MSD Ignition Advance Curve - Corvette Curves

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
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).

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."

As mentioned earlier, some engines will "like" more initial timing than others, while some combinations will want more total timing. In any case, you can adjust where the advance starts, the rate of advance (slope), as well as the total amount of advance. It will take some time to sort through the timing maze; just be certain that you record all changes. Those notes will be a valuable guide when setting up the curve for your particular combination.