In a racing application, the vacuum advance is removed, or in the case of vintage Corvettes, it was never installed in the first place. Typically, a vacuum advance system can advance the total timing by 50 degrees (or more) under certain circumstances. A good example is a car cruising at 65 mph. The increase in vacuum advance can improve the fuel economy (by significant margins) without causing detonation. In a race car, there's little interest in fuel economy-and little or no part-throttle (or high vacuum) operation. The real catch is the moveable breaker plate. With the vacuum advance system hooked up, there is a chance the plate can move, which in turn, can create inconsistent spark timing. The accompanying photos show how the vacuum advance is disconnected.

Total Timing
When the initial timing and the centrifugal advance are added together, you come up with total timing. As an example, if your engine has 12 degrees of initial timing (dialed into the distributor by way of the timing marks on the harmonic damper) and it has another 25 degrees of centrifugal timing, then the total timing is 37 degrees. Some people also factor the vacuum advance into this figure as well. Assuming that the vacuum advance mechanism adds another 12 degrees, you have 49 degrees total advance in the system.

Reworking the Advance Curve
The results of tinkering with the advance curve on any engine can be remarkable. For example, many stock distributors were set up to bring the advance all in at engine speeds of 4,000 rpm or more. Bringing the curve in sooner can result in startling performance improvements. MSD describes the process: "The function of the advance curve is to match the ignition timing to the burning rate of the fuel and the speed (rpm) of the engine. Any factor that changes the burning rate of the fuel or the engine speed can cause a need for an ignition timing change."

MSD also offers the following tips on selecting an advance curve:
Use as much initial advance as possible without encountering excessive starter load or engine kickback.
Start the centrifugal advance just above the idle rpm.
The starting point of the centrifugal advance curve is controlled by the installed length and tension of the spring.
How quickly the centrifugal advance (slope) comes in is controlled by the spring stiffness.
The stiffer the spring, the slower the advance curve.
The amount of advance is controlled by the advance bushing. The bigger the bushing, the smaller the amount of advance.

Here are a couple of extra curve tips I've come up with over the years:
Automatic transmission cars almost always need a quicker, shorter curve than stick shift cars; however, total timing should still be the same.
Automatic transmission cars use more initial advance than stick shift cars.
By using a separate starter and ignition switch, you can overcome adverse starter load by spinning the engine first, then clicking on the ignition switch.

The very best re-curves are those that use the car as a test bed. All that is required is a degreed damper (or a timing tape), a timing light, a reliable tach, a note pad, and a bit of patience. The main idea when re-curving a distributor is to bring the curve in as quickly as possible without the engine detonating. In other words, play with the springs until you reach the optimum curve for your application. Some cars may require one very light spring and a heavy spring, certain combinations will require a pair of medium springs, while others can get away with a pair of light springs. I've even seen Corvette Delco applications that required just one spring. The other weight was used "springless."