Why exactly is pre-ignition so destructive? I know that sounds like a dumb question. But really, how different is it if the vapor is ignited by a spark or by the ambient heat? I'm assuming it has something to with the fact that the motor is probably still in its compression stroke when it ignites. Or is it because it possibly ignites throughout the cylinder simultaneously while spark starts a burn that moves through the combustion chamber?

 

DETONATION (KNOCK)
-A/F ignited after spark due to heat and pressure
-burns about 6x faster when this happens
-caused by too much spark advance, too low octane
-occurs under low load/low RPM

The extra heat created by this will melt pistons and spark plugs and crack the cylinder walls. Also, the shockwaves created by the force of this explosion rattles the pistons around in their bores creating the knocking sound.

Here is a good thread to get you going: http://www.honda-tech.com/zerothread?id=328862

 

Two words......Uncontrolled Burn....
Piston on it's way up and and unexpected explosion (meant to force the piston down) happens something has to give...piston, rod, or as usual the ring land.....(the weakest point)....
Two explosions per stroke insted of one......thats going to do wonders for your motor.