I know of no chemical phenomena that allows the air fuel ratio to
suddenly and unexpectedly increase burn speed rapidly.  Can you point to any
reference that suggests that?

There definately is a difference between pre-ignition and detonation - 
hence the need for 2 terms.  My understanding is similar to Paul's. 
(it comes from my read of "Fuel Injection" available through ACS).

In pre-ignition, a normal(ish) burn of the mixture occures, sometimes
 because of a hot spot on the piston or elsewhere.  It is a normal type
 of burn that just starts before the spark plug has triggered it - 
something else triggered it.

Detonation occures when the conditions are right (temperature, 
pressure, and mixture) for spontaneous ignition of the mixture.  
Since these conditions are present throughout the chamber (more or less) 
then all the fuel in the chamber decides to spontaneously combust at 
(more or less) the same time.  Thus the very violent and destructive event.

Perhaps when you were doing those experiements you were 
producing pre-ignition and not detonation.

Dave Leonard

 
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