Setting Ignition Timing Curves

Having a car with outrageous low-end throttle response and top-end horsepower is something every gearhead strives for. The correct combination of parts will get you close, but it’s the tune-up that truly makes the difference. This month we are taking a look at ignition timing and how it can be used to improve an engine’s midrange torque, throttle response, fuel mileage, and temperament.

Ignition timing is crucial to every engine combination. Remember that it’s average power that accelerates your car, not the peak. Optimizing the ignition timing curve is a cheap and easy way to gain power across the entire rpm band.

Unfortunately, like anything else, there is only so much gain that can be achieved before detrimental effects begin to occur. Advancing the timing of an engine causes the ignition system to ignite the compressed air/fuel mixture as the piston attempts to squeeze the mixture into the chamber of the cylinder head. Starting the combustion process before the piston has reached top dead center (TDC) may seem hazardous to the engine because the piston has to work against negative force. However, because the combustion process takes time to occur, this advance improves power at that particular rpm. The amount of advanced ignition timing is relative to an engine’s bore/stroke compression ratio, fuel octane, and a dozen other variables. Because you cannot run excessive amounts of ignition timing at all times, distributors are designed to employ timing curves that progressively induce timing into the cylinders.

A distributor is capable of setting a timing point that is advanced by two independent methods. First, a Chevrolet V-8’s initial timing >> must be set at idle by rotating the distributor clockwise for retard or counter-clockwise to advance. This initial timing point acts as a base point for the engine’s ignition system. Once initial timing is set, there are two ways of adding additional timing to an engine. One method uses a vacuum canister designed to work off a ported vacuum source (Holley carburetors locate this on the metering block), which is typically referenced from just above the closed throttle blades. As the throttle begins to open, the engine displays its highest level of vacuum and causes the distributor’s vacuum canister to advance the timing.

Mechanical advance is the second method of ignition timing advance. As the distributor spins fast enough to activate the mechanical-advance weights, the engine receives initial timing, mechanical timing, and vacuum timing under part-throttle conditions. As the engine accelerates to wide-open throttle (WOT), the vacuum drops, eliminating the vacuum canister’s timing. For example, part-throttle total timing would look something like this: 10 degrees initial + 10 degrees vacuum + 20 degrees mechanical = 40 degrees of total timing. At WOT, there is no vacuum present and the canister timing is eliminated, giving your engine a total of 30 degrees timing. The reason your engine is able to sustain more timing at part-throttle is because only a limited amount of air and fuel make it into the cylinder at part-throttle. Lower cylinder pressures enable the combustion process to start sooner and help improve part-throttle response by increasing torque. This additional part-throttle timing improves efficiency and torque.

Ideal ignition settings will allow your engine to run the maximum amount of timing at all engine speeds without detonation. Now that you know how distributor timing works, you can manipulate it to improve your torque and horsepower curves.

Time to Vacuum

Vacuum canisters control part-throttle timing. By igniting the spark sooner during part-throttle operation, the combustion process is aided. When vacuum canisters first appeared on distributors, the factory designed them to employ a nonadjustable amount of predetermined advance at maximum engine vacuum. As an engine accelerates and vacuum decreases, the canister slowly pulls timing from the engine until it reaches zero vacuum advance. The factory vacuum canisters were designed to work with individual engine combinations. HEI systems were designed to employ less mechanical advance to help control emissions, while point-type distributors featured high amounts of mechanical advance. When engines are altered and modified, their timing demands also change, which is why Crane Cams and Moroso designed adjustable vacuum-advance kits. By simply inserting a 3/32-inch Allen wrench into the end of the canister, the internal vacuum-advance springs can be adjusted to control the engine’s rate of vacuum advance. The system is also designed to work with a vacuum-timing limiter plate. This plate allows its user to preset the total amount of vacuum timing at maximum engine vacuum.

Lifting Weights

The mechanical-advance system inside a distributor uses springs to determine an engine’s rate of advance. There are a total of two springs, two weights, and four pins. Each spring is attached to one weight pin and one timing pin. As the engine accelerates, centrifugal forces attempt to pull the weights from the timing plate. The rate at which the springs let the advance weights move determines the timing curve. Using higher- or lower-load springs will bring timing in at a lower rpm. For example, two light-tension springs would allow the mechanical timing curve to start at a low engine speed such as 1,000 rpm. By 2,500 rpm, the maximum amount of mechanical advance would be employed and provide the engine with an extremely quick advance curve.

A timing slot in the mechanical-advance mechanism limits total engine timing advance by using a pin inside a slot. In order to increase an engine’s total timing, this slot must be elongated with a carbide cutter. If the total timing needs to be limited, the slot will have to be welded shut. Many aftermarket distributors (such as MSD) supply offset bushings that allow the total timing to be increased or decreased without any cutting or welding.