SPARK ADVANCE TIMING:

To obtain full power from combustion, the maximum pressure must be reached just as the piston passes top dead center, and combustion must be completed by approximately 23 degrees after top dead center.

The fuel mixture ignited in the combustion chamber does not explode. It burns rapidly until the fuel is consumed. The time required for complete combustion is a small fraction of a second. For this reason ignition must take place before the piston passes top dead center.

As the engine speed increases, the piston moves through the compression stroke more rapidly, but the burning rate of the fuel mixture remains virtually the same. To compensate for the high piston speed, ignition must occur earlier in the compression stroke.

During the combustion process at 120 rpm, the crankshaft travel through 41 degrees of rotation, from the point of ignition, to 23 degrees after top dead center. The spark must occur at 18 degrees before top dead center. The same engine running at 3600 rpm will require 63 degrees of crankshaft rotation to complete combustion by 23 degrees after top dead center. This would require that the spark occur 40 degrees before top dead center. This is why spark advance is such an important factor in efficient engine operation.

AS ENGINE SPEED INCREASES SPARK MUST BE TIMED EARLIER