[SactoJim]

Detonation is basically the uncontrolled ignition of unburnt fuels typically after top dead center. It is not good, but many engines can withstand some detonation. Mfrs. even take this into account when designing components for engines.

Pre-ignition is the worse and basically uncontrolled ignition of fuel too far before top dead center. So the rotation assembly has momentum turning in one direction and pre-ignition is trying to make it turn the opposite way......very bad and most engines will not live long with this.

Bottom line is that both aren't good and should be avoided with proper tuning and the correct octane rated fuels.