I’ve always thought that vacuum advance was something just for stock engines and something you didn’t use with a performance engine. In fact, I’ve heard guys say they’ve locked out their advance and just run a fixed timing amount. Is this a better way to go with a street engine? I have a small-block 350 Chevy in a ’67 Chevy pickup with a TH350 trans and 3.73:1 gears. It’s a nice little cruiser and not really a hot performance engine. Right now the engine has an Edelbrock Performer intake, a 600 cfm Holley four barrel carb, and an HEI distributor. What kind of timing should I be running and should I disconnect the vacuum advance?
Jeff Smith: You’ve addressed several issues, but they are all related to ignition timing. First, on a street-driven engine there is no good reason to lock out your ignition timing. If we had to use a generic maximum advance that seems to work for most pump gas engines, somewhere around 34 to 36 degrees before top dead center (BTDC) is a good number. But with this much initial timing in the engine it will be difficult for the starter motor to crank the engine. You can get around this with an ignition cut-out so that the engine cranks and then you hit the ignition circuit, but this is unnecessarily complex for a street car.
The better plan is to start with a decent initial timing of roughly 10 to 14 degrees. This can be checked with the engine running at idle with a timing light. Make sure the vacuum advance connection is removed, and now rev the engine up to around 2,500 to 2,800 rpm. Ideally, the timing now should be somewhere around 34 to 36 degrees to total advance. With 34 degrees of total mechanical advance and 14 degrees initial timing, you have 20 degrees of mechanical advance—14 + 20 = 34.
Now connect the vacuum advance from the carburetor to the distributor and read the timing on the harmonic balancer while revving the engine to 2,800 rpm. This number will now be greater than 34 degrees with the addition of the vacuum advance. Let’s say it now reads 49 degrees, which would mean the vacuum advance is worth another 15 degrees. These are typical numbers.
First, let’s look at the mechanical advance portion of the timing—the 34 degrees total. This is determined by the weights and springs spinning around inside the distributor. This establishes the amount of timing the engine will see at wide open throttle (WOT). As you are probably aware, at part throttle the engine will create a certain amount of vacuum in the intake manifold. This is because the throttle is mostly closed and the engine is pulling against this restriction. Because the throttle is mostly closed, very little air is moving into the cylinders. So at light cruise, such as running down the highway, the engine is making much less power than it would at this same rpm at WOT.
With less air and fuel in each cylinder, the air-fuel mixture is not as densely packed compared to WOT. This less-dense mixture requires more ignition timing to complete the combustion because it takes longer to complete the combustion process. So we need a way to increase the amount of timing based on the load on the engine. This is how vacuum advance works. At part throttle, high manifold vacuum moves the diaphragm in the vacuum advance canister on the distributor to add more timing. But at WOT, the vacuum drops to near zero and vacuum advance is removed and the total timing then is established by the initial plus the mechanical advance.
So there are significant advantages to retaining the vacuum advance on your distributor. In the case of the HEI, you can actually purchase an adjustable vacuum advance canister that will allow you to custom-tune the amount of timing advance. For example, Pertronix offers an adjustable vacuum advance canister that bolts in place of the stock canister. Using a 7/32-inch Allen wrench placed in the vacuum nipple, you can change the amount of advance. Generally, the canister comes adjusted with roughly half of the total advance possible. The canister easily installs with two screws and then you can drive your truck with the unit adjusted as delivered. Each clockwise turn of the Allen wrench will add about 1.5 degrees of additional advance above 5 to 7 inches of manifold vacuum. The maximum is about 14 degrees of vacuum advance.
If too much advance is added, the engine will either start to knock or ping or perhaps it may surge slightly at very light throttle opening with high vacuum. If so, back the adjustment (counterclockwise) one to three turns and you will be very close to the ideal timing at part throttle. Once tuned properly, you may notice the engine does not require as much throttle to cruise at the same speed. This tuning should produce slightly better fuel mileage—assuming you can keep your foot out of the throttle now that it runs better!
So based on this, you can see that having a curve in the distributor along with vacuum advance is a good thing as it reduces low-speed timing where the engine might detonate with too much timing but it can also benefit from more timing at part throttle than what is required at wide open throttle.