One thing I did not mention. When I was running 13.25 to 1 pistons I had a head that I polished the combustion chambers on. With the 12.5 to 1 pistons the "pre ignition" seemed to be worse even though I had a little less timing. Dose polishing the chambers help that much?
I think you are attributing the change in behavior to the wrong thing. The 12.5:1 piston likely had less squish (and less mixture motion) which strongly impacts burn speed. The higher compression of the 13.25:1 would also speed up combustion, and I would suspect due to the higher compression ratio had better mixture motion. The polishing certainly helped, but it was not the only factor.
Detonation is strongly affected by both the ignition timing and the burn speed of the mixture. That is one of the reasons engines require less octane at high rpm than they do at lower rpm. Highest octane demand is usually at the peak torque rpm, because by definition that is the point the engine has its highest cylinder pressures (and most efficient intake volumetric efficiency).
Fuel air mixture also controls burn speed, with mixtures of gasoline near 12.5:1 - 13:1 having highest burn speeds in most cases (varies with specific fuel and engine design). The burn speed slows down if you go either rich or lean of what ever the best burn speed mixture is for your engine.
Looking at your pictures I think you had both pre-ignition and detonation. Detonation tends to break things (piston picture) and pre-ignition tends to torch pistons and heads (cylinder head picture). I suspect you had detonation that rapidly progressed to pre-ignition which only stopped when the cylinder pressure blew the top out of the piston.
For detonation to occur you need to hold the fuel air mixture at high pressure and temperature for a period of time for the detonation sensitive precursors to form as the hot fuel breaks down. This is why octane demand drops at high rpm, there is less time for the detonation reactions to take place.
At some critical heat pressure and time value that fuel air mix that has not yet burned normally, suddenly ignites and burns at near detonation velocities (speed of sound in the fuel air mixture) rather than burning normally along a flame front, causing huge over pressure spikes that break things. The high pressure shock waves strip off the thin insulating layer of gases that helps hold temperatures in check on piston crowns, valves and spark plugs and temperatures of the surfaces skyrocket even though the actual combustion temperatures are lower. This can very quickly make any sharp edge in the combustion chamber like a valve pocket cut out, piston relief edge or spark plug electrode get up to melting temperatures.
The burning/detonation process of knock happens so fast that even at 200,000 frames a second high speed camera speeds NACA had difficulty recording the actual detonation process.
Larry
