We spend a lot of time preaching about building horsepower through improved combinations of cylinder heads, camshafts, intakes, and so on, and if the mail is any indication, so do you. Bigger, better parts are cool and all, but the pursuit of the trick-of-the-month can cloud one's focus on the little things that can make all the difference. We're talking about the basic adjustments that affect an engine's state of tune. Sessions on the engine dyno have demonstrated time and again the effects of ignition timing on engine output, but wide-open-throttle pulls with a constant load don't tell the whole story. On an engine dyno, we're typically only dealing with total timing, not the advance curve itself, and it's the curve that you'll be feeling every time you take your street car out for a spin, whether it's to the grudge night or to pick up a loaf of bread. In this article, we'll examine how a distributor's advance mechanisms operate and how to dial them in for improved performance.
What's An Advance Curve?In the basic design of an internal combustion gasoline engine, an air/fuel mixture burns in the combustion chambers to force the pistons down, which in turn, spins the crankshaft. This converts the energy released by the combustion events into a rotational output that can be used for propulsion. In a gasoline-powered engine, an electrical spark arcs across the electrodes of the spark plugs to initiate the combustion of the air/fuel mixture; this is the nucleus of our discussion. The precise timing of that spark is critical for optimum engine performance, but since automobile engines are required to perform under widely varying loads and conditions, the timing of the spark must also vary to keep the engine performing at its best at all times.
So when is the ideal time for the spark to occur? On the surface, it might seem like the logical point to light off the combustion would be just after the piston has reached top dead center (TDC), so that the force of the explosion would serve to drive it down, and while that is the goal, the spark actually has to occur a little earlier in the piston's travel to effectively exert the force of combustion on the crankshaft. The reason is that combustion doesn't actually occur all at once, despite the impression an uncorked gasoline engine might project with its sudden explosive sounds and lightning-fast flashes. Although undetectable to the naked eye in real-time, the burn begins at the spark plug electrode, which triggers a sort of chain reaction that then moves outward; the direction or pattern of the flame front-the leading edge of combustion-is determined by multiple factors, including combustion-chamber shape, spark-plug position, and piston-dome shape. The goal when determining ideal spark timing is to synchronize the moment of peak cylinder pressure with the optimum point in the piston's down-stroke to direct the force of combustion against the crankshaft with maximum efficiency. If combustion is triggered after the piston has passed TDC, the combustion events will occur too late to effectively act on the piston, and in turn, the crankshaft. This is why it is necessary to begin combustion prior to TDC, or "before top dead center" (BTDC).
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.
Total Advance: This is the sum of ignition advance in crankshaft degrees from all sources. Most V-8s operate best with between 30-36 degrees of total advance, so if the initial timing is set for 8 degrees BTDC, and the mechanical advance adds another 24 degrees; you'd have 32 degrees total advance, also referred to as total timing. Note that vacuum advance does not figure into total advance, since there is no vacuum at wide-open throttle.
Advance Rate: This is how "quickly" the additional advance is applied, usually expressed in terms of rpm, including the point where advance begins and the point where timing is "all in." The ideal rate depends on a number of factors, including transmission type and ratios, rear-axle ratio, and vehicle weight. Most drag racers like to get the advance in quickly to make maximum power, but many stock passenger vehicles can't apply maximum advance as quickly without triggering detonation. Most vehicles begin to advance just after 1,000 rpm; performance-tuned drag cars may have all the timing in shortly after 2,000 rpm, while stock smog-era cars may not realize total advance until nearly 4,000 rpm. Determining the precise rate and amount requires testing and tuning for individual applications, and razor-sharp race tunes may even vary with fuel quality and weather and ambient temperature conditions.
Advance Curve: All the factors that make up the ignition timing for a particular application-initial, mechanical, vacuum, total timing, the advance rate, and so on, work together to make up the advance curve. The curve refers to the performance characteristics of a particular vehicle's ignition timing advance system.
Crankshaft Degrees vs. Distributor Degrees: Ignition timing is usually expressed in terms of crankshaft degrees, since the relationship that is being tuned is between the spark timing and the position of the crank. However, a potentially confusing aspect of ignition tuning is that advance curves are often described in terms of distributor degrees. When using distributor degrees, simply multiply by 2 to get crank degrees, since the distributor rotates at half the speed of the crank when installed in the engine.
