Detonation is the spontaneous combustion of the air/fuel mixture ahead of the flame front (combustion by explosion rather than controlled burning). It occurs after the combustion process has started and is usually located in the last area to burn. As the flame front advances across the chamber, the pressure (and temperature) in the remaining unburned mixture rises. If the autoignition temperature is exceeded, this remaining mixture explodes. The audible ping (metallic clanking noise) is the explosion's shockwave.
Detonation is very destructive. No metals in existence, forged pistons, special head gaskets can withstand sustained detonation. If you hear detonation your best measure is to temporarily back off, before looking for its cause.
Detonation can be caused by the following:
1. Poor Fuel. A fuels octane rating is a measure of its resistance to spontaneous combustion. The greater the octane, the greater the resistance. For extreme boost levels it is recommended you use either an octane booster or some type of RACEGAS.
2. Ignition Timing. Improper ignition timing can cause detonation. A lot of the late model turbo cars have knock sensors built in that automatically retard the ignition when knock is sensed, to prevent damage.
3. Lean air/fuel ratio. Lean running will promote detonation. Lean mixtures increases heat which increases the chances of detonation.
4. Exhaust Gas back pressure. A build up of back pressure can cause heat to build up, which can agravate detonation.
5. Intercooler. Anything that lowers the intercoolers efficiency will increase the chances of detonation. eg paper covering intercooler.
6. Ambient Heat. High performance turbo systems often run near the detonation threshold. Hot days can push the car over this threshold.
