Well there are two kinds of timing...valvetrain timing (when the valves open in relation to crank rotation) and spark timing(when a spark ignites the air/fuel mixture in a given cylinder, also measured in degrees of crank rotation)
Most of the time, when people talk about timing, they are talking about spark timing...Valvetrain timing is usually within 4-degrees of TDC (Top Dead center)of the number one cylinder.
Spark timing is an interesting beast. Thespark must ignite the air/fuel mixture at the right moment..too early, and you have pre-ignition (also called detonation) when the air/fuel mixture is ignited when the piston is still on its upward compression stroke..this tries to push the piston down even though it want to continue traveling up.
If the spark ignites too late, you lose valuable compression of the air/fuel mixture and loose potential power...
Ideally, at idle, the plug will spark just before or AS the piston reached the top of its compression stroke (the top of the compression stroke is also called TDC)
You set your timing by rotating your distributor. A timing light is used identify where the spark occurs in terms of crankshaft rotation. On my TBI 350, at idle, I have 0-degrees of advance. This means that when the number one cylinder is at the EXACT top of its compresion stroke, the spark occurs
As with many things there is a catch...as the engine speeds up, the spark must occur sooner and sooner. This is because the spark takes a split second to ignite the air/fuel mixtue. As engine speeds increase, the plug must spark sooner and sooner so the actual combustion takes place when intended (just as the piston reaches TDC)...Therefore, the engine must have a way of Advancing the timing...newer, fuel injected engines use computer comtrolled distributors to advance the timing..Older engines use either vacuum advance or mechanical advance to advance the timing curve.
Chris /forums/images/graemlins/thumb.gif