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overhead camshaft
Overhead camshaft, commonly abbreviated to OHC,〔〔 is a valvetrain configuration which places the camshaft of an internal combustion engine of the reciprocating type within the cylinder heads ('above' the pistons and combustion chambers) and drives the valves or lifters in a more direct manner compared to overhead valves (OHV) and pushrods.
== Overview ==
Compared to OHV pushrod systems with the same number of valves, the reciprocating components of the OHC system are fewer〔 and have a lower overall mass.〔 Though the system that drives the camshafts may be more complex, most accept that added complexity as a trade-off for better engine performance and greater design flexibility. The fundamental reason for the OHC valvetrain is that it offers an increase in the engine's ability to exchange induction and exhaust gases. (This exchange is sometimes known as 'engine breathing'.〔 ) Another performance advantage is gained as a result of the better optimised port configurations made possible with overhead camshaft designs. With no intrusive pushrods, the overhead camshaft cylinder head design can use straighter ports〔 of more advantageous cross-section and length. The OHC design allows for higher engine speeds than comparable cam-in-block designs, as a result of having lower valvetrain mass. The higher engine speeds thus allowed increases power output for a given torque output.〔
Disadvantages of the OHC design include the complexity of the camshaft drive, the need to re-time the drive system each time the cylinder head is removed, and the accessibility of tappet adjustment if necessary. In earlier OHC systems, including inter-war Morrises and Wolseleys, oil leaks in the lubrication systems were also an issue.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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