a drop to take the engine out of its power RPM zone. For instance, if the shift is executed at 8000 RPM, the engine falls to about 6200 RPM, where it will generate a lot less power. The climb from 6200 will be a slow, labored acceleration. By contrast, shifting up from a 1.15 gear to a 1.0 gear represents only a 13% drop in engine revolution speed. Executed at 8000 RPM, the shift will achieve nearly 7000 RPM, just at the low end of the example engine's power band, allowing the car to continue accelerating quickly.


Pseudo-close-ratio transmissions

One way to obtain some of the benefits of a close-ratio transmission, without the compromises, is simply to cram more gears into the transmission. In fact, some six-speed gearboxes available in consumer vehicles are labelled as "close-ratio".

Whether a six speed transmission can be legitimatelly called "close-ratio" depends on whether, compared to a five speed model, it adds an extra high overdrive gear for leisurly freeway cruising, or whether it keeps the top gear about the same as in a comparable 5-speed model, and rather distributes more closely spaced ratios among the lower gears.

To simultaneously capture the advantages of a regular five-speed wide-ratio transmission as well as a five-speed close-ratio transmission, it can be argued that seven gears are required (like current Formula One cars and BMW M5). Six are too few, because a low first gear and a tall, smooth-cruising overdrive gear leave only four remaining gears. If truly close ratios are assigned to these, then second gear will be too tall.

Belt (mechanical)

Belts are used to mechanically link two or more rotating items. They may be used as a source of motion, to transmit power at up to 98% efficiency between two points, or to track relative movement.

As a source of motion, a conveyor belt is one application where the belt is adapted to continually carry a load between two points. A belt may also be looped (or crossed) between two points so that the direction of rotation is reversed at the other point.

Power transmission is achieved by specially designed belts and pulleys. The demands on a belt drive transmission system are large and this has led to many variations on the theme.

The earliest was the flat belt, used with line shafting. It is a simple system of power transmission that was well suited to its time in history. The Industrial Revolution soon demanded more from the system, as flat belt pulleys need to be carefully aligned to prevent the belt from slipping off. The flat belt also tends to slip on the pulley face when heavy loads are applied. In practice, such belts were often given a half-twist before joining the ends (forming a Möbius strip), so that wear was evenly distributed on both sides of the belt.

Round belts are a circular cross section belt designed to run in a pulley with a circular (or near circular) groove. They are for use in low torque situations and may be purchased in various lengths or cut to length and joined, either by a staple, gluing or welding (in the case of polyurethane). The early sewing machines utilized a leather belt, joined either by a metal staple or glued, to great affect.

Vee belts (also known as v-belt or wedge rope) are an early solution that solved the slippage and alignment problem. The V-belt was developed in 1917 by John Gates of the Gates Rubber Company. The "V" shape of the belt tracks in a mating groove in the pulley (or sheave), with the result that the belt cannot slip off. The belt also tends to wedge into the groove as the load increases — the greater the load, the greater the wedging action — improving torque transmission and making the vee belt an effective solution. They can be supplied at various fixed lengths or as a segmented section, where the segments are linked (spliced) to form a belt of the required length. For high-power requirements, two or more vee belts can be joined side-by-side in an arrangement called a multi-V, running on matching multi-groove sheaves. The strenght of these belts is obtained by reinforcements with fibers like steel, polyester or aramid (e.g. Twaron).

Timing belts, (also known as Toothed, Notch or Cog) belts are a positive transfer belt and can track relative movement. These belts have teeth that fit into a matching toothed pulley. When correctly tensioned, they have no slippage and are often used to transfer direct motion for indexing or timing purposes (hence their name). Camshafts of automobiles and stepper motors often utilize these belts.

Timing belts with a helical offset tooth design are available. The helical offset tooth design forms a chevron pattern and causes the teeth to engage progressively. The chevron pattern design is self-aligning. The chevron pattern design does not make the noise that some timing belts make at idiosyncratic speeds.

Belts normally transmit power only on the tension side of the loop. However, designs for continuously variable transmissions exist that use belts that are a series of solid metal blocks, linked together as in a chain, transmitting power on the compression side of the loop.

Super Select (Active-Trac)

Super Select is the brand name of a four-wheel drive system produced by Mitsubishi Motors, used worldwide except for North America, where it is known as Active-Trac. It was first introduced in 1991 with the then-new second generation of the Mitsubishi Pajero.

The system offers a choice of four rear- or four-wheel driving modes, selected using a lever mounted alongside the gear shift, and can be changed while the vehicle is in motion. In 2H mode the front axle is disconnected and the vehicle is rear-wheel drive. Reduced frictional losses in the powertrain mean that fuel economy improves while noise levels are reduced. 4H is a part-time four-wheel drive mode using a viscous coupling unit (VCU) and center differential to direct drive to the front wheels when the rear axle loses traction, and is capable of handling a wide variety of road conditions and speeds. 4HLc locks the center differential to provide extra traction for sandy, snowy or poorly surfaced roads in "high range" mode, while 4LLc, the "low range" mode, also offers a much lower gearing, providing the maximum amount of traction. Changing between 4HLc and 4LLc is only possible with the vehicle stationary.

The system is used on Mitsubishi's Pajero iO mini SUV, while its larger Pajero, Challenger, Triton and Delica models use a more complex system dubbed Super Select II (SS4-II). In most respects the two are the same, although the torque-split in SS4-II is 33/67 front/rear, meaning two thirds of the torque is channelled to the rear axle. In Super Select (SS4i) the torque-split is an equal 50/50. SS4-II also offers an option to lock the rear differential, offering greater traction to the rear axle.

Corvair engines Trivia and arcana

Many Corvair engine fans acquired a second life after the demise of their engines, mounted bottom side out on the outside of the wheels of Corvettes involved in road-racing, in order to pull air through the brakes and keep them cool. Lightweight and cheap, they were perfectly sized.

The single carburetor on each head of the two carburetor engine was not mounted symmetrically in the center of the intake manifold, where it might be intuitively placed, but offset from the center, between the middle and end cylinders. Although sometimes erroneously cited as an engineering error, this was in fact an example of clever attention to detail; had the carburetor been placed in the center of the manifold, the center cylinder would have received a significantly greater air/fuel charge then either end cylinder. Instead, the carburetor was situated so that the firing order required the air flow to reverse itself when filling either of the nearer cylinders, whereas the airflow to the far cylinder was merely an extension of the airflow to the center cylinder, which was just prior in the firing order. This allowed for a more balanced filling of the three cylinders, and smoother operation.

High performance parts manufacturer Edelbrock made available a set of larger bore aluminum cylinder barrels (with cast iron liners to withstand wear); when combined with their aluminum pushrods, the rate of thermal expansion of all parts of the valve train became compatible, so that solid valve lifters could be used, rather than the hydraulic lifters required by the stock cast iron cylinders. This in turn allowed the engine to run to higher RPMs; in conjunction with the increased torque resulting from the increase in cylinder bore, this resulted in a substantially more powerful engine.

In addition, "stroker" crankshafts with longer stroke were quickly made available for the original engine. When Chevrolet increased the stroke of the stock engine, however, there was no longer room to increase it any further.

Immediately after the car became available with the original two carburetor engine, a number of manufacturers began to sell conversion kits for attachment of four carburetors, with either two stock carburetors, two of the ubiquitous Stromberg 97 carburetors, or a Rochester two barrel carburetor for each bank of cylinders. The means of attachment varied from simple two into one adapters, to machining off the entire top surface of the intake manifold (cast as part of the head), enlarging the internal passages of the manifold, and attaching a new upper surface incorporating the appropriate mounting pads for the new carburetors. Similarly, aftermarket manufacturers provided several means of supercharging the original engine, including belt driven centrifugal, axial flow, or rotary vane type compressors. Chevrolet, seeing the marketing opportunity available in these aftermarket options, of course went on to offer its own four carburetor and turbocharged versions.

Corvair engines swapped into Volkswagens

Initially, the cooling fans were designed with a twist to the vanes, so that they were only efficient when rotating in the correct direction. Early on, however, the vanes on the fan became vertical and radial, so that the fan functioned identically in either rotation. Whatever the reason for this change, one effect was that the engine could easily be configured to run in the direction opposite from stock. This proved useful for those who swapped the engine into Volkswagen Beetles and dune buggies, since the Corvair engine's normal direction of rotation was opposite to that of the Volkswagen (and most other automobiles). Otherwise, the ring gear of the Volkswagen differential had to be flipped over by 180 degrees to allow the transmission's forward and reverse directions to be correct.

This swap was fairly common at the time, with the Corvair engine serving to give a power boost to Volkswagen Beetles, dune buggies, and Karmann Ghias. Excessively vigorous use of first gear would break the transaxle (the prudent driver would avoid first gear altogether), and the engine cover of the Karmann Ghia would not close completely with a Corvair engine in place, but otherwise the swap was relatively problem free, as such things go.

Chevrolet Corvair engine Problems

The Corvair engine design was so unique that good dealer service and maintenance was spotty. Mechanics, unused to the aluminum head and crankcase, would frequently overtighten threaded fasteners and spark plugs, stripping the threads out of the aluminum, requiring extensive repair.

Due to the greater thermal expansion of aluminum, hydraulic valve lifters were used to maintain correct lash as the engine expanded. These were trouble free and did not require periodic adjustment. Tuning issues related to the dual (or quadruple) carbs in non-turbocharged Corvairs sometimes led to erroneous diagnosis of valve issues in Corvairs- in fact, the Corvair had top quality valve materials in all models and valve jobs were almost never required. In fact, the valve train in most engines usually functioned perfectly for the life of the car.

Early engines were subject to occasional failures of the head gasket, between the heads and the cylinder barrels; this was addressed in later models by increasing the width of the sealing area and redesigning the gasket material and cross section, eliminating any issues.

The large cooling fan located on top of the engine required the fan belt to bend from the vertical plane of the crankshaft to the horizontal plane of the fan, causing additional stress. Chevrolet engineers designed a unique fan belt, which many owners and dealers replaced with an inappropriate design. The correct fan belt, properly installed to proper tension, worked well, while other belts even of proper size installed loose or tight would break frequently, giving the engine fan and belt design an undeserved bad reputation. Since failure of the cooling fan on an air-cooled engine leads to immediate overheating much more quickly than in a water-cooled engine (within 15 seconds at the high RPMs when the belts were likely to fail), mechanically inclined owners would routinely carry a spare belt and the 9/16 inch box wrench needed to change the belt, in addition to adding a large and eye catching warning light in parallel with the normally sized factory generator/alternator warning light. Aftermarket manufacturers made available differently sized pulleys which reduced the fan speed to 1.3 or 1.2 times engine speed, rather than the stock 1.5; this reduced the tendency to throw or break a fan belt for engines which spent most of their time at higher RPMs.

The pushrods were located below the cylinders, each in a separate metal tube between the crankcase and the head; these tubes also served to return oil from the head to the crankcase, and were fitted with neoprene O-rings at each end. After a short time, the neoprene exposed to the intense heat of the head lost resilience and developed a tendency to leak oil which became characteristic of Corvairs; unfortunately, since engine cooling air was diverted to the interior heater, this caused an unpleasant odor. Improved elastomer O-rings with much greater durability became available from aftermarket suppliers.

To address fuel slosh and cut-out issues in very hard cornering, some owners acquired an aftermarket kit to rotate the carburetors through ninety degrees and attach the now colinear throttle shafts of the two carburetors on each side together. However, this also eliminated the progressive feature of the stock carburetor linkage, so that performance could not be optimized both at low to midrange rpm and at high rpm.

Other owners replaced the four single-barrel carburetors with a single four-barrel carburetor, centrally mounted on a manifold with four long arms that attached to the original carburetor mounting pads on the heads. While this caused the carburetor and manifold to be slow to warm up to operating temperature and therefore caused problems with flooding and cold temperature operation, it eliminated linkage problems, simplified tuning the carburetor, and provided access to the large variety of four-barrel carburetors available on the market. This modification was especially ill-suited to models with Powerglide.

A factor which would have, in itself, led to the demise of the air cooled engine design was the rapid and relatively large temperature variation of the air-cooled engine with variations in load and rpm; this would have made meeting the upcoming emissions requirements of the 1970s difficult. Engine temperatures on lower performance Corvairs with the AIR system were comparable to the Turbocharged models in some situations- head temperatures under full throttle could exceed 600F.

Chevrolet Corvair engine

The Chevrolet Corvair engine was a flat-6 (or boxer engine) piston engine used exclusively in the 1960s Chevrolet Corvair automobile. It was a highly unusual engine for General Motors: It was air-cooled, used a flat design, with aluminum heads (incorporating integral intake manifolds) and crankcase, and individual iron cylinder barrels. The heads were modeled after the standard Chevrolet overhead valve design, with large valves operated by rocker arms, actuated by pushrods run off a nine lobe camshaft (exhaust lobes did double duty for two opposing cylinders) running directly on the crankcase bore without an inserted bearing, operating hydraulic valve lifters (which eliminated low temperature valve clatter otherwise seen with that much aluminum in the engine, due to its high degree of thermal expansion).


The flat horizontally opposed ("flat engine") air-cooled engine design, previously used by Volkswagen and Porsche as well as Lycoming aircraft engines, offered many advantages. Unlike inline or V designs, the horizontally opposed design made the engine inherently mechanically balanced, so that counterweights on the crankshaft were not necessary, reducing the weight greatly. Eliminating a water-cooling system further reduced the weight, and the use of aluminum for the heads and crankcase capitalized on this weight reduction; so that with the use of aluminum for the transaxle case, the entire engine/transaxle assembly weighed under 500 pounds (225 kilograms). In addition, the elimination of water-cooling eliminated several points of maintenance and possible failure, reducing them all to a single point; the fan belt. As with the Volkswagen and Porsche designs, the low weight and compact but wide packaging made the engine ideal for mounting in the rear of the car, eliminating the weight and space of a conventional driveshaft.


Two years after its 1960 debut, the Corvair engine gained another unusual attribute: it was the second production engine ever to be equipped from the factory with a turbocharger, released shortly after the Oldsmobile Jetfire V8.


Aircraft hobbyists and small volume builders, perhaps seeing the Corvair engine's similarity to Lycoming aircraft engines, very quickly began a cottage industry of modifying Corvair engines for aircraft use, which continues to this day. The Corvair engine also became a favorite for installation into modified Volkswagens and Porsches, as well as dune buggies and homemade sports and race cars.




140

The Corvair's innovative turbocharged engine; The turbo, located at top right, takes in air through the large air cleaner at top left, passes it through the sidedraft carburetor in between, and feeds pressurized fuel/air mixture into the engine through the chrome T-tube visible spanning the engine from left to right.
The Corvair's innovative turbocharged engine; The turbo, located at top right, takes in air through the large air cleaner at top left, passes it through the sidedraft carburetor in between, and feeds pressurized fuel/air mixture into the engine through the chrome T-tube visible spanning the engine from left to right.


The initial Corvair engine displaced 140 in³ (2.3 L) and produced 80 hp (60 kW). The high performance optional "Super TurboAir" version, introduced mid 1960 with a special camshaft and revised carburetors and valve springs produced 95 hp (70 kW).



145

In 1961, the engine received its first increases in size via a larger bore. The engine was now 145 in³ and the base engine was said to produce the same 80 hp (60 kW). The new high performance engine was rated at 98 hp (73 kW). In 1962 the high performance engine was rated at 102 hp (76 kW). The high compression 102 HP heads were added to the Monza models equipped with Powerglide when the standard engine was ordered, giving an 84 HP engine rating. 1962 engines returned to automatic chokes after a one year only manual choke on 1961 models.


The ultimate performance was found in the Spyder model, which became available with a turbocharged engine rated at 150 hp (112 kW). The turbocharger was mounted on the right side of the firewall behind the rear seat, fed by both exhaust manifolds; a single sidedraft carburetor mounted on the left side of the firewall fed directly into the turbocharger's intake, with a chromed pipe leading from the turbocharger's outlet to what would otherwise be the carburetor mounting pads on the intake manifolds, which were integral parts of the heads. The turbocharged heads received some valve upgrades to improve durability. Exhaust valves on turbocharged engines were made from a non-ferrous material used in jet engine turbine buckets, called 'Nimonic 80-A'. All other Corvair engines had slight upgrades in valve and valve seat materials as well for 1962.



164

The engine was stroked out (from 2.6" to 2.94") displacing 164 in (2.7 L) for 1964. Power output was boosted to 95 hp (70 kW) for the base model and 110 hp (80 kW) in the high performance normally aspirated engine, while the Turbocharged engine remained rated at 150 hp for this year. This increase in stroke was the maximum the engine could tolerate, to the point that the bottoms of the cylinder barrels had to be notched to clear the big end of the connecting rods.


For the 1965 model year, all engines had the head gasket area between the cylinder and the head widened, with a new design folded "Z" section stainless steel head gasket virtually eliminating any risk of head gasket failure. A 140 hp (104 kW) version with 4 single barrel carburetors, and a progressive linkage was introduced in 1965 as option L63 'Special High Performance Engine' and was standard equipment on the Corsa model. The carburetors consisted of a single barrel primary and a single barrel secondary on each head, connected by a progressive linkage; in addition, the heads featured a 9.25:1 compression ratio, and the cars received dual exhaust systems. Engines supplied with the automatic transmission after spring 1965 were modified with a camshaft from the 95 Horsepower base engine, and a special crankshaft gear that retarded its timing 4 degrees- the former to increase torque and smooth idle with the Powerglide transmission, the latter to restore some of the peak HP lost at higher engine speeds by the economy contoured camshaft with short timing.


1966 engines were basically carryover from the 1965 models, however Corvairs sold in California (except Turbocharged models) now featured the General Motors Air Injection Reactor System (AIR), and emissions control system consisting of an engine driven air pump that drew filtered air from the air cleaner, and injected a metered amount into the exhaust manifolds via tubing to promote complete oxidation and combustion of exhaust gasses to lower emissions. Specially calibrated carburetors and slight changes to the ignition timing and advance curves were part of the package. The AIR system had an unfortunate effect of sustantially raising exhaust gas, valve and head temperatures, particularly under heavy loads and this was a drawback on the Corvair where engine cooling could not be easily improved to cope with the higher temperatures. Nonetheless, performance and drivability were not noticably effected in most circumstances. In 1968, all Corvair (and other GM) engines got the AIR system for every market.


The 140 HP engine was officially discontinued for '67, but became optional in 1967 as COPO 9551-B, not a regular production option. Chevrolet sold 279 of these engines in the 1967 model year, 232 with manual transmissions, and 47 with Powerglide transmissions. Only six were sold with the four carburetor engine and the AIR injection system required by California emissions standards. These figures include 14 Yenko Stingers and 3 Dana Chevrolet variants of the Stinger.


Both the 140 HP engines and the Turbocharged engines had many special quality features not shared with lesser Corvairs- Moly insert top rings, stellite tips and faces on the valves, a Tufftrided (cold gas hardened) crankshaft, and Delco Moraine '400' aluminum engine bearings- the quality of the 140HP Corvair engine for materials is directly comparable to the Rolls Royce V8 of that era, item for item. It was a fabulous bargain for the $79 premium it commanded over the basic 95HP engine. Performance of the 140HP engine was better than you might expect, with a 5200 rpm peak horsepower output, it offered road performance in a Corvair comparable to contemporary Cadillac models of the day.


The turbocharged engine now developed 180 hp (134 kW). Contemporary reviews describe a similarity in power between the turbocharged and four-carburetor engines throughout the low and mid rpm range, with the turbocharged engine being superior only when it was possible to sustain boost continously. The turbocharged engines long suit was highway acceleration, flooring the accelerator at turnpike speeds produced ferocious acceleration in the upper speed ranges as the turbocharger began to boost, reaching manifold pressures approaching 15 PSI. No wastegate was used on the Corvair turbocharged engine, boost was controlled by careful balancing of exhaust restriction, mostly via the muffler, and intake restrictions from the smallish Carter YH carburetor used. Preignition and knock under boost was controlled using a novel 'pressure retard' device, essentially a modified vacuum advance device, on the specially curved distributor, as boost pressures built, ignition advance was progressively reduced to preclude detonation.

Variable geometry turbocharger

The Variable geometry turbocharger (VGT) exists in several forms, usually designed to allow the effective A/R ratio of the turbo to be altered as the conditions change. This is done as the optimum A/R at low engine speeds is very different to the optimum at high engine speeds. If too large an A/R ratio is used, the turbo will fail to create boost until a relatively high engine speed. However, if too small an A/R ratio is used, the turbo will choke the engine at high speeds, leading to large exhaust manifold pressures, high pumping losses and ultimately lower power. By altering the geometry of the turbine housing as the engine accelerates, the turbo's A/R ratio can be maintained at its optimum. Because of this, VGT turbochargers have a minimal amount of lag, have a low boost threshold and are very efficient at higher engine speeds. In many setups these turbos don't even need a wastegate. This however depends on whether the fully open position is sufficiently open to allow boost to be controlled to the desired level at all times. Some VGT implementations have been known to over-boost if a wastegate is not fitted.

The most common implementation is a set of several aerodynamically-shaped vanes in the turbine housing near the turbine inlet. As these vanes move, the area between the tips of them change, thereby leading to a variable A/R ratio. Usually, the vanes are controlled by a membrane actuator identical to the one on a wastegate, although electric servo actuated vanes are becoming more common.

The first production car to use these turbos was the limited-production 1989 Shelby CSX-VNT, equipped with a 2.2L petrol engine. The Shelby CSX-VNT utilised a turbo from Garrett, called the VNT-25 because it uses the same compressor and shaft as the more common Garrett T-25. This type of turbine is called a Variable Nozzle Turbine (VNT). Turbocharger manufacturer Aerocharger uses the term 'Variable Area Turbine Nozzle' (VATN) to describe this type of turbine nozzle. Other common terms include Variable Turbine Geometry (VTG), Variable Geometry Turbo (VGT) and Variable Vane Turbine (VVT).

The 2006 Porsche 911 Turbo has a twin turbocharged 3.6-litre flat six, and the turbos used are BorgWarner's Variable Turbine Geometry (VTGs). This is significant because although VGTs have been used on advanced turbo diesel engines for a few years and on the Shelby CSX-VNT, this is the first time the technology has been implemented on a high production petrol car (only 500 Shelby CSX-VNTs were ever produced). Exhaust temperatures in petrol cars are much higher than in diesel cars and this normally has adverse effects on the delicate, moveable vanes of the turbo. BorgWarner engineers however have managed to combat this problem with the new 911 Turbo.

Radial tire

A radial tire (more properly, a radial-ply tire) is a particular design of automotive tire (in British English, tyre). The design was originally developed by Michelin in 1946 [1] but, because of its advantages, has now become the standard design for essentially all automotive tires.

Tires are not fabricated just from rubber; they would be far too flexible and weak. Within the rubber are a series of plies of cord that act as reinforcement. All common tires (since at least the 1960s) are made of layers of rubber and cords of polyester, steel, and/or other textile materials. This network of cords that gives the tire strength and shape is called the carcass.

In the past, the fabric was built up on a flat steel drum, with the cords at an angle of about +60 and -60 degrees from the direction of travel, so they criss-crossed over each other. They were called cross ply or bias ply tires. The plies were turned up around the steel wire beads and the combined tread/sidewall applied. The green (uncured) tire was loaded over a curing bladder and shaped into the mould. This shaping process caused the cords in the tire to assume an S shape from bead to bead. The angle under the tread stretched down to about 36 degrees. This was called the Crown Angle. In the sidewall region the angle was 45 degrees and in the bead it remained at 60 degrees. The low crown angle gave rigidity to support the tread and the high sidewall angle gave comfort.

By comparison, radial tires lay all of the cord plies at 90 degrees to the direction of travel (that is, across the tire from lip to lip). This design avoids having the plies rub against each other as the tire flexes, reducing the rolling friction of the tire. This allows vehicles with radial tires to achieve better fuel economy than vehicles with bias-ply tires. It also accounts for the slightly "low on air" (bulging) look that radial tire sidewalls have, especially when compared to bias-ply tires.


Construction

As described, a radial tire would not be sufficiently strong and the surface in contact with the ground would not be sufficiently rigid. To add further strength, the entire tire is surrounded by additional belts that are oriented along the direction of travel. First made of tire cord, these belts were made of steel (hence the term "steel-belted radial") by 1948, and subsequently aramid fibers such as Kevlar.

In this way, radial tires separate the tire carcass into two separate systems:

* The radial cords in the sidewall allow it to act like a spring, giving flexibilty and ride comfort.

* The rigid steel belts reinforce the tread region, giving high mileage and performance.

Each system can then be individually optimized for best performance.

Filter (oil)

Many items requiring lubrication by petroleum products need the lubricant to be especially clean. The oil filter is a device used for this purpose, particularly in automotive and other applications for internal combustion engines.

Early automobiles did not have any way of filtering oil. For this reason, along with the low standards to which lubricating oil was generally refined in the era, very frequent oil changes, of the order of every 500 miles (800km) or 1000 miles (1600 km) were often specified for early vehicles. As automotive technology advanced, the first oil filtration devices were developed, becoming widespread by the late 1920s. Early automotive oil filters were largely of the cartridge type, generally consisting of a pleated paper element, surrounded by a metal canister perforated with many holes inside a sheet metal housing.

Cartridge-type oil filters were a considerable advance over the previous practice, of the oil going unfiltered through the engine but were still only partially effective, in that much of the oil bypassed the filter, which was located on an entirely separate oil line and, hence, went unfiltered. By the 1950s, the 'spin-on' or 'full flow' filter had become widespread. This device attaches directly to the side of the engine block, by a threaded fitting and was positioned so that all of the engine's oil capacity eventually had to pass through it during the course of normal operation. This type of filter is now used almost exclusively in modern passenger cars and, in recent years, has gained in use even in heavy-duty uses such as large truck engines and non-road going equipment such as bulldozers. Oil quality and filtering capabilities have now advanced so far that some manufacturers such as Mobil sell engine oils and filters that claim to have up to a 15,000 mile change interval. Many vehicle manufacturers recommend replacing the fitler each and every time the oil is changed. A dirty filter can quickly contaminate clean oil.

Some spin-on filters incorporate an integrated pressure relief valve. If the filter becomes completely blocked due to a lack of maintenance, this valve allows oil to bypass the blocked filter, which protects the bearings from oil starvation. The valve may also open in very cold conditions if a high viscosity oil is used.

Major brands of oil filters available in the U.S. include Fram (an Allied Signal brand), Wix, AC Delco (a General Motors brand) and Motorcraft (a Ford Motor Company brand). Some brands, such as Ford's Motorcraft, are manufactured by other companies (i.e. Purolator for Motorcraft) but are generally designed and quality tested by the brand selling them. Many of the brands manufature filters for a wide variety of makes and models of vehicles. For instance, Motorcraft sells oil filters that fit GM, Chrysler, Honda, and Toyota vehicles, in addition to Fords. The manufacturer usually provides a list of what makes and models they supply filters for.

Some have argued that there is a major difference in quality of various oil filter brands, and some studies have proven it. Generally speaking, those branded by automotive manufactuers (such as Motorcraft and AC Delco as listed above) usually meet higher standards without costing significantly more than cheaper-made (and poorer performing) brands such as Fram or Penzoil brand. Very expensive brands such as Mobil and K&N perform excellently, but cost a lot more than traditional brands.

Many major autoparts stores (such as AutoZone, which sells the Valucraft brand and NAPA, which sells NAPA and NAPA GOLD) offer their own brands of oil filters, but these are usually also made by one of the other major oil fiter makers.

Oil filters are not limited to automotive use. Power generating stations use upwards of 40,000 gallons of turbine lube oil to lubricate large bearings. Hydraulic lines are used in industry for many purposes. All of this oil needs to be filtered and the level of filtration is much more stringent than that of standard automobile filtration. In these applications many times a resin impregnated glass fiber filtration media down to even 1um is used, whereas in automobile filtration it is always cellulose which has a micron rating of 50um or more. Industrial applications do not "change their oil" frequently as changing tens of thousands of gallons of oil @ $10 a gallon quickly adds up. This is why much higher quality filters are usually used. Subsequently the cost for an industrial grade oil filter can be anywhere from $50 to $1000 (depending on size). You can not purchase an industrial grade filter and expect it to fit on your car, as these filters are sometimes 6" in diameter and upwards of 60" long. Nor would you want to, as in automobile filtration problems often result from the additives package breaking down, more so than particle contamination. Major players in industrial oil filtration are Pall, Donaldson, Parker, Kaydon, and Vickers. The industrial oil filtration market is full of retrofitted or will-fit filter elements. Every major manufacturer has a filter element that will fit in another manufacturers housing. Some manufacturers specialize in only retro-fitting other manufacturers filters elements, usually for 1/4 to 1/2 the cost.

Nissan VG engine

The VG engine family consists of V6 piston engines designed and produced by Nissan for several vehicles in the Nissan lineup. The VG series started in 1983 becoming Japan's first mass produced V6 engine. VG engines displace between 2.0 L and 3.3 L and feature an iron block and aluminum head. The early VG30 featured SOHC, 12 valve heads. A Later revision featured a slightly different block, and DOHC, 24 valve heads with Nissan's own version of variable valve timing for increased high RPM efficiency. The block is a particularly strong design with a single piece main bearing cap, and is capable of reliably supporting more than 1000hp. The production blocks and production head castings were used successfully in the Nissan IMSA cars in the 80's and 90's.

The VG series engine found its way into thousands of Nissan vehicles, starting in 1984. The VG design was retired in 2004, as all models received the VQ series engine instead.


VG20E

The VG20E is a 2 L (1998 cc) engine produced from 1984 on. It produces 126 hp.

Applications:

* Nissan Gloria/Nissan Cedric
* Nissan Leopard
* Nissan Fairlady Z


VG20ET

The VG20ET is the same as the VG20E, but with turbocharger. The VG20ET produces 170 hp.

It was used in the following vehicles:

* Nissan 200Z (Z31)
* Nissan 200ZG (Z31)
* Nissan 200ZS (Z31)


VG20DET

The VG20DET is an 2,0L engine with DOHC and a turbocharger. It produces 210 hp.

It was used in the following vehicles:

* Nissan Leopard (F31)
* Nissan Gloria/Nissan Cedric (Y31)


VG20P

The VG20P is the autogas LPG (Liquified petroleum gas) version of the VG20. It produces 99 ps @5600 rpm and 149 nm @2400 rpm. It is an OHC 12 valve engine.

It is used in the following vehicles:

1987-1991 Nissan Cedric Y31


VG30i

The VG30i is a 3 L (2960 cc) engine produced from 1986 through 1989 and featured a throttle body fuel injection system. It has a long crank snout, a cylinder head temperature sensor positioned behind the timing belt cover, and a knock sensor in the cylinder valley.

Applications:

* 1986-1989 Hardbody Truck
* 1986-1989 Nissan Pathfinder


VG30E

The 3.0 L (2960 cc) VG30E produced 153 hp and 182 ft.lbf. Bore is 3.43 in (87 mm) and stroke is 3.27 in (83 mm). In 300ZX form, it produced 160hp and 174lb-ft. 1988 saw the 300ZX gain 5 more horses for a total of 165; however, torque ratings remained the same. In 1989, the Maxima received the 160hp rating, but also used a variable intake plenum that let it make 182lb-ft@3200rpm. Strangely, the 300ZX never received the variable intake plenum.

It was used in the following vehicles:

* 1984–1989 Nissan 300ZX/Nissan Fairlady Z
* 1987–1988 Nissan 200SX SE
* 1984–1994 Nissan Maxima
* 1990–1991 Infiniti M30/Nissan Leopard
* 1990–1996 Hardbody Truck
* 1990–1995 Nissan Pathfinder/Nissan Terrano
* 1992–1999 Nissan Gloria/Nissan Cedric (179 hp)
* 1992–1995 Nissan Quest/Mercury Villager (modified to become a non-interference design)


VG30ET

The 3.0 L (2960 cc) VG30ET was available in early production with a single Garrett T3 turbocharger and a 7.8:1 compression ratio. The USDM version produced 200 hp and 227 ft.lbf. In 1988 it changed to a single Garrett T25 turbocharger and an 8.3:1 compression ratio to reduce turbo lag, and was bumped to 205 hp and 227 ft.lbf. No VG30ET was ever factory equipped with an intercooler as they featured low boost pressure for fast response.

It was used in the following vehicles:

* 1984–1989 Nissan 300ZX Turbo
* 1984–1986 Nissan Fairlady Z
* Nissan Leopard
* Nissan Gloria/Nissan Cedric


VG30DE

The 3.0 L (2960 cc) VG30DE produces 222 hp and 198 ft.lbf. Bore is 3.43 in (87 mm) and stroke is 3.27 in (83 mm).

It is used in the following vehicles:

* 1990–1996 Nissan 300ZX
* 1987–1999 Nissan Fairlady Z
* 1988 Nissan 200zx (similar chassis to the 300zx, different panels
* 1993–1998 Infiniti J30 and Nissan Leopard J. Ferie
* 1992–1995 Nissan Gloria and Cedric
* 1989–1991 Nissan Cima


VG30DET

The VG30DET is the same as the VG30DE, but with a single ceramic turbo. It produces 255 hp. It was used in the Nissan Leopard (F31)

As well as the Nissan Cima, Gloria, Cedric (y31).


VG30DETT

The 3.0 L (2960 cc) VG30DETT produces 280 hp and 283 ft.lbf when mated with a 4 speed automatic transmission. When used with a 5spd manual transmission, it was rated at 300hp and 283tq. It featured twin T25 turbochargers, twin intercoolers and variable valve timing.

It is used in the following vehicles:

* 1990–1996 Nissan 300ZX Twin Turbo
* 1990–1999 Nissan Fairlady Z Twin Turbo


VG33E

The VG33E is a 3.3 L (3275 cc) version built in Smyrna, TN. Bore is 91.5 mm and stroke is 83 mm. Output is 180 hp (134 kW) at 4800 RPM with 202 ft.lbf (274 Nm) of torque at 2800 RPM. It has a cast iron engine block and aluminum SOHC cylinder heads. It uses SFI fuel injection, has 2 valves per cylinder with roller followers and features forged steel connecting rods, a one-piece cast camshaft, and a cast aluminum intake manifold.

It is used in the following vehicles:

* 1996–1999 Nissan Pathfinder
* 2000–2004 Nissan Frontier
* 2000–2004 Nissan Xterra
* 1998–2004 Nissan Elgrand
* 1999–2002 Nissan Quest/Mercury Villager


VG33ER

The 3.3 L (3275 cc) VG33ER or VG33S is supercharged and produces 210 hp (157 kW) at 4800 RPM with 246 ft.lbf (334 Nm) of torque at 2800 RPM.

It is used in the following vehicles:

* 2001–2004 Nissan Frontier SC
* 2001–2004 Nissan Xterra SC

Mazda RX-7 Motorsport

Racing versions of the first-generation RX-7 were entered at the prestigious 24 hours of Le Mans endurance race. The first outing for the car, equipped with a 13B engine, failed by less than one second to qualify in 1979. The next year, a 12A-engine car not only qualified, it placed 21st overall. That same car did not finish in 1981, along with two more 13B cars. Those two cars were back for 1982, with one 14th place finish and another DNF. The RX-7 Le Mans effort was replaced by the 717C prototype for 1983. In 1991, Mazda became the first (and so far, only) Japanese manufacturer to win the 24 hours of Le Mans. The car was a 4-rotor prototype class car, the 787B. Le Mans outlawed rotary engines shortly after this win.

Mazda began racing RX-7s in the IMSA GTU series in 1979. That first year, RX-7s placed first and second at the 24 Hours of Daytona, and claimed the GTU series championship. The car continued winning, claiming the GTU championship seven years in a row. The RX-7 took the GTO championship ten years in a row from 1982. The RX-7 has won more IMSA races than any other car model.

The RX-7 also fared well at the Spa 24 Hours race. Three Savanna/RX-7s were entered in 1981 by Tom Walkinshaw Racing. After hours of battling with several BMW 530i and Ford Capri, the RX-7 driven by Pierre Dieudonné and Tom Walkinshaw won the event. Mazda had turned the tables on BMW, who had beaten Mazda's Familia Rotary to the podium eleven years earlier at the same event. TWR's prepared RX-7s also won the British Touring Car Championship in 1980 and 1981, driven by Win Percy.

Canadian/Australian touring car driver Allan Moffat was instrumental in bringing Mazda into the Australian touring car scene. Over a four year span beginning in 1981, Moffat took the Mazda RX-7 to victory in the 1983 Australian Touring Car Championship, as well as a trio of Bathurst 1000 podiums, in 1981 (3rd with Derek Bell), 1983 (second with Yoshima Katayama) and 1984 (third with former motorcycle champion Gregg Hansford). Australia's adoption of international Group A regulations, combined with Mazda's reluctance to homologate a Group A RX-7, ended Mazda's active participation in the touring car series at the end of the 1984 season.

The RX-7 even made an appearance in the World Rally Championship. The car finished 11th on its debut at the RAC Rally in Wales in 1981. Group B received much of the focus for the first part of the 1980s, but Mazda did manage to place third at the 1985 Acropolis Rally, and the Familia 4WD claimed the victory at Swedish Rally in both 1987 and 1989.

The RX-7 is considered as a popular choice in import drag racing, during the late nineties toward 2004 Abel Ibarra raced a spaceframe FD which averaged no less than high 6 seconds passes, until he replaced it with a spaceframe RX-8, the FD was later to shipped and sold to an Australian.

The FC and FD is considered a popular choice for drifting contests, given the long wheelbase and an average of 450bhp. Youichi Imamura won the D1 Grand Prix title in 2003 and Masao Suenaga narrowly lost his in 2005, both in FDs.

The RX-7 is a popular choice among autocross drivers,

In Japan, the RX-7 has always been a popular choice in domestic events, competing in Group 5 based Formula Silouette to its modern day incarnation, the Super GT series from when the Japan Sport Sedan series would become the GT300 category which it had been competing in. Its patience would pay off as in 2006, RE Amemiya Racing Asparadrink FD3S won the GT300 class championship.


Notes

Recently, Mazda has revived the rotary engine in the form of the RX-8. It produces approximately 232HP naturally aspirated (the FD3S produces 255hp stock with two Hitachi turbochargers), while the Japanese market version produces around 250HP.


Movies and TV

The car appeared in the Japanese anime series, Initial D, driven by the Takahashi brothers; Keisuke and Ryosuke, with Keisuke driving a yellow FD3S and Ryosuke driving a white FC3S.The FD was featured in several Need for Speed games as well as Japanese imported racing computer games. Many who played Need for Speed: Underground praised the FD for its versatile capabilities.[verification needed] The RX-7 has made an appearance in every Gran Turismo game to date, which includes many types of the different generations, including specially tuned racing versions by the various 'shops' in 'Gran Turismo mode'. The RX-7 has also been featured in every film in the 'Fast and the Furious' franchise so far.


Video Game Appearances

* Auto Modellista features all three generations of the RX-7, which can also be customized and driven.
* Driving Emotion Type-S features the FC and FD RX-7.
* D1 Grand Prix features the FC and FD models which can be customized and driven.
* Enthusia Professional Racing features all three generations of the RX-7.
* Forza Motorsport features both the FD and FC RX-7.
* Full Throttle (known as Top Speed outside Japan) was the first game to feature an FC3S in its 1987 release.
* Initial D features the FC and FD RX-7 which can be customized and driven.
* Juiced features the stock FD RX-7.
* Gran Turismo features racing and stock versions of the FC and FD RX-7.
* Gran Turismo 2 features more racing and stock versions of the RX-7.
* Gran Turismo 3: A-Spec features more racing and stock versions of the RX-7 and also offers the LM version as a prize car.
* Gran Turismo 4 features more racing and stock versions of the RX-7. Some RX-7s are also available in the Used Car Shops. The LM version again makes an appearance with notable engine sound effects.
* Gran Turismo HD will feature the Veilside RX-7.
* The Need for Speed features the 1993 FD RX-7.
* Need for Speed: Underground features an FD RX-7 that can be unlocked and then driven and modified in the game.
* Need for Speed: Underground 2 features the RX-7 with more parts available.
* Need for Speed: Most Wanted features the RX-7.
* Need for Speed: Carbon features the RX-7.
* R:Racing Evolution features the FC and FD RX-7.
* Sega GT also features the last two generations of the RX-7.
* Sega GT 2002 and its Online incarnation feature all three generations of the RX-7.
* Street Racing Syndicate features a customisable FD RX-7.
* Tokyo Xtreme Racer: Zero features all three generations of the RX-7, which can also be customized and driven
* Tokyo Xtreme Racer: 3 features all three generations of the RX-7, which can also be customized and driven
* Import Tuner Challenge Features the Mazda RX7 (1993)
* The Fast and The Furious Features the Mazda RX7 Infini and the original RX7

Mazda RX-7

The Mazda RX-7 (also called the Ẽfini RX-7) is a sports car produced by the Japanese automaker Mazda since 1978. The original RX-7 competed in the affordable sports car segment with the likes of the Nissan Fairlady Z. The styling was inspired by the Lotus Elan 2+2. It featured a unique twin-rotor Wankel rotary engine and a sporty front-midship, rear-wheel drive layout, making it well balanced and appropriate for racing. The RX-7 was a direct replacement for the RX-3 (both were sold in Japan as the Savanna) and subsequently replaced all other Mazda rotary cars with the exception of the Cosmo.

The original RX-7 was a true sports coupé design, as opposed to a sports car like the Triumph TR6 or a sedan with sporting intentions. The compact and light-weight Wankel engine, also known as a rotary engine is situated slightly behind the front axle. It was offered in America as a two-seat coupé, with four seats being optional in Japan, Australia, and other parts of the world.

The RX-7 made Car and Driver magazine's Ten Best list five times. In total, 811,634 RX-7s were produced.




First generation (SA/FB)

* Series 1 (1979–1980) is commonly referred to as the "SA22C" from the first alphanumerics of the vehicle identification number- although RX-7 tech site Rotorhead.ca points out that the chassis code used by Mazda was 'P642'. This series of RX-7 had exposed steel bumpers and a high-mounted license plate located in an indented part of the rear of the car, famously criticized by Werner Buhrer of Road & Track magazine as a "Baroque depression."

* Series 2 (1981–1983) had smoothly integrated plastic-covered bumpers, wide black rubber body side moldings, wraparound taillights and updated engine control components. The GSL package provided optional 4-wheel disc brakes and clutch-type rear limited slip differential (LSD). Known as the "FB" in North America after the US Department of Transportation mandated 17 digit Vehicle Identification Number changeover. Elsewhere in the world, the 1981-1985 RX-7 is technically still an 'SA22C' among enthusiasts.

* Series 3 (1984–1985) featured an updated lower front fascia and different instrument cluster (the S3 RX-7 is the only rotary-engined car to not have a centrally mounted tachometer). GSL package was continued into this series, but Mazda introduced the GSL-SE sub-model. The GSL-SE had a fuel injected 1.3 L 13B RE-EGI engine producing 135 hp (101 kW) and 135 lb-ft. GSL-SEs had much the same options as the GSL (clutch-type rear LSD and rear disc brakes), but the brake rotors were larger, allowing Mazda to use the more common lug nuts (versus bolts), and a new bolt pattern of 4x114.3 (4x4.5"). Also, they had upgraded suspension with stiffer springs and shocks, and a new, heavy duty oil cooler.

The 1984 RX-7 S has an estimated 29 highway miles per gallon (12.33 kilometres per litre)/19 estimated city miles per gallon (8.08 k/l). According to Mazda, its rotary engine, licensed by NSU-Wankel allowed the RX-7 S to accelerate from 0 to 50 (80kph) in 6.3 seconds. Kelley Blue Book, in its January-February 1984 issue, noted that a 1981 RX-7 S retained 93.4% of its original sticker price.

The handling and acceleration of the car were noted to be of a high caliber for its day. This generation RX-7 had "live axle" 4-link rear suspension with Watt's linkage, a 50/50 weight ratio, and weighed under 2600 lb (1180 kg). It was the lightest generation of RX-7 ever produced. 12A-powered models accelerated from 0–60 mph in 9.2 s, and turned 0.779 lateral Gs on a skidpad. The 12A engine produced 100 hp (75 kW) at 6000 rpm, allowing the car to reach speeds of over 120 mph (190 km/h). Because of the smoothness inherent in the Wankel rotary engine, little vibration or harshness was experienced at high rpm, so a buzzer was fitted to the tachometer to warn the driver when the 7000 rpm redline was approaching.

Options and models varied from country to country. The gauge layout and interior styling in the Series 3 was only changed for North American versions. Additionally, North America was the only market to have offered the first generation RX-7 with the fuel injected 13B. A turbocharged (but non-intercooled) 12A engine was available for the top-end model of Series 3 in Japan.

Sales were strong, with a total of 474,565 first generation cars produced; 377,878 were sold in the United States alone. In 2004, Sports Car International named this car #7 on its list of Top Sports Cars of the 1970s. In 1983, the RX-7 would appear on Car and Driver magazine's Ten Best list for the first time.



Second generation (FC)


* Series 4 (1986–1988) was available with a naturally aspirated, fuel-injected 13B-VDEI producing 146 hp (108 kW). An optional turbocharged model, known as the Turbo II, had 182 hp (141 kW).

* Series 5 (1989–1992) featured updated styling and better engine management, as well as lighter rotors and a higher compression ratio, 9.7:1 for the naturally aspirated model, and 9.0:1 for the turbo model. The Turbo II monicker was dropped, and the turbocharged model was simply dubbed Turbo. The naturally aspirated Series 5 FC made 160 hp (119 kW), while the Series 5 Turbo made 200 hp (147 kW).

The second generation RX-7 ("FC", VIN begins JM1FC3 or JMZFC1), still known as the "Savanna RX-7" in Japan, featured a complete restyling reminiscent of the Porsche 928. Mazda's stylists, lead by Chief Project Engineer Akio Uchiyama, actually focused more on the Porsche 944 for their inspiration in designing the FC because the new car was being styled primarily for the American market where the majority of first generation RX-7's had been sold. This strategy was chosen after Uchiyama and others on the design team spent time in the United States studying owners of earlier RX-7's and other sports cars popular in the American market. The Porsche 944 was selling particularly well at the time and provided clues as to what sports car enthusiasts might find compelling in future RX-7 styling and equipment. While the SA22/FB was a purer sports car, the FC tended toward the softer sport-tourer trends of its day. Handling was much improved, with less of the oversteer tendencies of the FB. Steering was more precise, with rack and pinion steering replacing the old recirculating ball steering of the FB. Disc brakes also became standard, with some models (S4: GXL, GTU, Turbo II, Convertible; S5: GTUs, Turbo, Convertible) offering four-piston front brakes. The revised independent rear suspension incorporated special toe control hubs which were capable of introducing a limited degree of passive rear steering under cornering loads. The rear seats were optional in some models of the FC RX-7, but are not commonly found.

Though about 80 lb heavier and more isolated than its predecessor, the FC continued to win accolades from the press. The FC RX-7 was Motor Trend's Import Car of the Year for 1986, and the Turbo II was on Car and Driver magazine's Ten Best list for a second time in 1987.

In 1988, a convertible version started production in atmospheric and turbocharged form, proving an instant success. This sleek, clean lined model featured a cabriolet design and was introduced to the American market in splashy television advertisements featuring Hollywood actor James Garner. Several leading car magazines at the time also selected the convertible as the best ragtop available on the market, and it was the star of auto shows around the globe. The convertible's well orchestrated introduction caused a notable public sensation and heavy demand for these vehicles. Dealers took full advantage of the situation, charging up to $5,000 above Mazda's suggested retail selling price with buyers happy to pay the premium. It is believed Mazda exported approximately five thousand convertibles to the United States in 1988 and fewer in each of the next three model years, although it is difficult to confirm these figures, as Mazda USA did not keep RX-7 import records by model type. Despite production ceasing in October 1991, Mazda built a limited run of 500 convertibles for 1992 as "specials" for the domestic market only. In Japan, the United Kingdom, and other regions outside the US, a turbocharged version of the convertible was available. Being former "dream cars", it now appears a nascent collectors market is developing for these classic, semi-exotic sports cars.

In the Japanese market, only the turbo engine was available; the atmospheric version was allowed only as an export. This can be attributed to insurance companies penalising turbo cars (thus restricting potential sales). This emphasis on containing horsepower and placating insurance companies to make RX-7's more affordable seems ironic in retrospect. Shortly after the discontinuance of the second generation RX-7's in 1991, an outright horsepower "arms race" broke out between sports car manufacturers, with higher and higher levels of horsepower required to meet buyer demands.

Overall, the second generation was the most successful for Mazda sales wise, with 86,000 units sold in the US alone in 1986, its first model year. The FC model is believed to have achieved its peak in sales in 1988..



Third generation (FD)

* Series 6 (1992–1995) was exported throughout the world and had the highest sales. In Japan, Mazda sold the RX-7 through its Efini brand as the Efini RX-7. Only the 1993–1995 model years were sold in the U.S. and Canada.

* Series 7 (1996–1998) included minor changes to the car. Updates included a simplified vacuum routing manifold and a 16-bit ECU allowing for increased boost which netted an extra 10 hp. In Japan, the Series 7 RX-7 was marketed under the Mazda brand name. The Series 7 was also sold in Australia, New Zealand and the U.K. Series 7 RX-7s were produced only in right-hand-drive configuration.

* Series 8 (January 1999– August 2002) was the final series, and was only available in the Japanese market. More efficient turbochargers were installed, while improved intercooling and radiator cooling was made possible by a revised frontal area. The seats, steering wheel, and front and rear lights were all changed. The rear wing was modified and gained adjustability. The top-of-the-line "Type RS" came equipped with a Bilstein suspension and 17" wheels as standard equipment, and reduced weight to 1280 kg. Power was officially claimed as 280 ps (276 hp, 208 kW) (with 330 N·m (243 ft·lbf) of torque) as per the maximum Japanese limit, though realistic power was more likely 220–230 kW (290–308 hp). The Type RZ version included all the features of the Type RS, but at a lighter weight (at 1270 kg). It also featured custom gun-metal colored BBS wheels and a custom red racing themed interior. Further upgrades included a new 16-bit ECU and ABS system upgrades. The improved ABS system worked by braking differently on each wheel, allowing the car better turning during braking. The effective result made for safer driving for the average buyer. Easily the most collectible of all the RX-7s was the last 1,500 run-out specials. Dubbed the "Spirit R", they combined all the "extra" features Mazda had used on previous limited-run specials and all sold within days of being announced. They still command amazing prices on the Japanese used car scene years later.

-There are three kinds of "Spirit R"s: the "Spirit A", "Spirit B", and "Spirit C". The "Spirit A", which accounts for 1,000 of the 1,500 "Spirit" models produced, has a 5-speed manual transmission, and is said to have the best performance of the three models. The "Spirit B" is a four-seater, and sports a 5-speed manual transmission. "The Spirit C" is also a four-seater, but has a 4-speed automatic transmission.

There is also a "Touring Model" which includes a sun roof, and Bose stereo system. Compared to the R1 and R2 which both don't have a moon roof, and they have an extra front oil cooler in the front bumper, and other race modification equipment

The third and final generation of the RX-7, FD (with FD3S for the JDM and JM1FD for the USA VIN), was an outright, no-compromise sports car by Japanese standards. It featured an aerodynamic, futuristic-looking body design (a testament to its near 11-year lifespan). The 13B-REW was the first-ever mass-produced sequential twin-turbocharger system to export from Japan, boosting power to 255 hp (190 kW) in 1993 and finally 280 ps (276 hp, 208 kW, the Japanese manufacturers' gentlemen's agreement on engine power) by the time production ended in Japan in 2002.

The FD RX-7 was Motor Trend's Import\Domestic Car of the Year. When Playboy magazine first reviewed the FD RX-7 in 1993, they tested it in the same issue as the [then] new Dodge Viper. In that issue, Playboy declared the RX-7 to be the better of the two cars. It went on to win Playboy's Car of the Year for 1993. The FD RX-7 also made Car and Driver magazine's Ten Best list for 1993 through 1995.

The sequential twin turbocharged system was a very complex piece of engineering, developed with the aid of Hitachi and previously used on the domestic Cosmo series (JC Cosmo=90–95). The system was comprised of two small turbochargers, one to provide torque at low RPM. The 2nd unit was on standby until the upper half of the rpm range during full throttle acceleration. The first turbocharger provided 10 psi of boost from 1800 rpm, and the 2nd turbocharger was activated at 4000 rpm and also provided 10 psi (70 kPa). The changeover process was incredibly smooth, and provided linear acceleration and a very wide torque curve throughout the entire rev range.

Handling in the FD was regarded as world-class, and it is still regarded as being one of the finest handling and best balanced cars of all time. The continued use of the front-midship engine and drivetrain layout, combined with an 50:50 front-rear weight distribution ratio and low center of gravity made the FD a very competent car at the limits.

In North America, three models were offered; the "base", the touring, and the R models. The touring FD had a sunroof, leather seats, and a complex Bose Acoustic Wave system. The R (R1 in 1993 and R2 in 1994–95) models featured stiffer suspensions, an aerodynamics package, suede seats, and Z-rated tires.

Australia had a special high performance version of the RX-7 in 1995, dubbed the RX-7 SP. This model was developed as a homologated road-going version of the race car used in the 12hr endurance race held at Bathurst, New South Wales, beginning in 1991. An initial run of 25 were made, and later an extra 10 were built by Mazda due to demand. The RX-7 SP produced 204 kW (274 hp) and 357 N·m (263 ft·lbf) of torque, compared to 176 kW (236 hp) and 294 N·m (217 ft·lbf) on the standard version. Other changes included a race-inspired nose cone, race-proven rear wing, a 120 L fuel tank (as opposed to the 76 L tank in the standard car), a 4.3:1-ratio rear differential, 17 in diameter wheels, larger brake rotors and calipers. An improved intercooler, exhaust, and modified ECU were also included. Weight was reduced significantly with the aid of carbon fibre; a lightweight bonnet and seats were used to reduce weight to just 1218 kg (from 1310 kg). It was a serious road going race car that matched their rival Porsche 968CSRS for the final year Mazda officially entered. The formula paid off when the RX-7SP won the title, giving Mazda the winning trophy for a fourth straight year. A later special version, the Bathurst R, was released in 2001.

A popular modification to the 3rd Gen RX-7 is the substitution of a 20B (2.0 litre) 3-rotor engine taken from the Eunos Cosmo in place of the stock 13B (1.3 litre) 2-rotor engine. Many aftermarket performance houses sell conversion kits with the 20B engine, such as Stillen and Pettit Racing. Such 3-rotor configurations typically produce 550hp and a top speed of well over 200 MPH. While critics claim that any 13B 2-rotor RX-7 can be highly tuned to achieve this level of performance, the difference is in daily drivability and reliability that makes the 20B conversion superior to the stock 13B motor.

Front mounted intercooler

Front mounted intercooler, an IC mount position, which involves mounting the intercooler at the front of the engine, usually in the bumper. Often found in high performance cars, although many manufacturers and racing teams use TMICs.

FMICs generally require open bumpers, and front spoilers, which will force air into the bumper and provide downforce as well, are also beneficial. In general, because of the location, a front mount intercooler tends to cool air more efficiently than a similarly sized TMIC (top mount intercooler) or a SMIC (side mount intercooler). FMICs have some disadvantages, however. One obvious drawback is the vulnerable position of the intercooler in front of the car - any moderately serious frontal impact will significantly damage the FMIC. Secondly, FMICs, by virtue of their siting in front of the radiator, block airflow to the radiator, as the air that passes through the intercooler is several degrees hotter than the air on the other side. While on most piston engines, this is not too major a concern, on hot-running engines, and rotary engines in particular, this can lead to problems. Thirdly, FMICs also require the most plumbing of any intercooler setup, which means that there is much more volume that the turbocharger or supercharger must pressurise before it can deliver positive boost. Because of this, many manufacturers opt to use SMICs or TMICs to avoid excessive turbo lag (acceptable for a personally modified car, but a major detriment to a stock car). One of the very few manufacturers to offer an FMIC setup in their factory street cars is Mitsubishi, in the Lancer Evolution series.

Top mounted intercooler

A top mounted intercooler (TMIC) is an automotive intercooler mounted within the engine bay, above the engine. Because of restricted airflow to this location, a hood scoop is virtually a necessity for a TMIC.






TMIC is circled in red


Advantages

* The TMIC may be placed close to the turbocharger and/or supercharger compressor and to the engine's intake. As a result, the intake tubing can be kept short. The longer the path from the intercooler to the engine, the more air must be pressurised within the hoses when a change in pressure is demanded - and the greater the lag imposed. When used in combination with quick-spooling turbochargers, such as ball bearing turbochargers, the result is a more responsive engine.

* Unlike front-mounted intercoolers, TMICs do not block any airflow to the radiator and/or oil cooler, allowing better engine cooling.


Disadvantages

* Heat from the engine may be conducted through to the intercooler, usually while trying to escape through an open hood vent. For this reason hot, cramped or poorly ducted engine bays (commonly the case with rotary engines) negatively affect the performance of TMICs.

* TMICs tend to be less efficient than similarly sized front mount intercoolers, due to the smaller amount of cold air flow through the hood scoop compared to the front grille area in most car designs.


Applications

TMICs are used in many street cars, such as all current intercooled Subarus, the MINI Cooper'S and also in older cars such as the Mazda RX-7 (86-91 model).

A properly designed top mount intercooler's advantage in responsiveness is preferred over more lagging front mount intercoolers in situations where responsiveness is more important than total power - notably in rallying, drifting, autocross and touge.


V-Mounted Intercoolers

The V-Mounted Intercooler is a hybrid system, developed to provide superior air cooling to a front mounted intercooler, yet still retain the short intake piping and radiator airflow of the TMIC. In this case, the intercooler is mounted horizontally, directly in front of the engine (although it can be at an angle). Most VMIC setups place the radiator below the intercooler, at a great angle, tilted back until it is almost touching the motor. Ducts are used in the front of the car to duct air through the intercooler, creating a ram-air effect, while the remainder of the air flows over the radiator, normally. The air is usually removed via a hood vent (a vent recessed into the car's hood near the front of the car; if it is mounted too far back, it will actually suck air into the engine bay), although in the case of a bottom-mounted intercooler, the air is allowed the exit underneath the car (although this is dangerous because is places the intercooler at extreme risk to damage from bumps and rocks). VMIC setups are typically utilised on Front Midship cars, as the location of the engine, far back in the engine bay, allows room for the system.

VMICs were pioneered on the Mazda RX-7, because rotary engines have a tendency to run hot. It was intended to be a compromise between a TMIC or a side-mounted intercooler (2nd Generation and 3rd Generation RX-7, respectively) and a FMIC. An intercooler in the stock position would not support high airflow (and thus limit top power, or create severe detonation in the engine, which damages rotary engines more easily than piston engines), while FMICs would block airflow to the radiator, leading to overheating. The RX-7 is the only car that currently has a VMIC kit available for it. VMICs on other cars are custom made, usually used on track cars and require significant investment and fabricating skills to properly set up and tune.

Torque converter

A torque converter is modified form of a hydrodynamic fluid coupling, and like the fluid coupling, is used to transfer rotating power from a prime mover, such as an internal combustion engine or electric motor, to a rotating driven load. As with the fluid coupling, the torque converter takes the place of a mechanical clutch. Unlike a fluid coupling, however, a torque converter is able to multiply torque when there is a substantial difference between input and output rotational speed, thus providing the equivalent of a reduction gear. The most widespread usage of torque converters is in automobile, bus and light truck automatic transmissions. Torque converters are also found in marine propulsion systems and industrial applications.



Function

Torque Converter Elements

A torque converter is a type of hydrodynamic drive whose function is very similar to that of a fluid coupling. The principal difference is that whereas a fluid coupling is a two element drive that is incapable of multiplying torque, a torque converter has at least one extra element—the stator—which alters the drive's characteristics during periods of high slippage, producing an increase in output torque. It is suggested to the reader that he or she become familiar with the principles of hydrodynamic drives before continuing by reading the fluid coupling article.

In a torque converter there are at least three rotating elements: the pump, which is mechanically driven by the prime mover; the turbine, which drives the load; and the stator, which is interposed between the pump and turbine so that it can alter oil flow returning from the turbine to the pump. The classic torque converter design dictates that the stator be prevented from rotating under any condition, hence the term stator. In practice, however, the stator is mounted on an overrunning clutch, which prevents the stator from counter-rotating the prime mover but allows for forward rotation.

Modifications to the basic three element design have been periodically found, especially in applications where higher than normal torque mutiplication is required. Most commonly, these have taken the form of multiple turbines and stators, each set being designed to produce differing amounts of torque multiplication. For example, the Buick Dynaflow automatic transmission was a non-shifting design and, under normal conditions, relied solely upon the converter to multiply torque. The Dynaflow used a five element converter to produce the wide range of torque multiplication needed to propel a heavy vehicle.

Although not strictly a part of classic torque converter design, many automotive converters include a lock-up clutch to improve cruising power transmission efficiency. The application of the clutch locks the turbine to the pump, causing all power transmission to be mechanical, thus eliminating losses associated with fluid drive.


Operational Phases

For the purposes of explanation, a torque converter can be considered to have three stages of operation:

* Stall. The prime mover is applying power to the pump but the turbine cannot rotate. For example, in an automobile, this stage of operation would occur when the driver has placed the transmission in gear but is preventing the vehicle from moving by continuing to apply the brakes. At stall, the torque converter can produce maximum torque multiplication if sufficient input power is applied (the resulting multiplication is called the stall ratio). The stall phase actually lasts for a brief period when the load (e.g., vehicle) initially starts to move, as there will be a very large difference between pump and turbine speed.

* Acceleration. The load is accelerating but there still is a relatively large difference between pump and turbine speed. Under this condition, the converter will produce torque multiplication that is less than what could be achieved under stall conditions. The amount of multiplication will depend upon the actual difference between pump and turbine speed, as well as various other design factors.

* Coupling. The turbine has reached approximately 90 percent of the speed of the pump. Torque multiplication has ceased and the torque converter is behaving in a manner similar to a fluid coupling. In modern automotive applications, it is usually at this stage of operation where the lock-up clutch is applied, a procedure that tends to improve fuel efficiency.

The key to the torque converter's ability to multiply torque lies in the stator. In the classic fluid coupling design, periods of high slippage cause the fluid flow returning from the turbine to the pump to oppose the direction of pump rotation, leading to a significant loss of efficiency and the generation of considerable waste heat. Under the same condition in a torque converter, the returning fluid will be redirected by the stator so that it aids the rotation of the pump, instead of impeding it. The result is that much of the energy in the returning fluid is recovered and added to the energy being applied by the pump itself. This action causes a substantial increase in the mass of fluid being directed to the turbine, producing an increase in output torque. Since the returning fluid is initially traveling in a direction opposite to pump rotation, the stator will likewise attempt to counter-rotate as it forces the fluid to change direction, an effect that is resisted by the one-way stator clutch.

Unlike the radially straight blades used in a fluid coupling, a torque converter's turbine and stator use angled and curved blades. The blade shape of the stator is what alters the path of the fluid, forcing it to coincide with the pump rotation. The matching curve of the turbine blades helps to correctly direct the returning fluid to the stator so the latter can do its job. The shape of the blades represents a bit of a black art in converter design, as minor variations can result in significant changes to the converter's performance.

During the stall and acceleration phases, in which torque multiplication occurs, the stator remains stationary due to the action of its one-way clutch. However, as the torque converter approaches the coupling phase, the energy and volume of the fluid returning from the turbine will gradually decrease, causing pressure on the stator likewise decrease. Once in the coupling phase, the returning fluid will reverse direction and now rotate in the direction of the pump and turbine, an effect which will attempt to forward-rotate the stator. At this point, the stator clutch will release and the pump, turbine and stator will all (more or less) turn as a unit.

Unavoidably, some of the fluid's kinetic energy will be lost due to friction and turbulence, causing the converter to waste heat (dissipated in many applications by water cooling). This effect, often referred to as pumping loss, will be most pronounced at or near stall conditions. In modern designs, the blade geometry minimizes oil velocity at low pump speeds, which allows the turbine to be stalled with the engine at idle speed for long periods with little danger of overheating.


Efficiency and Torque Multiplication

A torque converter cannot achieve 100 percent coupling efficiency. The Twin Turbine Dynaflow torque converter has an efficiency curve that resembles an inverted letter J: zero efficiency at stall, maximum efficiency at approximately 50 percent coupling, and very low efficiency at maximum coupling while the classic three element torque converter has an asymptotical efficiency curve that resembles an inverted U. The loss of efficiency as the converter enters the coupling phase is a result of the turbulence and fluid flow interference generated by the stator, and as mentioned above, is commonly overcome by mounting the stator on a one-way clutch.

Even with the benefit of the one-way stator clutch, a converter cannot achieve the same level of efficiency in the coupling phase as an equivalently sized fluid coupling. Some loss is due to the presence of the stator (even though rotating as part of the assembly), as it always generates some power-absorbing turbulence. Most of the loss, however, is caused by the curved and angled turbine blades, which do not absorb kinetic energy from the fluid mass as well as radially straight blades. Since the turbine blade geometry is a crucial factor in the converter's ability to multiply torque, trade-offs between torque multiplication and coupling efficiency are inevitable. In automotive applications, where steady improvements in fuel economy have been mandated by market forces and government edict, the nearly universal use of a lock-up clutch has helped to eliminate the converter from the efficiency equation during cruising operation.

The maximum amount of torque multiplication produced by a converter is highly dependent on the size and geometry of the turbine and stator blades, and is generated only when the converter is at or near the stall phase of operation. Typical stall torque multiplication ratios range from 1.8:1 to 2.5:1 for most automotive applications (although multi-element designs as used in the Buick Dynaflow and Chevrolet Turboglide could produce more). Specialized converters design for industrial or heavy marine power transmission systems are capable of as much as 5.0:1 multiplication. Generally speaking, there is a trade-off between maximum torque multiplication and efficiency—high stall ratio converters tend to be relatively inefficient below the coupling speed, whereas low stall ratio converters tend to provide less possible torque multiplication.

While torque multiplication increases the torque delivered to the turbine output shaft, it also increases the slippage within the converter, raising the temperature of the fluid and reducing overall efficiency. For this reason, the characteristics of the torque converter must be carefully matched to the torque curve of the power source and the intended application. Changing the blade geometry of the stator and/or turbine will change the torque-stall characteristics, as well as the overall efficiency of the unit. For example, drag racing automatic transmissions often use converters modified to produce high stall speeds to improve off-the-line torque, and to get into the power band of the engine more quickly. Highway vehicles generally use lower stall torque converters to limit heat production, and provide a more firm feeling to the vehicle's characteristics.

A design feature once found in some General Motors automatic transmissions was the variable-pitch stator, in which the blades' angle of attack could be varied as much as 75 degrees in response to changes in engine speed and load. The effect of this was to vary the amount of torque multiplication produce by the converter. At the normal angle of attack, the stator caused the converter to produce a moderate amount of multiplication but with a higher level of efficiency. If the driver abruptly opened the throttle, a valve would switch the stator pitch to a different angle of attack, increasing torque multiplication at the expense of efficiency.

Some torque converters use multiple stators and/or multiple turbines to provide a wider range of torque multiplication. Such multiple-element converters are more common in industrial environments than in automotive transmissions, but automotive applications such as Buick's Triple Turbine Dynaflow and Chevrolet's Turboglide also existed. The Buick Dynaflow utilized the torque-multiplying characteristics of the its planetary gearset in conjunction with the torque converter for low gear and bypassed the first turbine, using only the second turbine as vehicle speed increased. The unavoidable trade-off with this arrangement was poor efficiency and eventually these transmission were discontinued in favor of the more efficient three speed units with a conventional torque converter.

Lock-up Torque Converters

As described above, pumping losses within the torque converter reduce efficiency and generate waste heat. In modern automotive applications, this problem is commonly avoided by use of a lock-up clutch that physically links the pump and turbine, effectively changing the converter into a purely mechanical coupling. The result is no slippage, and therefore virtually no power loss.

The first automotive application of the lock-up principle was Packard's Ultramatic transmission, introduced in 1949, which locked up the converter at cruising speeds, unlocking when the throttle was floored for quick acceleration or as the vehicle slowed down. This feature was also present in some Borg-Warner automatics produced during the 1950's. It fell out of favor in subsequent years due the extra complexity and cost it added to the transmission. However, in the late 1970's lock-up clutches started to reappear in response to demands for improved fuel economy. They are now nearly universal in automotive applications.


Capacity and Failure Modes

As with a fluid coupling, the theoretical torque capacity of a converter is proportional to r(N^2)(D^5), where r is the mass density of the fluid, N is the impeller speed, and D is the diameter. In practice, the maximum torque capacity is limited by the mechanical characteristics of the materials used in the converter's components, as well as the ability of the converter to dissipate heat (often through water cooling). As an aid to strength, reliability and economy of production, most automotive converter housings are of welded construction. Industrial units are usually assembled with bolted housings, a design feature that eases the process of inspection and repair, but adds to the cost of producing the converter.

In high performance, racing and heavy duty commercial converters, the pump and turbine may be further strengthened by a process called furnace brazing, in which molten brass is forced into seams and joints to produce a stronger bond between the blades, hubs and annular ring(s). Because the furnace brazing process creates a small radius at the point where a blade meets with a hub or annular ring, a theoretical decrease in turbulence will occur, resulting in a corresponding increase in efficiency.

Overloading a converter can result in several failure modes, some of them potentially dangerous in nature:

* Overheating: Continuous high levels of slippage may overwhelm the converter's ability to dissipate heat, resulting in damage to the elastomer seals that retain fluid inside the converter. This will cause the unit to leak and eventually stop functioning due to lack of fluid.

* Stator Clutch Seizure: The inner and outer elements of the one-way stator clutch become permanently locked together, thus preventing the stator from rotating during the coupling phase. Most often, seizure is precipitated by severe loading and subsequent distortion of the clutch components. Eventually, galling of the mating parts occurs, which triggers seizure. A converter with a seized stator clutch will exhibit very poor efficiency during the coupling phase, and in a motor vehicle, fuel consumption will drastically increase. Converter overheating under such conditions will usually occur if continued operation is attempted.

* Stator Clutch Breakage: A very abrupt application of power can cause shock loading to the stator clutch, resulting in breakage. When this occurs, the stator will freely counter-rotate the pump and almost no power transmission will take place. In an automobile, the effect is similar to a severe case of transmission slippage and the vehicle is all but incapable of moving under its own power.

* Blade Deformation and Fragmentation: Due to abrupt loading or excessive heating of the converter, the pump and/or turbine blades may be deformed, separated from their hubs and/or annular rings, or may break up into fragments. At the least, such a failure will result in a significant loss of efficiency, producing symptoms similar (although less pronounced) to those accompanying stator clutch failure. In extreme cases, catastrophic destruction of the converter will occur.

* Ballooning: Prolonged operation under excessive loading, very abrupt application of load, or operating a torque converter at very high RPM may cause the shape of the converter's housing to be physically distorted due to internal pressure and/or the stress imposed by centrifugal force. Under extreme conditions, ballooning will cause the converter housing to rupture, resulting in the violent dispersal of hot oil and metal fragments over a wide area.