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Again, this is wrong information. At wide open throttle regardless if it is ported or manifold vacuum there will be no vacuum or very little vacuum. The ported vacuum is there for just idle or very close to idle conditions. Once you are on the road there is very little difference between the two. I still think that using ported vacuum is better for more situation as long as the distributor advance slots have been shortened to allow much more mechanical advance then is the norm on the stock dist.
Ported vacuum is shut off at idle and the same as manifold vacuum as soon as the throttle plates start opening.
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As I said, ported vac is generated by the flow of air thru the venturi, the higher the flow, the more vac. That is exactly how the end carbs on six packs and the secondaries on Vac secondary carbs are able to open at high/full throttle. If they used manifold vac, they would never open. That GM guy gives a good explanation, it has always been a confusing subject.
No. Ported vacuum does not come from the venturi. It comes from right above the throttle plates so that as soon as you come off idle it is the same as manifold vacuum. You are correct about the vacuum source for opening a sixpack or for that matter any vacuum secondary Holley carb is from the venturi. Two entirely different things.
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The ONLY difference between "ported" and manifold is the ported passage is just above the throttle blades. When the throttle is closed it closes the port = no vacuum = no vacuum advance at idle. As soon as you crack the throttle past the port it then sees manifold vacuum. This was an emissions era invention.
The manifold vacuum port passage is below the blades so it sees vacuum or the lack of it all the time.
This is correct.
Last edited by GomangoCuda; 08/07/11 12:42 PM.
In theory, there is no difference between theory and practice. In practice, there is.
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