[mikedsilva]

Originally Posted by Dr J

The physics of this is as follows:
Best power is obtained when the maximum cylinder pressure happens around 15 deg after top dead center (ATDC) (due to mechanical leverage of rod to crank). When the spark plug lights up, it takes some time for the flame front to travel across the cylinder. As RPMs go up, you advance timing to fire earlier, say 25 deg before top dead center (BTDC). This gives more time for the flame front to travel so that max cylinder pressure happens 15 deg ATDC.

With a single plug in the center, the flame front has to travel half the diameter in all directions. In our cars, the plug is off center, so it takes even longer to travel in one direction.

With twin plug, you start two flame fronts so you get peak cylinder pressure faster because the flame front travels half the distance. The result is that you do not need to fire so early. Hence, you decrease advance. In fact, keeping the same advance can cause the engine to work against itself, generating peak pressure when the piston is still traveling up or around TDC, where you are stressing the piston, rod, and crankshaft rather than using that pressure to turn them.

For the sake of completeness of this explanation, I would mention that other things that affect the speed of the flame front are air fuel ratio, manifold pressure from turbo/supercharging and things like water-methanol injection.

Thanks for this John.