The results of tinkering with the advance curve on any engine (race or street) can be remarkable. For example, many stock production line Delco 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. Refer to the previous chart for some of the factors that will affect engine timing."
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 we've discovered over the years:* Automatic transmission cars (particularly foot brake, non-trans brake applications) need a quicker-but-shorter curve than stick shift cars, however, the total timing should still be the same.* Automatic transmission cars (particularly foot brake, non-trans brake applications) can 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.
So far so good. The very best recurves are those that use your vehicle 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 recurving a distributor is to bring the curve in as quick 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; others can get away with a pair of light springs. We've even seen Delco applications that required but one spring. The other weight was used "springless."
When experimenting with the curve, set the initial timing and make a note of it. Then record the spring combination in the distributor. Increase the engine speed, and log the speed at which the curve begins along with the speed at which the curve ends (where the curve is "all in"). You can also graph the results by checking the timing at 200-rpm intervals (correlating the advance shown on the harmonic damper to the engine speed). Test the results and begin again. Trial and error plays a major role in the selection of a proper timing curve.
What you have to obtain is good throttle response along with detonation-free timing. As mentioned earlier, some powerplants 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. Take the time to sort through the timing maze and be certain that you record all changes in the notebook. It will become a valuable guide when setting up the curve for your particular combination.
As you can see, in the world of ignition timing is not perfect. Even when Chevys are seemingly equal, they might need a different curve. It's all a matter of trial and error.
For a closer look at the mechanics of ignition advance curves, check out the following photos.