The good old distributor has served its purpose in controlling the ignition advance curve of the engine for almost as long as engines have been around. Advancements in technology have made the use of distributors as we know them redundant many years ago. They have been replaced by electronics which still do the same job but far more accurate in controlling the centrifugal advance curve than a distributor ever could. This article is aimed at basic control that the distributor can achieve. The concept of ignition advance is simple enough, ignite the spark plug at the correct time so that the resulting expansion of burning gases produces maximum pressure in the cylinder just After Top Dead Centre (ATDC). With the exemption of only a few, most engines spark timing is always Before Top Dead Centre (BTDC) This is the relationship of the piston in crankshaft degrees before it reaches the top of its stroke. The compressed air / fuel mixture does not instantly ignite, it takes time to burn and reach its maximum effective pressure, therefore the process of igniting the spark plug must start to happen earlier to ensure the pressures are at their peak at the right time. ie. just ATDC. If the spark is started late (retarded) the piston literally runs away from the expanding gases with a dramatic loss of power. If started to early (advanced), the effective pressure can be reached before the piston reaches TDC, causing severe pressure shock to pistons and related engine parts. Generally this pressure shock can be easily heard and sounds like a thousand crickets or a bag of marbles clanging together in a bag. This is know as detonation or pinging and can destroy engines. You must immediately back off the throttle when you hear detonation and correct the cause of the problem as soon as possible. Generally detonation only occurs at half to full throttle under load, so the car can still be driven under light load conditions without destroying anything. The graph below demonstrates the affects of ignition timing versus cylinder pressure when the engine is at Wide Open Throttle (WOT). As you can see by the green curve the ideal peak cylinder pressure is reached just ATDC.
Lets look at the conventional distributor operation. There are two mechanisms to control the ignition advance. First I will discuss centrifugal advance. This mechanism operates by sensing engine RPM only, well actually camshaft RPM. This is accomplished with a set of rotating weights that react to centrifugal force. A set of springs are attached to the distributor camshaft which control the rate of advance. Centrifugal force moves the weights outward which in turn move the distributor cam forward in rotation thereby advancing the ignition timing. By adjusting the tension on the springs we are able to roughly control the rate of ignition advance relative to engine RPM as it increases. In most cases there are two springs, a primary and a secondary. The primary spring has light tension and controls the starting point of advance, the rate of advance from the start point is controlled by the spring stiffness. The secondary spring is used to slow the rate of advance in the mid to higher RPM range. The total advance available is set by the distributor and engine manufacturers using a mechanical stop built into the design of the cam shaft. This can be easily modified by either a rat tail file to increase the total or brazing to reduce the total. Brazing is required when modifying the advance for LPG use. By changing springs and / or adjusting the tension, a suitable ignition advance can generally be found to suit the engine. When checking your initial ignition timing, you should also check that the centrifugal advance is within specification. A centrifugal advance modification is always required when engine modifications are done, eg. performance cam, changing compression ratios, stroking, carburation and similar mods that are used to increase performance.
The second device incorporated into the distributor is the vacuum advance unit. This unit compensated ignition timing according to engine load. Vacuum in the intake manifold is higher under lower loads such as cruising and since the burn rate of a lean mixture is slower, more timing advance is required. The high vacuum pulls on a diaphragm which is connected to the ignition points base plate causing it to rotate in the opposite direction to the distributor shaft rotation. This in effect advances the ignition timing. As vacuum drops off due to acceleration the diaphragm returns to its static position and only centrifugal advance is engaged. The vacuum advance unit depending on make has little to no adjustment and therefore cannot be easily modified when engine modifications are done. When large cams or intake manifolds are utilised the load signal (vacuum) becomes unreliable. In these cases the engine tuner usually removes the vacuum unit or source of vacuum and relies totally on centrifugal advance for the ignition advance curve.
Modifying an advance curve for a performance engine is really a hit and miss affair. Generally the advance rate is increased with the total advance reached earlier than a stock motor. With the use of a dyno an advance curve will be found that is optimal for an engine without having to test drive the vehicle several times to test for detonation. An optimal curve is one just short of detonation and this is usually just a few degrees. Once an optimal curve is found it will definitely be worthwhile to write it down for future reference.