Recurving Your Distributor for Power

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 be able to light the fire. Kaboom. Job done. But this isn't a perfect world, and neither is the case of ignition advance curves.

Why Spark Advance Is Necessary

Let's look at why advance curves are required in the first place. Spark is most often introduced into the cylinder prior to the piston reaching TDC. This simply gives the spark sufficient time to light the air-fuel mixture. As engine speed increases, then 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, however, where you can dial-in too much advance into the ignition system. Too much advance and the engine will detonate. This is caused when the "explosion" in the engine is timed too early.

Initial Advance

Initial advance is the base timing that is 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 the altitude increases so does the need for additional spark advance. Why? More advance helps to compensate for the lack of oxygen. There are other factors as well. Check out the chart from MSD:

If you take a look at the chart, you can see that the variables can change throughout the range of the 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 there is no one perfect curve. Each engine will be different and, consequently, 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 catch are "HD" Corvettes such as the L88 and ZL1 - they 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. This causes 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. The way they operate is different. 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 and 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. As a result, there is no chance of detonation (or pinging).In the mid-'60s, vacuum advance mechanisms changed to suit emission requirements. The vacuum source was changed from the manifold to the carburetor venturi. This is called "spark ported vacuum." Spark ported vacuum is lowest at idle, and then increases as the throttle is opened. This is completely opposite to 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).

In a racing application, the first thing that gets modified is the vacuum advance. It gets removed, or in the case of vintage Chevy HD Corvettes, never installed in the first place. Typically, a vacuum advance system can increase the total timing to 50-degrees (or more) advanced 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 knocking or pinging. In a race car, there's little interest in fuel economy and, besides, there is 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. In the accompanying photos, we'll show you 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 (dialed into the distributor by way of the timing marks on the harmonic damper), and it has another 25 degrees of timing in the centrifugal, 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 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 production line distributors were setup 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, but shorter, curve than stick shift cars, however, the 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.

So far so good. The very best recurves are those that use the car as a testbed. All that is required is a degreed damper (or a timing tape), a timing light, a reliable tach, a notepad, and a bit of patience. The main idea when recurving 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 but one spring. The other weight was used "springless."

When playing with the curve, set the initial. Make a note of the initial timing. Then make a note of the spring combination in the distributor. Increase the engine speed and make a note of 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 curve. What you have to obtain is good throttle response along with detonation free timing. 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. Take the time to sort through the timing maze and be certain 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, it's not a perfect world. Even when cars are seemingly equal, they might need a different curve. It's all a matter of trial and error.