[Excaliber]

One more time...

When the spark plug fires it ignites the mixture of fuel\air which has been COMPRESSED by the piston moving up in the cylinder bore and the valves are closed. There is no escape for the fuel\air, it is trapped and compressed and ignited by the plug. BOOM, big fire inside the motor, GREAT pressure now being exerted on the piston. But this is NOT an 'explosion', it is a very accellerated and yet 'controlled burn'.

Because it takes a little bit of 'time' for the 'burn' to reach maximum pressure we want to start the 'burn' a little bit before the piston actually reaches TDC (top dead center). We want to fire the spark plug maybe 15 to 18 degress BTDC (BEFORE top dead center). As the engine rpm increases we need to fire the fuel\air mixture even earlier than that. Because, well the piston is really flying now, things are happening REAL fast, we need to stay ahead of the game! 30-35 degrees TOTAL timing is in the ball park for max rpm operation. We want MAXIMUM cylinder pressure to be developed precisely at the point where the piston is at TDC and ready to go DOWN. BUT not before that, which is 'detonation'.

This is a simplified version. In fact we want the pressure build up to CONTINUE even AS the piston is moving down, so not 'exactly' at TDC. But I hope you get the point...