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Detonation, similar to preignition.

detonation definition: your shit's too hot. Due to excess heat the air and fuel is igniting from a hot spot at nearly the same time as the spark plug is igniting the air and fuel. The resulting double flame fronts cause too rapid of a pressure increase. This rapid pressure increase stresses bearings and other parts and makes an audible 'ping' sound. This also causes pits too form on the combustion chamber and piston dome.

preigniton definition: your shit's waaay too hot, hotter than detonation. Due to excess heat the air and fuel is igniting from a hot spot way before the spark plug should ignite the air and fuel. This will cause "run on" of a engine when you attempt to shut it off. It's similar to the way a diesel engine works.

"I have a head temp gauge, how hot is too hot?" It differs from engine to engine. Sparkplug heat index, compression ratio, squish velocity and other factors determine what temperature the air/fuel mixture will prematurely auto ignite. Thus head temp gauges are just that, a gauge and not a sure fire way to know when you're engine is too hot.

You want to have as much compression ratio as possible, thus on the verge of detonation, for maximum horse power but how do you tell when overheating(detonation) is occurring?

While the engine is hot and you suspect detonation hit your kill switch while still maintaining throttle position. Ideally it should instantly cut off and decelerate very smoothly. If it decelerates smoothly but also pops very loudly once or twice from the exhaust(way louder than four stroking sputter) you're pipe is so hot it's igniting the unburned air and fuel which means you are close in temperature to incurring detonation. If it jerks violently then slows down you're into detonation but not hot enough to keep autoignition going. If it continues to pull until you shut the throttle you're engine is WAYYYY TOO HOT and into preignition.

In this author's experience detonation will always occur prior to seizing an engine. The only exceptions to this is a seize caused from insufficient lubrication or catastrophic failure. So in conclusion, USE YOUR FUCKING KILL SWITCH!!!