There is NO such thing as Pre-Detonation !!!!!! Every time I see that term tossed around, I cringe. and yes I AM an engineer. There is such a thing as Pre-IGNITION, and there is Detonation which are not the same thing, but Pre-Detonation does NOT exist. Pre-Ignition is generally caused by hot-spots in the combustion chamber and can, but not usually be caused by too low fuel octane. Detonation results from too much spark advance for the octane of the fuel used or conversely not enough fuel octane for the desired spark advance. Both are loosely described as "spark knock" and tend to sound similar when audible, but are NOT the same event.