valve lead

The number of degrees of the crankshaft before the top or bottom center, at which the intake or exhaust valves open or close. For example, if the intake valve opens when the crankshaft is 25° before the piston reaches the top of the exhaust stroke, the intake valve lead is 25°. Early opening of the valve ensures that it is fully open at the beginning of the induction stroke and that there is no time lag between the piston moving down and the mixture flowing into the cylinder, as would otherwise occur as a result of the inertia of the mixture.

Aviation dictionary. 2014.

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