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An overhead valve (OHV) engine, also called 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.

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 lbf·ft torque), LS7 Chevrolet Corvette 7.0 L V8 Engine (505 hp, 475 lbf·ft torque) and LS4 5.3 L DOD V8 (303 hp, 323 lbf·ft torque). Few pushrod type engines remain in production. This is a result of few manufacturers wanting to design both OHV and OHC engines, and competitively OHC racing engines have an advantage in power due to rpm limits. 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.

Contents 1 History 2 Advantages 3 Limitations 4 1994 Mercedes/Ilmor Indianapolis 500 engine 5 Comparison of engine configurations and types 5.1 See also 6 External links

In automotive engineering, an overhead valve internal combustion engine is one in which the intake and exhaust valves and ports are contained in the cylinder head.

The original overhead valve or OHV piston engine was developed by the Scottish-American David Dunbar Buick. It employs pushrod-actuated valves parallel to the pistons and this is still in use today. This contrasts with previous designs which made use of side valves and sleeve valves.

Nowadays, side-valves have virtually disappeared (except perhaps in lawn-mower engines) and valves are almost all "overhead". However most are now driven more directly by the overhead camshaft system and these are designated OHC instead (either SOHC or DOHC).

Pushrod engines have become less common in recent years, serving primarily as either truck engines or as budget V6 models for General Motors, though Chrysler's HEMI engines and GM's LS series are a notable exception. Pushrod engines are nearly extinct among other automakers.

In contrast, pushrod engines have specific advantages:

• Smaller overall packaging - Because of the camshaft's location inside the engine block, pushrods are 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. 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. In addition, failure of the timing belt or chain can sometimes result in the pistons colliding with the open valves, resulting in severe damage to the engine.

• Reduced major servicing times - Operations which require the cylinder head to be removed (such as head gasket replacement) can be performed without removal of the camshaft and therefore the camshaft drive does not need to be replaced or re-timed, saving both time and expense. If the head and block are cast iron, this can be achieved very quickly as full cooling of the parts may not be necessary.

Two specific problems remain with pushrod engines:

• Limited engine speeds or rpm - Pushrod engines have more valvetrain moving parts (the pushrod itself) thus more valvetrain inertia, suffer more easily from valve "float" due to the innate valve actuation rocker design, and exhibit a tendency for the pushrods, if too long, to flex or snap at very high engine speeds. Therefore, a pushrod engine cannot revolve ("rev") at engine speeds as near as high as an OHC design. Modern pushrod engines are usually limited to about 6,000 rpm while many modern OHC engines can have rev limits from 6,000 rpm to 9,000 rpm in road legal vehicles and up to 19,000 rpm in current Formula One race engines. High-revving pushrod engines have also been developed, albeit solid (mechanical) lifter designs, flat and roller. In 1969, Chevrolet offered a Corvette, Camaro Z28, and other models with a solid lifter cam pushrod V8, the ZL1, that could rev to 8,000 rpm. The Volvo B18 and B20 engines can rev to more than 7,000 rpm with their solid lifter camshaft. However, the LS7 of the C6 Corvette Z06 is the first production hydraulic roller cam pushrod engine to have a redline of 7100 rpm. Various pushrod racing engines are capable of varying from 9,000 to 10,500 rpm.

• Limited cylinder head design flexibility - The biggest benefit that an OHC design has is the ease of using multiple intake and exhaust valves and variable valve timing. Most modern pushrod engines have two valves per cylinder, while many OHC engines can have 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. The GM 3900 was the first mass-produced pushrod engine to feature variable valve timing. The system adjusts both intake and exhaust timing between only two settings, it can not vary the intake and exhaust timing independently. Presently there is even a company called Arao Engineering, formerly Dominion Performance, that has developed, patented, and sold a 4-valve per cylinder aluminum cylinder head for various pushrod engines like the small/big block Chevrolet engines, Ford small/big block engines and others.

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 engines 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. Mercedes-Benz knew this beforehand, deciding that the cost of engine development was worth one win at Indianapolis.

Comparing engines is not an exact science. This table shows the comparison of some of the most important features when looking at an engine

Comparison of naturally-aspirated engines for race and road legal track day cars

• Valvetrain
• Overhead camshaft
• Cam-in-block

• Pushrod (OHV), SOHC and DOHC engine animated diagrams
• LS7 torque and power by rpm chart
• Ferrari Enzo: The Engine