Normal fuel burning is like the ripples caused when a pebble is dropped into a puddle- - - - -concentric waves of flames beginning at the spark plug tip and moving outward until they encounter an obstruction like the edges of the combustion chamber. Then the pressure wave is applied to the top of the piston to push it down the cylinder. Ideally, the pressure wave should encounter the piston right at the top of its travel so no energy is wasted trying to push the piston backwards, or chasing it down the cylinder if the spark happens too late. Since the flame front speed of a correct fuel/air mixture is pretty constant, the spark needs to happen sooner as the engine speed increases. The advance can be done by centrifugal weights, manifold vacuum, or a combination of both, plus the position of the distributor housing (initial timing) that determines when the whole process begins. Electronic ignition timing by computer is much more flexible than the old mechanical systems can be, sometimes altering timing several times a second. There is a practical limit to how much timing advance can be dialed in, and even one degree "too early" robs as much power as five degrees "late". I've proved that fact on dozens of dyno runs. Trying to nurse that last bit of torque out of an engine by tinkering with the ignition timing can cause serious problems. Volumes of information have been written on the causes and effects of preignition and detonation, and a soon as someone starts bloviating about it, someone else will dispute his findings. Don't get greedy about spark advance, or running a too-lean fuel mixture, and most of the problems will go away.
Jerry
