Here's where it gets tricky: As the engine spins faster, it becomes necessary to initiate combustion earlier to ensure that peak cylinder pressure occurs at the optimum point in the piston's stroke. This is why typical road-going automobile engines need a means of automatically varying the amount of ignition advance relative to rpm (and sometimes other factors). The amount of additional advance and the rate at which it is administered is referred to as an advance curve. Modern computer-controlled vehicles handle this electronically, but prior to this technology, ignition advance had to be applied mechanically. Ignition timing is just as important on late-model vehicles, but alterations to such systems are typically done with a computer, while dialing in vintage iron still requires you to get out the handtools. For the remainder of this article, we'll discuss older distributors-both breaker-point and electronic-that use mechanically controlled advance systems.
Advance Terms ExplainedInitial Advance: Even at an idle, the ignition spark needs to be triggered prior to TDC, and exactly how far is usually determined by the initial advance setting. The specification for most typical V-8 engines generally falls somewhere between 6-12 degrees BTDC, which allows the engine to run smoothly and cleanly at idle while not creating too much resistance during starting. The distributor's shaft is interfaced with the camshaft, but the housing can be rotated independently to alter the amount of spark advance or retard relative to the cam, and therefore, the crank. Once the timing has been set, a distributor housing hold-down locks the housing in place.
Mechanical Advance: Since engines require additional timing advance as they climb the rpm scale, a mechanical-advance mechanism is built in to most automotive distributors. Usually, this is made up of a pair of hinged weights mounted to a plate on top of the distributor shaft and retained with springs. As the distributor shaft spins faster, centrifugal force causes the weights to overcome the spring tension and swing outward. The weights are attached to a separate plate that mounts either the breaker points or the electronic pickup, and when the weights swing out, the breaker plate rotates a predetermined number of degrees, effectively advancing the ignition timing. Altering the tension of the springs as well as the shape and weight of the weights will alter the rate that advance is applied. Altering the length of the slots or guides that control the rotational travel of the breaker plate changes the total amount of advance applied.
Vacuum Advance: While mechanical advance controls the rate that spark lead changes relative to engine rpm, most automotive engines need to be able to adapt to varying throttle inputs and engine loads, both of which directly affect intake-manifold vacuum. A vacuum-advance mechanism uses engine vacuum to overcome a spring-loaded diaphragm, which is in turn, linked to the breaker plate. When activated, the breaker plate is again rotated to add more ignition advance. Engine vacuum is strongest at idle, but most vacuum-advance units are connected to a ported vacuum source, which means no vacuum is present until the throttle is opened. This way, the engine runs only on initial advance at idle, and then gets a nearly instant boost in timing as soon as the throttle is tipped in. However, as the throttle is depressed further, load increases, vacuum decreases, and the amount of additional advance from the vacuum unit drops. By adding additional timing under light load and reducing it as engine load increases, a vacuum-advance unit allows the engine to run with "aggressive" timing under most conditions without experiencing detonation from too much advance. Most factory advance units are set for a predetermined advance rate, though adjustable aftermarket units are available.