Pushrod engine

A pushrod engine or I-head engine is a type of piston engine that places the camshaft in the cylinder block (usually beside and slightly above the crankshaft in a straight engine or directly above the crankshaft in the V of a V engine) and uses pushrods or rods to actuate rocker arms above the cylinder head to actuate the valves. Lifters or tappets reside in the engine block between the camshaft and pushrods.

This contrasts with an overhead cam (OHC) design which places the camshafts above the cylinder head and drives the valves directly or through short rocker arms. In an OHC engine, the camshafts are normally part of the cylinder head assembly, while in an I-head engine the camshaft (rarely more than one) is part of the main engine block assembly.

Pushrod engines are perceived to be "old fashioned" by the modern automotive press, the cause is historical: While both layouts are over 100 years old, the I-head engine came first. OHC engines were developed as more expensive high-performance engines and have largely replaced the pushrod design in countries where cars are taxed based on engine displacement. In 1949, Oldsmobile introduced the Rocket V8. It was the first high-compression I-head design, and is the archetype for most modern pushrod engines. General Motors is the world's largest pushrod engine producer with engines such as the 3800 Series III Supercharged V6 (260 hp, 280 lb/ft torque), LS7 Chevrolet Corvette 7.0L V8 Engine (505 hp, 475 lb/ft torque) and LS4 5.3L DOD V8 (303 hp, 323 ft·lbf torque). Fewer pushrod type engines remain in production, a result of the fact that it has become difficult to achieve competitive engine performance with the configuration. However, in 2002, Chrysler introduced a new pushrod engine: a 5.7L Hemi engine. The new Chrysler Hemi engine presents advanced features such as variable displacement technology and has been a popular option with buyers. The Hemi was on the Ward's 10 Best Engines list for 2003 through 2007.



Limitations

Three specific problems remain with pushrod engines:

* Limited engine speeds or rpm - Pushrod engines have more reciprocating mass, suffer more easily from valve "float", and exhibit a tendency for the pushrods themselves to flex or snap at high engine speeds. Therefore a pushrod engine cannot revolve ("rev") at engine speeds as high as an OHC design. Modern pushrod engines generally rev to 6,000 rpm: compare this to modern OHC engines that can easily rev from 7,000 rpm in average engines to near 20,000 rpm in Formula One racing engines. High-rev pushrod engines have also been developed — in 1969, Chevrolet offered a Camaro Z28 with a pushrod V8 that could rev to 8,000 rpm and the Volvo B18 and B20 engines can rev to more than 7,000 rpm. Various pushrod racing engines are capable of reaching from 9,000 in some series to 10,500 rpm in others.

Because overall power is computed by multiplying torque by revolution speed (HP = (RPM x LB-FT) / 5252), an engine capable of revving higher will produce more power from the same amount of torque than one revving lower. A pushrod engine therefore needs to have a larger displacement to match the power output of an OHC engine. A good comparison would be the 3.9L GM 3900 Engine to the 3.0L Honda J-series V6. The smaller (in displacement) Honda Engine produces more power, while requiring less torque to do so.

* Difficulty in using crossflow cylinder heads in straight engine configurations - A few straight pushrod engines have been manufactured with crossflow heads, such as the six cylinder Humber Super Snipe. These engines combined much of the performance of the overhead camshaft with the ease of service of the pushrod, but were more expensive to manufacture than either competing design.

* Limited design flexibility - The biggest benefit of an OHC design is the use of multiple intake and exhaust valves and variable valve timing. Most modern pushrod engines have two valves per cylinder, while many OHC engines use three, four or even five valves per cylinder to achieve greater efficiency and power. Recently, however, GM has begun offering a pushrod V6 with VVT, and Cummins' ISB is a 4-valve pushrod straight-6. For the 2006 model year, General Motors will introduce the Vortec 6200. This is the first mass-produced pushrod engine to feature variable valve timing. The system adjusts both intake and exhaust timing between two settings. There is even a company called Arao Engineering that has developed and patented a 32-valve aluminum cylinder head for various pushrod engines like the small/big block Chevrolet engines and Ford small/big block engines.


Advantages

In contrast, pushrod engines have two specific advantages:

* Smaller overall packaging - Because of the camshaft's location inside the engine block, pushrods are generally more compact than an overhead cam engine of comparable displacement. For example, Ford's 4.6 L OHC modular V8 is larger than the 5.0 L I-head Windsor V8 it replaced and GM's 4.6 L OHC Northstar V8 is slightly taller and wider than GM's larger displacement 5.7 to 7.0 L I-head LS V8. The Ford Ka uses the venerable Kent Crossflow pushrod engine to fit under its low bonnet line.

* Less complex drive system - Pushrod engines have a less complex drive system when compared with OHC engines. Most OHC engines drive the camshaft or camshafts using a timing belt, a chain or multiple chains. These systems require the use of tensioners which add some complexity to the engine.


1994 Mercedes Indianapolis 500 engine

The Indy 500 race in Indianapolis each year bears some vestige of its original purpose as a proving ground for automobile manufacturers, in that it once gave an advantage in engine displacement to engines based on stock production engines, as distinct from out-and-out racing engines designed from scratch. One factor in identifying production from racing engines was the use of pushrods, rather than the overhead cams used on most modern racing engines; Mercedes-Benz realized before the 1994 race that they could very carefully tailor a purpose-built racing engine using pushrods to meet the requirements of the Indy rules and take advantage of the 'production based' loophole but still design it to be state of the racing art in all other ways, without any of the drawbacks of a real production-based engine. They entered this engine in 1994, and, as expected, dominated the race. After the race, the rules were changed in order to reduce the amount of boost pressure allowed to be supplied by the turbocharger. The inability of the engine to produce competitive power output after this change caused it to become obsolete after just the one race, as Mercedes-Benz knew it would when deciding a victory at Indy was worth it.