There are two slightly different combustion conditions, "preignition" and "detonation". The way it was explained to me many decades ago was like this: "Preignition" is the beginning of a second flame front in the combustion chamber, one that was not originated by the spark plug. This can be caused by a variety of things, such as a hot spot like a glowing particle of carbon, a too-fast flame front travel that pushes the pressure wave to Diesel levels, poor combustion chamber design, or fuel that's too low octane. Additives that raise the octane rating slow down the burn rate of the fuel and turn the pressure wave into a steady push instead of a sharp punch. The more resistance to piston movement there is (lugging) the more likely preignition will happen. Running a 5% Diesel fuel mix can raise the octane rating of the fuel, sometimes as much as 5 points. That's one gallon of Diesel to 19 gallons of gas.
Once the second flame front begins, "detonation" is the noise made by the two flame fronts slamming together at supersonic speed. That's the "ping" that a driver hears. It's extremely destructive. Detonation has more power than dynamite, nitroglycerine, or any other explosive short of nuclear, when the weight of the reactive components is compared on a one to one basis. It produces extreme heat and pressure, to the point of melting piston material and spark plug electrodes, shattering porcelain spark plug insulators, and blowing holes in the tops of pistons, breaking rings, etc. By the time a driver hears that little ping or rattle, the pistons and other parts are getting sledgehammer blows, several times a second.
Retarding the vacuum advance under load is done to try to minimize preignition and detonation. If an otherwise good-running engine still pings and rattles on a long uphill pull at full throttle, try the next higher octane fuel, or retard the initial timing a little. That's why the GM engineers put the "octane selector" on stovebolt engines, so drivers could adjust the timing accurately if a bad tank of gas was encountered, then go back to the normal setting once that gas was used up.
Jerry
