The Advance CurveAll engines require an ignition curve based both on the requirements of engine rpm and cylinder filling (volumetric efficiency). This means that at 3,000 rpm with the throttle open only 10 percent, the engine's timing requirements will be different than at that same rpm at wide open throttle (WOT). This is because cylinder pressure is much higher at WOT than it is at part-throttle with a lower cylinder pressure requiring more time to complete the combustion process compared to a more highly pressurized cylinder.
Based on this variable timing requirement, all engines use a three-tiered formula for creating an optimized ignition curve. Combining initial timing with mechanical and vacuum advance creates a curve that will satisfy all of the engine's various operating requirements. Initial timing sets the base timing that is present as the engine cranks and at idle. Mechanical advance adds more timing as engine rpm increases. Vacuum advance is added to the combination of initial and mechanical at light engine loads when vacuum is highest to ensure optimal fuel mileage and efficiency.
Initial TimingAll engines start off with a base timing figure called initial timing. This is established with the physical position of the distributor in relationship to the engine. Ignition-timing figures are always delivered in crankshaft degrees and in relationship to the No. 1 piston at top dead center (TDC). This means that any ignition advance will be expressed in terms of degrees before TDC (BTDC). This is important because some enthusiasts get confused when viewing timing markers on engines where the timing tab may be embossed with ATDC and BTDC. The confusion here is that some people mistakenly think that the "A" in ATDC means "advanced" when it really means "after," as in "after top dead center."
Now that we have that straight, initial timing is the setting for the number of crankshaft degrees BTDC that the spark plug fires. For most performance street engines, that figure will generally be between 10 and 18 degrees BTDC. This is set by loosening the distributor holddown bolt and twisting the distributor housing while watching the timing mark on the crankshaft using a timing light. The key here is to twist the distributor in the proper direction. For most GM engines, the distributor turns clockwise. So to advance the initial timing, you must move the distributor against rotation (counterclockwise). To retard the timing, you would twist the distributor with rotation (clockwise). If you're not sure which direction the distributor turns, you can remove the distributor cap and crank the starter and watch the rotor.
Initial timing is added directly to both vacuum advance and mechanical advance. So if you have an engine that prefers more initial timing, this may require some adjustments to the mechanical-advance mechanism to accommodate the additional timing. Engines with long-duration camshafts and low-idle manifold vacuum benefit the most from more initial timing, while near-stock valve timing responds poorly to excessive initial timing. So the rule of thumb is more initial timing for engines with big cams and less for stock-cammed engines.
Mechanical AdvanceHere's where most of the magic lies. As engine speed increases, there is less time for the combustion process to take place. In order to create maximum cylinder pressure at higher engine speeds, its necessary to start the combustion process sooner.
This is accomplished by creating a mechanical-advance system based on engine speed. As engine speed increases, the "lead time" increases up to a point. If you recall the timing graph from earlier in this story, you can see that mechanical advance starts from the initial timing point and adds more timing as engine speed increases up to just under 3,000 rpm where it should level out. This is a typical street engine ignition curve.