In a perfect world, there would be no need for ignition advance curves. As soon as the piston in your vintage Vette reached top dead center and the engine built maximum compression, you'd light the fire. Kaboom. Job done. Unfortunately, this isn't a perfect world.
Why are advance curves required in the first place? Spark is most often introduced into the cylinder prior to the piston reaching TDC to give the spark sufficient time to light the air-fuel mixture. As engine speed increases, the time required to bring in the advance increases. Everything else being equal, bringing in the spark sooner creates more cylinder pressure and, consequently, increases low rpm torque. There's a trade-off though: As the engine speed increases (particularly in high gear), there is a loss of some top end power. That's why high gear spark retard systems show performance increases. There is a point, though, where you can dial too much advance into the ignition system, causing the air-fuel burn to take place too early. This is known as detonation (or pinging).
Initial advance is the base timing dialed into the engine before the centrifugal advance begins. How much initial do you need? It depends. For example, it's interesting to note that as altitude increases, so does the need for additional spark advance, since more advance helps to compensate for the lack of oxygen. There are other factors as well. Check out the accompanying chart from MSD.
Note that the variables can change throughout the range of engine operation. MSD notes that the timing mechanism of the distributor must make timing changes based upon these factors. It's also easy to see that there is no one perfect curve. Each engine will be different, so each curve will be different. More on curving the distributor later.
Two Types Of Advance
Generally speaking, there are two types of advance curves commonly used in a distributor: centrifugal and vacuum. Vintage Corvette distributors and aftermarket distributors used on street-driven cars usually incorporate both systems (the only exceptions are high-performance Corvettes such as the L88 and ZL1, which used mechanical advance-only distributors). Both systems function independently of one another. Centrifugal advance is based upon a set of governor weights and springs, which, in turn, are controlled by engine rpm. Essentially, centrifugal force moves the weights outward against the tension of the springs, causing the distributor cam, and consequently the spark timing, to advance.
Vacuum-advance arrangements are more complex. All vacuum-advance units operate on a system where the diaphragm reacts to the difference between atmospheric pressure and induction pressure. Early, pre-emission vacuum-advance units were typically linked to a manifold vacuum source. This meant that the vacuum was most often taken from a location below the carburetor throttle body. During idle and part-throttle operation, manifold vacuum is high. This advances the ignition timing under those conditions, which improves fuel economy. When the engine is operated at wide-open throttle, manifold vacuum is low. This means the vacuum mechanism does not advance ignition timing, so there is no chance of detonation.
In the mid '60s, vacuum advance mechanisms changed to suit emission requirements. The vacuum source was changed from the manifold to the carburetor venturi, which is called spark ported vacuum. Spark ported vacuum is lowest at idle, then increases as the throttle is opened. This is completely opposite of manifold vacuum. At idle, a spark ported vacuum system has no vacuum advance (in contrast, a manifold vacuum advance might have as much as 12 degrees extra timing).
Two Corvette motors, two very...
Two Corvette motors, two very different advance curve requirements. The dual four-barrel small-block most certainly needs a different curve than the '67 L88. Factor in the gasoline of today and you can appreciate how a custom curve is mandatory.
This is MSD's street distributor...
This is MSD's street distributor for a Chevy application. In terms of layout, it's much the same as a vintage Delco, where the mechanical advance mechanism is located under the rotor. This particular distributor is also equipped with a vacuum advance system.
In racing applications, there...
In racing applications, there is little need for a vacuum advance, although in a street-driven car, it's highly recommended. In the MSD distributor, there are two Allen head screws that hold the advance canister in place. These are removed along with the snap ring that holds the magnetic pickup assembly. By lifting up on the mag pickup plate, you can slide the vacuum canister out of the distributor body.