:agree 100%GTO TOO said:LarryM,..... This is more technical but there is no pre-detonation as a term as all spark before detonation is "pre". There is detonation and there is per-ignition ....... Normal combustion happens mostly after top dead center in an expanding cylinder area, ( even though the spark event is BTDC before top dead center ) even thought the partially unburned gases are being compressed as the mixture burns. During pre-ignition the piston is still moving upward under compression and the majority of the fuel is burned as the piston rises. It's like compressing a blow torch, with the same result. Pre-ignition does not last long before pistons are "holed". And pre-ignition cannot be heard.