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Reworking a Stock Distributor to Trigger MSD Ignition Systems Part 3
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We’re back with our series on reworking a stock point style distributor for use as a trigger for an MSD ignition system. This time around, we’ll look at the vacuum advance system in the distributor, along with other details. The vacuum advance system is very important and as a result, much of the tech provided below will be devoted to it. If you have a drag race only car, you can skip over the vacuum advance details. You can easily run without it; the reason is, your racecar spends most of its time running at wide-open throttle (zero manifold vacuum conditions). But if your car sees any street use at all, vacuum advance is particularly important.
Let’s go back and look at the basics of centrifugal advance: With centrifugal advance, spark timing functions with engine speed. Engine load isn’t taken into consideration by the system. As pointed out in the last article in this series, the amount of advance is determined by the weights and springs in the distributor.
Keep this in mind: Vacuum advance really has nothing to do with total timing at wide-open throttle. When you whack the throttle on your car, intake manifold vacuum drops to what is likely zero. The vacuum advance system has zero vacuum to function, so it doesn't. However, when the engine is idling, it needs more spark advance in order to fire the normally lean air-fuel mix condition. With the vacuum advance connected to manifold vacuum (where it should be connected in a performance car – ported vacuum was invented for use on smog applications and has no place in high performance), the stock vacuum advance system adds approximately 15 degrees to the initial timing (keeping in mind the centrifugal advance isn’t adding anything at idle). The same applies when you’re cruising down the road in high gear, except the vacuum advance adds timing to both the initial and the centrifugal. Here, you might end up with 50 degrees or so of total advance timing. While cruising at a constant speed, the engine is most likely lean. The extra advance provided by the vacuum advance system helps to burn the lean mixture.
There’s more to it: Let’s say you’re accelerating. Here, the air fuel mixture is enriched by the accelerator pump as well as the power valve. The rich mixture burns quicker and because of this, it doesn’t require additional spark advance. When you hit the throttle, the manifold vacuum drops and the vacuum advance goes to zero. The distributor reverts to the mechanical advance timing along with the initial advance timing only. What this really does is artificially retard the timing. Back off the throttle, and the carburetor leans out and vacuum advance system comes back into play, effectively adding timing to light the lean mixture. It’s a sound system and works incredibly well.
As you can see, the vacuum advance system effectively responds to engine load. It provides the correct degree of spark advance to deal with both rich and lean carburetor mixtures. Sure, it’s no computer controlled spark system, but it does a really good job of optimizing throttle response and fuel economy and improving cooling at idle. It also has no effect (nada – zero – nothing) on performance when the throttle is wide open. Keep that in mind.
Obviously, being able to fine-tune the vacuum advance system offers considerable benefits. With ACCEL’s adjustable vacuum advance system, you simply start by backing the advance off by turning the supplied Allen wrench counter-clockwise. ACCEL suggests you adjust the vacuum advance mechanism in two-turn increments until the car experiences a surge or a ping at cruise speeds. Typically (and dependent upon the amount of vacuum produced by the engine), one turn from counter-clockwise provides approximately 2 degrees of advance, while six turns from counter-clockwise provides approximately 18 degrees of advance.
For a closer look at the vacuum advance canister installation along with the point installation as well as the timing gear installation, check out the accompanying photos:
We’re back with our series on reworking a stock point style distributor for use as a trigger for an MSD ignition system. This time around, we’ll look at the vacuum advance system in the distributor, along with other details. The vacuum advance system is very important and as a result, much of the tech provided below will be devoted to it. If you have a drag race only car, you can skip over the vacuum advance details. You can easily run without it; the reason is, your racecar spends most of its time running at wide-open throttle (zero manifold vacuum conditions). But if your car sees any street use at all, vacuum advance is particularly important.
Let’s go back and look at the basics of centrifugal advance: With centrifugal advance, spark timing functions with engine speed. Engine load isn’t taken into consideration by the system. As pointed out in the last article in this series, the amount of advance is determined by the weights and springs in the distributor.
Keep this in mind: Vacuum advance really has nothing to do with total timing at wide-open throttle. When you whack the throttle on your car, intake manifold vacuum drops to what is likely zero. The vacuum advance system has zero vacuum to function, so it doesn't. However, when the engine is idling, it needs more spark advance in order to fire the normally lean air-fuel mix condition. With the vacuum advance connected to manifold vacuum (where it should be connected in a performance car – ported vacuum was invented for use on smog applications and has no place in high performance), the stock vacuum advance system adds approximately 15 degrees to the initial timing (keeping in mind the centrifugal advance isn’t adding anything at idle). The same applies when you’re cruising down the road in high gear, except the vacuum advance adds timing to both the initial and the centrifugal. Here, you might end up with 50 degrees or so of total advance timing. While cruising at a constant speed, the engine is most likely lean. The extra advance provided by the vacuum advance system helps to burn the lean mixture.
There’s more to it: Let’s say you’re accelerating. Here, the air fuel mixture is enriched by the accelerator pump as well as the power valve. The rich mixture burns quicker and because of this, it doesn’t require additional spark advance. When you hit the throttle, the manifold vacuum drops and the vacuum advance goes to zero. The distributor reverts to the mechanical advance timing along with the initial advance timing only. What this really does is artificially retard the timing. Back off the throttle, and the carburetor leans out and vacuum advance system comes back into play, effectively adding timing to light the lean mixture. It’s a sound system and works incredibly well.
As you can see, the vacuum advance system effectively responds to engine load. It provides the correct degree of spark advance to deal with both rich and lean carburetor mixtures. Sure, it’s no computer controlled spark system, but it does a really good job of optimizing throttle response and fuel economy and improving cooling at idle. It also has no effect (nada – zero – nothing) on performance when the throttle is wide open. Keep that in mind.
Obviously, being able to fine-tune the vacuum advance system offers considerable benefits. With ACCEL’s adjustable vacuum advance system, you simply start by backing the advance off by turning the supplied Allen wrench counter-clockwise. ACCEL suggests you adjust the vacuum advance mechanism in two-turn increments until the car experiences a surge or a ping at cruise speeds. Typically (and dependent upon the amount of vacuum produced by the engine), one turn from counter-clockwise provides approximately 2 degrees of advance, while six turns from counter-clockwise provides approximately 18 degrees of advance.
For a closer look at the vacuum advance canister installation along with the point installation as well as the timing gear installation, check out the accompanying photos:
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