There is a condition called "pre-ignition" which is caused by the pressure of the flame front traveling across the cylinder after the spark plug fires. If there is a sharp corner somewhere in the combustion chamber, or a glowing piece of carbon, etc. a secondary flame front gets started across from the spark plug. When the two flame fronts meet, they slam together to produce a shockwave known as "detonation". There is no mechanical contact of anything; the rattling noise we know as "spark knock" or other descriptive terms is actually the hammering noise of the two flame fronts hitting each other. Where the flame fronts meet, there is a huge spike in both temperature and pressure which can melt the piston crown, melt or blow the ground electrode off the spark plug, break piston rings or ring lands, etc. Gasoline engine detonation is the most destructive force known to mankind, short of nuclear. Ounce for ounce, detonating gasoline is more powerful than C-4, Dynamite, Nitroglycerine, or gunpowder.
A highly modified Chrysler 440 engine I was running under full load on a dyno one night developed a momentary lean condition in one cylinder. It was producing 700+ HP at 6500 RPM. I heard a detonation rattle begin, and before I could back off the throttle, it had blown a 2" hole in the top of one piston. Early timing, lean fuel/air mixture, or a combination of both destroys engines! Don't ignore a detonation rattle, especially if the engine is "lugging" a little, where more throttle opening doesn't make the RPM increase proportionately.
Jerry
