When something causes the air-fuel mixture to ignite prior to the spark plug firing, you have pre-ignition. Pre-ignition can be very damaging to your vehicle's engine. Detonation is the spontaneous combustion of the remaining mixture in the chamber after normal combustion is initiated by the spark plug. The key here is that detonation occurs after normal combustion. This often causes an engine "ping" that may indicate less than optimum performance.