what kind of fuel mixture causes detonation
Detonation (not to be confused with pre-ignition) is a spontaneous combustion of the end-gas (remaning air-fuel mixture) within the combustion chamber AFTER the spark/ignition. Combustion of the air-fuel mixture is supposed to be a smooth, controlled burn originating at the spark plug and spreading throughout the chamber, (relatively) steadily increasing pressure and tempurature to drive the piston down.