If anyone here is learning "How to tune 101" at my expense I have cut and pasted why I need vacuum advance as these guys are helping me with:
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.
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