(136) Valve Timing.
The exact time at which the valves of a four stroke cycle engine open and close depends to a great extent upon the speed of the engine, the fuel used, the compression pressure, and the relation of the bore to the stroke.
As these items vary in nearly every make of engine there has appeared in the technical press, a great mass of seemingly conflicting data. Engine speed is the principal factor in determining the timing.
Correct valve timing plays a considerable part in the output and efficiency of an engine, for if the inlet valve, for example, opens too late, the cylinder will not receive a full charge. If it opens too early the hot gases in the cylinder will ignite the gas in the carburetor and cause back-firing. Should the exhaust open too late, the retention of the hot gas in the cylinder is likely to cause overheating.
The timing of the valves is usually expressed in degrees of the circle described by the crank-pin, or the angle formed by the crank with the center line of the cylinder at the time the valve is to open or close.