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CATEGORIES (articles) > Engine, Gearbox > Technical > The job of the Pushrod

The job of the Pushrod

A pushrod or overhead valve (OHV) type piston engine places the camshaft below the pistons 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.

Pushrod engines are somewhat maligned as "old fashioned" by the modern automotive press, but pushrod designs have been quite successful. Two specific problems present themselves with pushrod engines:

1. Limited RPM - Pushrods add complexity and inertial mass, as well as a tendency to flex or snap at high RPM. Therefore, conventional wisdom says that a pushrod engine cannot rev as high as an OHC design. However, modern manufacturing and materials has all but eliminated this area of concern. Modern pushrod engines can rev well past 6,000 RPM.

2. Limited valve flexibility - The biggest benefit of an OHC design is the use of multiple intake and exhaust valves, and Variable Valve Timing.

No modern pushrod engine has had more than two valves per cylinder, while many OHC engines have used three, four or even five valves per cylinder to achieve greater efficiency. Rumours persist that General Motors is working on a 4-valve pushrod version of the LS2 V8, but this has yet to be seen.

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