Total advance

[Doug Flaten]

Re: Total advance

Well I am under the impression that the port is usually very near the closed plate position. (Again I may be wrong and getting it confused with the idle circuit ports) Anyway, I thought the plate lifts above the port at anything much above the idle. If that is the case, they would essentially be on the same side of the plate and the only difference between Manifold vac and any point upstream would be a very small amount of friction. Once the plates are openned a certain amount, I don't think there would be much of a localized venturi effect over the port. As the edge of the plate moves away from the wall of the bore, the venturi or choking effect of the throttle is going to decrease exponentially. Even if the port is well above the plate, it would differ from the Manifold vac by only a small amount of friction. But I am just am just an oil patch engineer. I am waiting to hear the rest of the story from one of our resident automotive engineers.

[John H.]

Re: Total advance

The manifold vacuum port is just below the edge of the closed throttle plate, and on a "ported" vacuum source carburetor, it's just above the edge of the closed throttle plate; that's as complex as it gets. Once the throttle plate on a "ported" source carburetor is opened off-idle, the port sees full manifold vacuum.

[Doug Flaten]

Re: Total advance

Thanks John,
I have seen posts on other boards where they had the belief that ported vac provided higher vac than manifold vac. I kept arguing that a point further away from the source could not have higher vac than the source. Anyway, I was trying to come up with a scenario where it could possible to get a very localized effect due to the edge of the plate. Kind of like downstream of an orifice in a gas meter run.