Pushrod engine
A pushrod engine or overhead valve (OHV) engine is a type of piston engine that places the camshaft below the pistons (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 lifters or tappets above the cylinder head to actuate the valves.
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 OHV engine the camshaft (rarely more than one) is part of the main engine block assembly.
Pushrod engines are maligned as "old fashioned" by the modern automotive press. The cause is historical: The OHV engine came first while OHC engines were developed as more expensive high-performance engines and have largely replaced the pushrod design, particularly in countries where cars are taxed based on engine displacement. In 1949, Oldsmobile introduced the Rocket V8. It was the first high-compression OHV design, and is the archetype for most modern pushrod engines.
Three specific problems remain with pushrod engines:
- Limited rpm - Pushrod engines have more rotational mass, suffer more easily from valve "float", and exhibit a tendency for the pushrods themselves to flex or snap at high rpm. Therefore, conventional wisdom says that a pushrod engine cannot rev as high as an OHC design. Modern pushrod engines generally rev to 6,000 rpm. Compare this to modern OHC engines that rev to 7,000 rpm in the case of ordinary engines, to 9,000 rpm in the case of high-performance engines like the one used in the Honda S2000, and even higher in specialty engines (Radical Motorsports engines are capable of revving past 10,000 rpm). However, high-rpm pushrod engines have also been developed — in 1969, Chevrolet offered a Camaro Z28 with a pushrod V8 that revs to 8,000 rpm.
- 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 valve 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 3-valve pushrod straight-6.
In contrast, pushrod engines have 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 4.9 L OHV 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 OHV LS V8.
- Peak torque - Pushrod engines develop maximum power at a lower rpm than overhead cam engines and also tend to be torquier. Combine this with lower gearing and that can add up to superior fuel economy. The LS6 V8, for example, makes significantly more torque and power than the Northstar while exhibiting similar fuel economy. Though some would consider this a questionable comparison since the Northstar engine is typically used in applications where engine output is sacrificed to improve NVH (Noise, Vibration and Harshness) characteristics.
- Complexity - By their nature OHC engines tend to be more complex than pushrod engines, particularly considering that an OHC V6 or V8 can have four camshafts. This adds cost, mass and increases the number of moving parts.
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 gives some advantages in engine size 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 universally used on 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 to prevent a recurrence, and the engine became obsolete after just the one race, as Mercedes-Benz knew it would when deciding a victory at Indy was worth it.