Suck, Squish, Bang, Blow Part Six: Ignition Timing Basics
By Mike Kojima
Ignition Timing is expressed as the number of degrees of crankshaft rotation in advance of Top Dead Center or TDC, when the sparkplug is fired. This kicks off the explosion of air and gasoline that pushes the piston down to drive the crank. Since even explosions take a few milliseconds to develop, the engine's ECU usually commands the sparkplug to fire a few degrees of crank rotation before TDC. This way the explosion will be fully underway by the time the piston reaches TDC and will be able to push the piston down with the greatest effect.
If the sparkplug fired after TDC the piston would already be going back down the bore and the explosion would not be able to give the piston the biggest push. Firing the spark retarded, or after TDC can also cause the engine to overheat by increasing the amount of time that the burning mixture can impinge upon the engine's internal parts as the piston travels downward. It can also dump a lot of heat out the exhaust valve, possibly damaging it and other exhaust components.
The number of degrees that the plug fires before TDC is called Advance. Generally the more advanced the spark is the bigger of a head start the combustion event can get before the piston reaches TDC. This allows for more pressure to be built up and a more powerful push to be put on the piston on its way back down the bore. It is possible to advance the timing too much and cause the engine to lose power because the explosion of a too far advanced spark will occur too early and actually hamper the pistons rise to TDC.
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