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May 3, 2007

Advantages and Disadvantages of Manual Transmission


* Manual transmissions typically offer better fuel economy than automatics. Increased fuel economy with a properly operated manual transmission vehicle versus an equivalent automatic transmission vehicle can range from 5% to about 15% depending on driving conditions and style of driving -- extra urban or urban (highway or city). There are several reasons for this:
o Mechanical efficiency. The manual transmission couples the engine to the transmission with a rigid clutch instead of a torque converter that introduces significant power losses. The automatic transmission also suffers parasitic losses by driving the high pressure hydraulic pumps required for its operation.
o Driver control. Certain fuel-saving modes of operation simply do not occur in an automatic transmission vehicle, but are accessible to the manual transmission driver. For example, the manual-transmission vehicle can be accelerated gently, yet with a fully open throttle (accelerator pedal to the floor), by means of shifting early to a higher gear, keeping the engine RPM in a low power band. By contrast, in an automatic transmission, the throttle position serves as the indicator of how fast the driver wishes to accelerate. If the accelerator pedal is floored, the transmission will shift to a lower gear, resulting in high engine RPM and aggressive acceleration. The thermodynamically efficient combination of open throttle and low RPMs is unavailable to the automatic transmission driver. Fuel-efficient acceleration is important to achieving fuel economy in stop-and-go city driving.
o Fuel cut-off. The torque converter of the automatic transmission is designed for transmitting power from the engine to the wheels. Its ability to transmit power in the reverse direction is limited. During deceleration, if the torque converter's rotation drops beneath its stall speed, the momentum of the car can no longer turn the engine, requiring the engine to be idled. By contrast, a manual transmission, with the clutch engaged, can use the car's momentum to keep the engine turning, in principle, all the way down to zero RPM. This means that there are better opportunities, in a manual car, for the electronic control unit (ECU) to impose deceleration fuel cut-off (DFCO), a fuel-saving mode whereby the fuel injectors are turned off if the throttle is closed (foot off the accelerator pedal) and the engine is being driven by the momentum of the vehicle. Automatics further reduce opportunities for DFCO by shifting to a higher gear when the accelerator pedal is released, causing the RPM to drop.[citation needed]
o Geartrain efficiency. Automatics may require power to be transmitted through multiple planetary gearsets before attaining the desired gear ratio. In comparison, manual transmissions usually transmit power through one or two gearsets at most.

* Manual transmissions are still more efficient than belt-driven continuously-variable transmissions.

* Manual transmissions are generally significantly lighter than torque-converter automatics.

* Vehicles with manual transmissions are typically cheaper than those with automatic transmissions.

* Manual transmissions generally require less maintenance than automatic transmissions.

* Manual transmissions normally do not require active cooling, because not much power is dissipated as heat through the transmission.
o The heat issue can be important in certain situations, like climbing long hills in hot weather, particularly if pulling a load. Unless the automatic's torque converter is locked up (which typically only happens in an overdrive gear that would not be engaged when going up a hill) the transmission can overheat. A manual transmission's clutch only generates heat when it slips, which does not happen unless the driver is riding the clutch pedal.

* A driver has more direct control over the state of the transmission with a manual than an automatic. This control is important to an experienced, knowledgeable driver who knows the correct procedure for executing a driving maneuver, and wants the machine to realise his or her intentions exactly and instantly. Manual transmissions are particularly advantageous for performance driving or driving on steep and winding roads. Note that this advantage applies equally to manual-automatic transmissions, such as tiptronic.
o An example: the driver, anticipating a turn, can downshift to the appropriate gear while the steering is still straight, and stay in gear through the turn. This is the correct, safe way to execute a turn. An unanticipated change of gear during a sharp turn can cause skidding if the road is slippery.
o Another example: when starting, the driver can control how much torque goes to the tires, which is useful for starting on slippery surfaces such as ice, snow or mud. This can be done with clutch finesse, or possibly by starting in second gear instead of first. The driver of an automatic can only put the car into drive, and play with the throttle. The torque converter can easily dump too much torque into the wheels, because when it slips, it acts as an extra low gear, passing through the engine power, reducing the rotations while multiplying torque. An automatic equipped with ESC, however, does not have this disadvantage. Some cars, such as the Saab NG900 Automatic transmission, have a special mode for low traction situations.
o Yet another example: passing. When the driver is attempting to pass a slower moving vehicle by making use of a lane with opposite traffic, he or she can select a lower gear for more power at exactly the right moment when conditions are right to begin the maneuver. Automatics have a delayed reaction time, because the driver can only indicate his intent by pressing the throttle. The skilled manual transmission driver has an advantage of superior finesse and confidence in such situations.

* Driving a manual requires more involvement from the driver, thereby discouraging some dangerous practices. The manual selection of gears requires the driver to monitor the road and traffic situation, anticipate events and plan a few steps ahead. If the driver's mind wanders from the driving task, the machine will soon end up in an incorrect gear, which will be obvious from excessive or insufficient engine RPM. Related points:
o It's much more difficult for the driver to fidget in a manual transmission car, for instance by eating, drinking beverages, or talking on a cellular phone without a headset. During gear shifts, two hands are required. One stays on the wheel, and the other operates the gear lever. The hand on the wheel is absolutely required during turns, and tight turns are accompanied by gear changes. If the hand leaves the wheel, the steering will begin to straighten. In general, the more demanding the driving situation, the more difficult it is for the manual driver to do anything but operate the vehicle. The driver of an automatic transmission can engage in distracting activities in any situation, such as sharp turns through intersections or stop-and-go traffic.
o The driver of a manual transmission car can develop an accurate intuition for how fast the car is traveling, from the sound of the motor and the gear selection. It's easier to observe the lower speed limits—like 30 km/h and 50 km/h or their U.S. and Imperial counterparts, 20 mph and 30 mph—without glancing at the instrumentation.

* Cars with manual transmissions can often be started when the battery is dead by pushing the car into motion or allowing it to roll downhill, and then engaging the clutch in third or second gear. This is commonly known as a "push start", "popping the clutch" (in the USA) or Bump starting, which in the UK describes the action of suddenly releasing the clutch pedal after putting it in gear.

* Manual transmissions work regardless of the orientation angle of the car with respect to gravity. Automatic transmissions have a fluid reservoir (pan) at the bottom; if the car is tilted too much, the fluid pump can be starved, causing a failure in the hydraulics. This could matter in some extreme off roading circumstances.

* It is sometimes possible to move a vehicle with a manual transmission just by putting it in gear and cranking the starter. This is useful in an emergency situation where the vehicle will not start, but must be immediately moved (from an intersection or railroad crossing, for example). It is also easier to put a car with a manual transmission into neutral, even when the transmission has suffered damage from an accident or malfunction. Many modern vehicles will not allow the starter to be run without the clutch fully depressed, negating this advantage, but some manufacturers have begun to add a clutch start override switch so that this advantage may still be enjoyed when necessary.


Many of the disadvantages of a manual transmission involve the driver interaction with the vehicle. While most of these can be overcome with practice and experience, they should be considered:

* Manual transmissions often require the driver to place their full and continuous attention on the road, which may be seen as a disadvantage. Some consider this an advantage, as it prevents the driver from other potential distractions like cell phone or radio use.

* Inexperienced drivers may place more of their attention on shifting the gears, potentially distracting them from the road surroundings.

* A driver may inadvertently shift into the wrong gear with a manual transmission, potentially causing damage to the engine and transmission, or the vehicle's body and its surroundings if the intended gear was reverse. However this can be offset with a lockout on the reverse such as found on many European cars.

* Manual transmissions require a learning curve as one must develop a feel for properly engaging the clutch.

* While it can easily be overcome with experience, manual transmission vehicles require good gas pedal application and clutch control when starting the car from a standstill. Too many RPM's causes the car to redline, whereas not enough RPM's upon clutch release causes the engine to stall, due to the lack of momentum required to sustain the engine.

* The smooth and quick shifts of an automatic transmission are not guaranteed when operating a manual transmission; such changes are dependent on the driver.

* Manual transmission places more work on the driver in heavy traffic situations since the driver is constantly clutching, in comparison with automatic transmission which merely require moving the foot from the gas pedal to the brake pedal and vice versa. Manual-transmission automobiles can also be slower to take off from traffic lights and roundabouts because of the subsequent gear changes required during the process of driving away from these.

* For a person with physical impairment, an automatic transmission might be the only available shifting option. The comparable systems for hand-operated clutch and brakes for a manual-transmission-equipped car are usable only by people with just lower body handicap. Retrofit of such a system also requires extensive modifications to the car.

* Vehicles with manual transmissions are more difficult to start from rest when stopped upward on a hill because the clutch must be depressed and the gas applied very quickly once the brake is removed to prevent slipping backward. However, this can be overcome with experience and/or the use of the handbrake.

* The clutch disc is a wear item and must be replaced periodically. While this is typically a labor intensive process that can be an expensive service, it shouldn't prove more expensive than periodic service to an automatic transmission in the long run


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