Thread: 78 Trans Am getting some love

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Lol. The kit is just a adjustable vacuum advance kit from MSD. Base timing is with the vacuum advance disconnected, I plug it in, it goes to 24 which I can adjust with the adjustable vacuum canister. my biggest thing is with the mechanical weights at the end. I can swap out springs to speed it up or slow it down but how do you actually change the timing to advance it more or retard it more, If I turn the distributor to adjust it, it throws the base timing out of whack.Another thing I do not quite understand is manifold vacuum versus ported vacuum.If I use ported vacuum, wouldn't that completely make the mechanical weights obsolete because the vacuum increases as you rev the engine up therefore advancing the timing?. I feel like I'm 15 again lol

I’m not sure if I understand your other question. Turning the distributer is how you adjust the base timing. Changing springs will affect at what RPM will the mechanical (centrifugal) come in. You probably want all of the centrifugal advance in by about 2500 to 3000 RPM.

Originally Posted by 53 Chevy5

If I use ported vacuum, wouldn't that completely make the mechanical weights obsolete because the vacuum increases as you rev the engine up therefore advancing the timing?

No, vacuum advance wether manifold or ported sourced will not effect your mechanical, they are two separate things.

And, the only difference between manifold and ported is when the vacuum advance starts. Manifold is at idle through cruise rpm, ported is off at idle and comes on at cruise rpm. Ported is also known as TIMED because it comes on later.

Simply put, vacuum advance is an economizer, it has nothing to do with performance. In theory, at idle and cruise speeds the motor can use more advance to burn leaner mixtures. However, I'm a big believer in giving the motor what the motor wants.

If the advance at idle brings the idle speed up too high, plug it into ported, you'll still have it at cruise for economy. If it idles good on manifold great, leave it there.