Originally Posted by bpmcgee
Just a minor elaboration -- detonation is an uncontrolled explosion BEFORE the spark plug sparks -- i.e. a combustion event working against the rotation of the engine.
And +1 to RMac.
Nope...you are actually referring to pre-ignition (ignition before the spark); it's related, but I assure you NOT the same as detonation. The terms are commonly used interchangeably when they technically should not be.
Detonation is the uncontrolled explosion of pockets of fuel/air mix inside the combustion chamber during
normal spark ignition that are a separate pressure front from the normal ignited mixture expansion. Pre-ignition (the worse of the two) is caused by ignition of the fuel/air mixture before the spark starts. Both can be initiated by similar things, hence their "relationship" and the fact that sometimes the noises they make are similar.
When pre-ignition occurs there is a heavy knocking/pounding along with a substantial loss of power while detonation makes the "ping" or "light rattle" noise. It is highly unlikely to find pre-ignition issues in a modern engine without a major defect or damage to the combustion chamber in the cylinder head or an obscene overheating condition.
EDIT: Forgot to add that when you get pre-ignition when the engine is running (not the run-on), you will likely experience component failure....engine failure...in short order. It isn't detectable until it happens.
I'm an old, long time engine builder and this subject along with "torque vs horsepower" discussions bristle me.