Ignition timing controls when the spark plug fires in relation to the position of the piston. For maximum power the spark emitted by the spark plug should ignite the air / fuel mixture in the cylinder just a couple of degrees before the piston is at the top of its compression stroke. This top most position is known and referred to as Top Dead Center (TDC for short).
With the mixture igniting at this position, by the time the piston has passed TDC and it starting its downward motion, the cylinder pressure is at the maximum point and the expanding gases force the piston downward . This is referred to as volumetric efficiency.
The number of degress of crankshaft rotation before TDC at which ignition takes place is called timing advance. If the timing is too advanced, meaning that the spark takes place too early , the igniting mixture and expanding gases would force the piston down while still on its upward motion. This causes detonation or what is better known as knocking. On the other hand, if the timing is too retarded, meaning that the spark and subsequent ignition takes place when the piston is beyond optimal ignition and / or TDC. This results in loss of power and bad fuel economy.
As engine RPM increases, likewise, the fuel consumption rises and the air / fuel ratio changes. Ignition timing also needs to change to compensate. To retain optimal performance the timing needs to further advance. There are two types of advance methods, namely mechanical and vacuum advance.
Mechanical ignition systems use a distributor mechanically linked to a rotating assembly to time the spark. This is known as Mechanical advance (also called centrifugal advance), whereby a set of weights and springs found under the distributor cap alter the timing as the engine speed varies. As the engine rpm increase the rotating assemble in the distributor cause the weights in the distributor to open up and advance the timing.
Vacuum advance changes timing as the engine vacuum changes as the engine rpm changes. This vacuum is measured at the carburetor or throttle body. At the pressure changes a diaphragm in the distributor causes the weights to open up advancing the spark time.
While the above mentioned timing advance systems are common for vehicles up to the early 80’s a move to electronic ignition system has changed the way ignition timing is controlled. It is now the job of a computer that has a pre-defined chart that will modify the ignition / spark timing at different engine RPM. The figures in this chart are normally set by the factory and cannot be changed by the vehicle owner / tuner unless programming tools and software controllers are used.