What is a Timing Curve?

The timing curve is the mechanical timing values plotted over an RPM curve. What this means is that at a given RPM based upon the weights and springs used in the distributor you will have a specified amount of timing advance. Remember, as your engine RPM increases, so does the inertia against the distributor weights within the distributor. The springs holding the weights is the opposite resistance force that controls the speed, or rate, that the weights move "out" causing the distributor advance plate to move which advances the timing. So, what happens is that at each RPM change there is a variance in the amount of timing advance up to a point in which the weights usually hit a "stop" that does not allow them to move out (advance the distributor) beyond that point.

Changing the springs to a lighter one (or both) will make the advance occur faster, and of course running a heavier (stronger) spring will slow the advance rate. You can also in most cases modify the weights (or stops) as to what the advance limit will be. Mixing springs to control the desired advance rate is how you modify your timing curve.