Thread: best vacuum ports for dist. advance and trany modulator

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Not arguing - just providing information . . .

From Car Craft Magazine:

Choose Your Vacuum

Where’s the right vacuum port to connect to the vacuum advance, you ask? Most street carbs are equipped with both ported vacuum ports and manifold vacuum ports. Vacuum is generated by the engine’s pistons traveling to the bottom of the cylinders—air is sucked in to fill the voids, which in turn creates a signal to the carburetor to meter fuel. Manifold vacuum is present anytime the throttle blades aren’t fully open, including idle when they’re nearly closed. Ported vacuum isn’t “seen” at all when the engine is idling, because it’s taken from a port above the throttle blades.

Some gearheads have near-religious views on ported versus manifold vacuum. (Ain't that the truth) Patterson connects the vacuum advance to either source, and chooses the “right” port after test-driving both hookups. He does say that full manifold vacuum should help tip-in response, and this setting made Smith’s Mustang feel crisper off-idle. But if your car idles shaky and “nervously” with manifold vacuum advance, it’s a sign that there’s too much initial advance. So give the ported vacuum a try.

From MSD's Chevy Distributor instructions:

Total Timing: This is the total of the initial timing plus the centrifugal advance added together. Example: 10° Initial + 25° centrifugal = 35° Total Timing. (When checking Total timing, disconnect the vacuum canister and plug the vacuum source.) Vacuum Advance: The vacuum advance will advance the timing up to 10° during partial throttle driving (with 15 lbs of vacuum). The vacuum line should be routed to a ported vacuum outlet above the throttle plates.

From Mallory's instructions:

ADJUSTING THE VACUUM ADVANCE
The vacuum chamber is adjusted at the factory to produce 12° of additional advance at steady speeds. This may be too much for some engines. Connect a vacuum line from the vacuum chamber to a ported vacuum source on the carburetor.