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MSD's Electronic Ignition System - Going Digital

A new wave in high-tech ignition systems

By Wayne Scraba

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 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 following chart from MSD:

If you take a look at this 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 that there is no one perfect curve. Each engine will be different and consequently, each curve will be different.

Generally speaking, there are two types of advance curves commonly used in a distributor; centrifugal and vacuum. OEM Chevy distributors and aftermarket distributors used on street cars usually incorporate both systems. 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. Centrifugal force moves the weights outward against the tension of the springs. This causes 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 Chevy 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 obviously, 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-to-late-'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, 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 dedicated racing application, the first thing that gets "modified" is the vacuum advance. It gets removed. 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 on a MSD distributor.

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.

By Wayne Scraba
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