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).