Total timing?

PKRWUD · 12-06-2002, 10:28 PM

Actually, as I read that, I don't think you have a clue about PORTED vacuum (the vacuum source for your vacuum advance).

Manifold vacuum is what exists underneath your throttle plates. When they are closed, manifold vacuum increases. If your vacuum advance was hooked up to this source of vaccum, you would be fully advanced at idle, and the timing would retard as you accelerated. This won't work. So, the engineers needed to figure out a way of having the opposite effect, but that meant creating vacuum as the throttle opened, and lessening it as the throttle closed. Quite a paradox. Until someone realized that the transfer slots, the grooves that are cut into the venturi above and below the throttle plate, worked on a similar principal. Rather than depending on the vacuum created when the throttle plates are closed, they used the vacuum created by the air rushing by a small port at the narrowest part of the venturi. The faster the air would rush by, the more vacuum would be created. So, at closed throttle, there would be no PORTED vacuum, whereas the farther the throttle was opened, the more PORTED vacuum there would be. It's just like a minature fuel feed, except it goes to the ported vacuum port instead of the bowl.

Get it?

Take care,
~Chris

12-06-2002, 10:28 PM	#11
PKRWUD Junior Member Join Date: Mar 1999 Location: Ventura, California Posts: 8,981	Actually, as I read that, I don't think you have a clue about PORTED vacuum (the vacuum source for your vacuum advance). Manifold vacuum is what exists underneath your throttle plates. When they are closed, manifold vacuum increases. If your vacuum advance was hooked up to this source of vaccum, you would be fully advanced at idle, and the timing would retard as you accelerated. This won't work. So, the engineers needed to figure out a way of having the opposite effect, but that meant creating vacuum as the throttle opened, and lessening it as the throttle closed. Quite a paradox. Until someone realized that the transfer slots, the grooves that are cut into the venturi above and below the throttle plate, worked on a similar principal. Rather than depending on the vacuum created when the throttle plates are closed, they used the vacuum created by the air rushing by a small port at the narrowest part of the venturi. The faster the air would rush by, the more vacuum would be created. So, at closed throttle, there would be no PORTED vacuum, whereas the farther the throttle was opened, the more PORTED vacuum there would be. It's just like a minature fuel feed, except it goes to the ported vacuum port instead of the bowl. Get it? Take care, ~Chris __________________ Webmaster: Rice Haters Club Jim Porter Racing Peckerwoods Pit Stop Support Your Local RED & WHITE!

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