Ignition timing is an often-misunderstood concept that has a big effect on performance. An air/fuel mixture burn rate doesn’t increase in direct proportion to engine rpm. Therefore, the faster the engine spins, the sooner the fuel must be ignited so that combustion is completed at roughly the same point in the cycle regardless of rpm. But if you ignite the mixture too soon (too much advance), you can cause engine-damaging detonation (pinging) because the piston is trying to move up as the burning gas is trying to force it down. The curve must also be tuned to prevent pinging as cylinder pressure increases with rpm.
The general terms used in timing are advance, retard, initial timing and timing curve. To advance timing is to fire the spark plug sooner or more degrees before top dead center (BTDC). To retard timing is to fire fewer degrees BTDC (for example, moving from 12 degrees BTDC to six degrees BTDC). And the timing setting at idle is known as initial timing. A timing curve is the rate at which timing advances with engine rpm.
For any given engine rpm, there is an optimum timing setting that provides maximum power. Most people set ignition timing by advancing the distributor until the engine pings under load, then backing off a few degrees. The only way that such a method will provide optimum timing is with a perfect timing curve, which a stock distributor probably doesn’t have. Ideally, you should set your curve for optimum power at 500-rpm increments. That can be done on an engine dyno, but the dyno can’t simulate the dynamics of accelerating a vehicle. Thus, it’s best to experiment at the dragstrip to find the curve that makes your combination run quickest.
Since the timing curve below your launch rpm is irrelevant to performance, you should be most concerned with your total mechanical advance and rate of advance. To determine optimum power at 2000 rpm, Dr. Chris Jacobs of Jacobs Electronics recommends finding the advance setting that makes the car accelerate quickest from 1000 rpm to 3000 rpm in First gear. Duplicate that procedure for each 500-rpm increment, then find the combination of advance weights and springs that most closely matches that curve.
A performance curve will usually call for 20 to 30 degrees of advance by 3000 rpm (with a total of between 30 and 40 degrees), so use a dial-back-to-zero timing light to set your total advance for best power, then alter the mechanical-advance curve so the initial timing allows for optimum idle quality. Finally, use an adjustable vacuum-advance canister to set up additional advance to keep the spark plugs clean.
