Author Topic: Ported vs Manifold Vacuum Advance  (Read 417 times)

Offline Cliff Ruggles

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Ported vs Manifold Vacuum Advance
« on: November 22, 2019, 04:33:18 AM »
OK, time for an education. Some controversy has come up about ported vs manifold vacuum advance. For some reason to this day some folks believe that a ported vacuum source will apply MORE vacuum than a manifold source.

They also believe that it applies MORE vacuum with increased throttle angles and at heavy and WOT throttle.

What really happens is that manifold vacuum applies what vacuum is available under the throttle plates at idle or any scenario when the primary throttle plates are fully closed, ported does not.

Right off idle thru the entire load/speed range they provide the same signal IF the ported source is correctly located just a few thousands above the closed throttle plates where it should be.

IF the ported source has a higher location it will NOT provide engine vacuum as soon nor as much of it, and it will actually fall off quicker/sooner as throttle angle is increased.

The pics below will show you why this happens.

The first pic is two baseplates showing both source locations and they are marked P for Ported and M for manifold. It is important to note how close together the two sources actually are and try to visualize at heavy and full throttle how one could possible have any different vacuum available to it than the other.

The second pic shows a baseplate with the throttle plates closed and location for the Ported vacuum advance slot. Notice that it is just a few thousands above the plates full closed. At idle the throttle plate will actually be just at it's lower edge and it will get uncovered with the slightest movement of the throttle and provide full manifold vacuum to the vacuum advance. This source location is EXTREMELY important as the engineers knew how beneficial manifold vacuum was to engine efficiency and wanted FULL vacuum in all driving scenarios but no vacuum at idle due to emissions for certain engines.

The next pic is a baseplate with ported vacuum designed for EGR. Notice that it has a slightly higher location as the engineers did NOT want a strong signal right off idle, so some delay. They also did NOT want full vacuum to the EGR or a weaker signal so it didn't cause a lean condition and engine running issues by putting too much inert matter into the combustion space at light engine load. So basically (and this goes against the folks who believe ported vacuum INCREASES with increased throttle angle) it will not provide as much vacuum as soon (as the source designed for the vacuum advance) nor will provide as high a reading on a vacuum gauge if you are hooked to both and reading them at the same time while throttle angle is increased.

It will also show the vacuum reading falling off sooner with increased throttle angle compared to the correctly located ported vacuum source. Yes, another important fact to get your head wrapped around, vacuum will fall off sooner with a higher source location, not continue to increase as some folks have tried to indicate.

As throttle angle and engine load continue to increase all three sources fall off at approximately the same rate and will go to zero or very close to it at WOT.

If there is some really high located source in a carburetor of any manufacturer used to provide ported vacuum to the advance which is located up in the venturi areas and it will continue to INCREASE vacuum at heavy and full throttle I have never seen one.

Continued...

Offline Cliff Ruggles

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Re: Ported vs Manifold Vacuum Advance
« Reply #1 on: November 22, 2019, 04:33:34 AM »

Anyhow, I've rebuild well over 10,000 carburetors in the past 20 years or so, of all types for these engines and all of them to date had vacuum source locations in the baseplate or bottom of the carb pretty much as shown in the pics below. If the factory wanted manifold vacuum to the VA on that engine the source will be found below the plates. If they wanted ported vacuum to the VA it will be very precisely located just above them when fully closed.

In actual use the ONLY difference the VA sees is it is adding timing idle and coasting with manifold vacuum. In all other driving scenarios off idle they both do the same thing........Cliff