Most domestic distributors create this curve by employing a pair of weights and small springs that are placed on a pedestal attached to the distributor shaft. As engine speed increases off idle, centrifugal force swings the weights away from their pivot point. This twisting movement also moves the position of the spinning pole points that swing past the electronic pickup, which advances the timing. The distance the weights travel is determined by a pin that travels through a slot. The distance the pin travels determines the total mechanical advance. For aftermarket distributors like an MSD, the pin is fitted with an interchangeable bushing. A thicker bushing limits the total amount of mechanical advance while a thinner bushing allows more.
The rate at which the total advance is achieved is determined by the strength of the two small springs. Heavier springs create a very slow curve while light springs allow a very rapid rate of timing increase. So the engine tuner has the opportunity to vary not only the total amount of timing, but also the rate at which the timing is achieved. As the graph shows, a typical street mechanical-advance curve will all be in by roughly 2,800 rpm.
For a street engine, you might think that more is better and that a quicker curve with very light springs is a good thing. But very light springs can cause other problems, such as causing clutch chatter when leaving from a stoplight, because the advance begins too soon and comes in too quickly.
Conversely, an engine with high static compression on pump gas might want a slightly slower advance curve to keep the engine out of detonation. Each engine has its own timing requirements and the only way to know for sure is to experiment with different springs to find a curve that works. The good news about this is that this only requires time and no expensive tools or parts.
Vacuum AdvanceUnder light-load cruise conditions, the mostly closed throttle minimizes cylinder filling. This creates reduced cylinder pressure. Ideal timing for this situation requires advanced timing in order to complete combustion before the exhaust valve opens. Since a mostly closed throttle also builds high manifold vacuum levels, vacuum can be used to produce additional advance through a diaphragm device called a vacuum-advance canister.
Before we get into the actual components, it's important to make sure the vacuum signal from the engine is correct. Most carburetors offer two different vacuum sources. The simplest is straight manifold vacuum, while the one you want for vacuum advance is called ported manifold vacuum. This source is designed so that the vacuum signal only occurs after the throttle blades are slightly open, which means there is no vacuum signal at idle. This ensures the engine idles on only initial timing and not the combination of initial and vacuum advance.
Most vacuum-advance cans add between 10 and 20 degrees of advance with the highest vacuum producing the most advance. As the vacuum signal drops (as the throttle opens), this reduces advance until WOT is achieved at which time minimal manifold vacuum is present and vacuum advance is zero.
Vacuum advance merely improves the part-throttle experience and is especially important in street driving since 90 percent of it is at part-throttle. Of course, it's possible to add too much advance. Generally, excessive timing will create a light surge or even detonation. Too little vacuum advance is not as clearly defined and will require experimentation to determine the ideal amount of vacuum advance.
Timing LightsThe best way to measure total mechanical and vacuum advance is with a dial-back timing light. These more expensive lights use a dial on the back to place the TDC mark on the harmonic balancer at the zero timing mark on the engine. Then you merely read the total amount of timing on the dial. These lights are more expensive and can sometimes introduce a slight delay in timing accuracy.