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Detonation and PreIgnition.

 - Excessive cylinder temperatures cause detonation and pre-ignition.
 - Maximum power amd economy is achieved just prior to detonation.
 - Detonation is uncontrolled combustion after the spark plug has fired.
 - Pre-ignition is uncontrolled combustion before the spark plug has fired.

Most engines perform best when cylinder pressure peaks at 12-14deg after TDC and performance is usually acceptable with a pressure peak as late as 18-20deg after TDC. Piston acceleration away from TDC is relatively slow, so at the time when combustion pressure is peaking, the combustion space is not rapidly increasing in volume. Because of this, pressure holds at a high level and gives the crankshaft a good shove, making high power.

However, when the charge temp is excessive, detonation occurs and a violent explosion occurs instead of a steady, progressive burn. Obviously this can lead to engine destruction and spark retard is often used to protect against this. But spark retard in itself can cause engine destruction so a good understanding of detonation and spark retard is essential for any engine tuner.

The positive effects of spark retard as follows…
The fuel burn is started as late as possible to ensure the cylinder peak pressure is reached well past 20deg after TDC. At the time peak burn pressure is reached, the piston has begun fast acceleration, thereby enlarging the combustion space. Cylinder pressure quickly tapers off as the fire chases the piston down the cylinder, which wards off harmful detonation – but also reduces horsepower.

However, combustion slows so much that as the exhaust valves open, the fire is still going. This increases exhaust valve temps and the exhaust side of the chamber becomes hotter along with the piston crown.

After this the inlet port opens and allows fresh fuel and air to enter but all the added heat affects the air charge, and it quickly expands, filling the chamber with low density air and leads to lost horsepower at best.

At worst, combustion continues on the down stroke and the piston crown, exhaust valve, and chamber have insufficient time to cool and soon reach dangerous temps and causes combustion occurs long before the plugs get fired. The intense heat generated by this pre-ignition situation acts like a blow torch and melts through the crown of the piston.