Originally Posted by
Connedrod
Once again not quite true. Whist the boundary layer protects the top of the piston detenation will at the extreme will and can blow down the side and top of the piston.
This is kind of true but not really very illuminating. Knock/detonation excites the resonant frequencies of the gas in the combustion chamber causing the gas to oscillate back and forth in varying patterns at the fundamental and higher order harmonic frequencies. There are many different modes to this oscillating pattern but the pressure wave that is generated needs an associated gas velocity to make it happen so yes, when a pressure peak arrives to one side of the piston crown the gas will be stuffed down the side of the ring land - that's what makes the pressure happen. It's the velocity of the gas that disturbs the boundary layer and causes the increased heat transfer rates associated with symptoms of higher CHTs.
The higher the knock pressure amplitude the faster the gas has to move so heavy knock is associated with high local gas velocities and thus high heat transfer. The local pressure of the wave against the combustion chamber can also fatigue the material (which at higher temperatures also has a lower tensile strength and therefore fatigue strength) causing pitting associated with knock damage.