Igniting the air/fuel mixture at the correct instant during the compression cycle is crucial to improving the efficiency of the engine in terms of power production and gas mileage. The idea is to have the pressure caused by the ignition of the air/fuel mixture peak just past the moment the piston reaches top dead center (TDC) so that maximum force is placed on the piston on the power stroke. Ignite the air/fuel mixture too soon and detonation (pinging) will occur as the peak pressure would be pushing down on the piston while it is still moving up in the compression stroke. Ignite the air/fuel mixture too late and peak pressure would only be reached while the downward movement of the piston is causing the pressure in the combustion chamber to fall. The optimal ignition timing will thus ensure that peak pressure is reached just past TDC, which will produce the lowest exhaust gas temperatures and maximum torque.
However, ignition timing is complicated by engine speed and the time the air/fuel mixture takes to burn. At higher RPMs the movement of the pistons requires the ignition process to occur earlier if peak pressure is to occur just past TDC, and the higher the RPM, the earlier ignition has to occur. This is because the rate at which the air/fuel mixture burns does not increase in direct proportion to the increase in engine speed. In addition, under light engine loads, such as during normal driving and cruising, the air/fuel mixture is leaner than it is under heavier engine loads, such as during heavy acceleration. A leaner air/fuel mixture burns slower and thus requires more time to produce peak pressure. The same goes for the intake air temperatures, where a cooler intake charge will burn slightly slower.
In addition, the combustion chamber design, the size and volume of the combustion chamber, the position of the spark plug in the combustion chamber, the camshaft timing and the size and number of valves all affect the rate at which the air/fuel mixture burns. Fortunately, these factors, with the exception of variable valve timing, are static.
The solution to all these factors affecting ignition timing is the ignition timing advance and the ignition timing curve. In a mechanical distributor, the timing advance and timing curve is controlled by a combination of a vacuum advance, taken from the intake manifold, and centrifugal force acting upon a set of weights whose free movement are dampened by springs. The latter is referred to as mechanical advance. Here the vacuum advance is responsible for reacting to engine load, and mechanical advance respond to engine speed. However, vacuum advance is somewhat problematic. Until full vacuum advance is reached, it allows air into the combustion cylinder and distorts the air/fuel mixture in which ever cylinder the vacuum is taken from, reducing power from that cylinder. This is one of the reasons why most racing and street performance ignitions do not have vacuum advance and why we would rather have the vacuum advance removed and blocked off. This means we only have the weights and springs of the mechanical advance mechanism to control the ignition advance. On modified engines, the stock weights may not produce the ideal timing curve that produces maximum power over the rev rage. In more advance distributorless ignition systems, the ignition timing can be mapped at different engine speeds and different engine loads, though the manual mapping of the timing curve might necessitate the use of a more advanced, programmable aftermarket engine control unit (ECU).
Determining the Timing Curve
The process of determining the ideal timing curve for a mechanical distributor and an ECU controlled distributorless system is quite similar. With a mechanical distributor the timing curve can be adjusted by replacing the weights and/or springs. Lighter weights would reduce the rate of timing advance as would stiffer springs, while heavier weights and lighter springs will increase the rate of advance. However, it might not be possible to find a set of weight and spring combinations that will produce that ideal ignition timing curve over the entire rev range but we can get as close as possible to it. A word of warning though, if we do need to compromise, it would be preferable that we err on the side of too much advance rather than too little as too little timing advance will result in detonation, and high RPM detonation can be very disastrous.
The timing curve can be determined either on a dynamometer (dynamo) or on a drag strip. We prefer to use the dynamo to set the initial ignition timing before moving to the drag strip as a rolling dynamo does not simulate the engine loads experienced under acceleration.
- If you have a mechanical distributor, start by cleaning and lubricating the moving parts of your distributor. Then set the initial timing on a dynamo by finding the ignition timing that produces the most power without causing detonation at a constant 1,500 RPM.
- Determine the best timing curve at 2,000 RPM on the drag strip by finding the timing advance that produces the best acceleration from 1,000 RPM to 3,000 RPM in first gear. This can be done by first determining the acceleration time with the current set up, then increasing the advance slightly if you have a programmable ECU, or using slightly weaker springs if you have a mechanical distributor. If the acceleration time comes down without causing detonation, increase the advance a bit more until the best acceleration time is reached without causing detonation. If increasing the advance lead to a slower acceleration time or causes detonation, reduce the timing advance until you reach the best acceleration time from 1,000 RPM to 2,000 RPM. If you have a mechanical distributor, make a note of which set of weights and springs produced the best acceleration, even if you only changed the springs.
- Now repeat this procedure for the rest of the rev range at increments of 500 RPM. In other words, to determine the best timing curve at 2,500 RPM, find the advance that produces the best acceleration from 1,500 RPM to 2,500 RPM in first gear; to determine the best timing curve at 3,000 RPM, find the advance that produces the best acceleration from 2,000 RPM to 3,000 RPM in first gear; etc. If you have a mechanical distributor, make a note of which set of weights and springs produced the best acceleration at each increment.
- Once you have determined the best advance over the entire rev range, and you have a programmable ECU, your work is done. If you have a mechanical distributor, you need to check which sets of weights and springs produced the best acceleration for each 500 RPM increment. More often than not, the set of weights and springs that results in the advance that produces the best acceleration at top RPM won't produce the best acceleration at 2,000 RPM and vice versa. This means you need to decide which set of weights and springs to go with. If more low down torque is required you should use a set of weights and springs that produce the best acceleration at lower RPMs. If you require more top end power, and the car will be drive hard at high RPM more often, it might be better to use a set of weights and springs that produce the best acceleration at high RPMs. However, the set of weights and springs that you use should not produce detonation over the entire rev range.
You can also read the spark plug to check if the ignition timing is in order at each of the rev ranges mentioned above. You can read the timing by the coloring, which is also called annealing, on the spark plug's ground strap. You are looking specifically for this coloring to be centered at the elbow of the earth strap. If it closer to the tip, your ignition timing is too far advance at that rev range. If it occurs closer to the spark plug's shell or there is no coloration, then your timing curve needs to be adjusted to advance the ignition timing. Note: when reading the spark plug, you need to shut the engine off at the RPM range you are testing and check each spark plug immediately.