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abbrv. OHC

A type of valvetrain layout used in internal combustion engines. In overhead cam engines, the camshaft is located physically above the cyclinder combustion chamber. This places it close to the valve stems, and often in a position of mechanical advantage. Lifters can be eliminated, reducing reciprocating mass. This permits overhead cam engines to be operated at higher RPM, all things being equal. Although American automobile makers converted to overhead cam designs fairly recently, they are not new. Henry Ford's 999 race car was the first to exceed 60 MPH on dirt, while driven by Barney Oldfield. The original engine was an SOHC design, though it was deleted before arriving at the Henry Ford Museum in Dearborn. The first really modern design was for a Peerless engine. The Grand Prix Peugeots of the teens sported a DOHC design, which was copied by everyone.

Overhead cam engines come in either single SOHC or dual DOHC designs. Either is capable of operating more than two valves per cylinder, though DOHC designs are preferred for high performance applications. SOHC designs require a rocker arm while in DOHC engines the cam may operate the valves directly, reducing reciprocating mass and parts count, which makes for a more reliable engine.

Overhead cam designs are preferred, and near ubiquitous for modern in-line automotive engines. However, with opposed or V type engines the cam drive becomes much more complex, and so overhead valve designs are common when raw performance is seen as less critical.

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