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First let me say I am not an engineer. This is what I've come to know over the years.
Two things that are commonly confused are pre-ignition and detonation.
Pre-ignition is caused by a "hot-spot", such as a glowing carbon deposit or spark-plug thread burr igniting the fuel-air charge BEFORE (pre) the actual ignition event. In other words, before the spark plug fires, something actually ignites the charge.
Detonation is a spontaneous ignition, generally caused by heat & pressure. (Two reasons why we have intercoolers, lower compression and control or limit boost.) Poor gas or gas of less than the required octane rating is usually involved. Big problem with detonation is, unlike a regular ignition event in which there is actually a controlled burn of the charge, there is an uncontrolled explosion and large shockwave at the flamefront which can damage piston crowns, pit valves etc. The collision of the detonation shockwave and the flamefront of the properly ignited charge is particularly damaging.
On a related note, Octane is used to slow the burn and prevent detonation. It allows higher cylinder pressures before detonation.
How does it feel? In my old '76 Ford Torino you could feel the sharp raps and hear the knocks. An engine under heavy load, like lugging up a hill with your foot in the throttle is more likely to experience detonation.
Again, I'm no engineer but I like to read about this stuff as much as I can. Hope it helps and if anyone out there can add to or correct me, please do!
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Erwin in Memphis, '95 855t - fast & comfy, '86 Guzzi LeMans - fast & loud, '76 Montesa 348 Cota - slow.
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