Posted by George in MW Mich on Tuesday, April 27, 2004 at 4:55PM :
I got a little brochure published by Chrysler in 1954 that discussed pinging (aka detonation) and preignition at last week's Carlisle meet.
Preignition occurs when burning starts before the spark. This is usually caused by very hot spark plugs, or a hot spot somewhere. You probably won't hear it and it causes big damage.
Detonation or pinging happens when the spark ignites the mixture correctly but then the pressure and heat caused by this ignition causes the rest of the charge to explode. The flame is supposed to follow an expanding front...not just go off at once. This causes the sound you hear and is usually not as damaging to the engine as preignition.
Hot plugs or very advanced timing are common causes of preignition. Advanced timing, low octane gas and carbon buildup (increasing the compression ratio) are common causes of pinging.
They discussed spark timing. They note that the timing in 1954 for the six was 2 degrees before TDC. They said that you can vary your timing plus or minus 4 degrees from this point....but no more... and that if you car runs right, you will get better milage and power if you advance timing to 6 degrees BTDC. This was part of a longer discussion on adjusting the timing to eliminate preignition or pinging.
There are lots of these little booklets around covering all sorts of things...pretty interesting