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Vacuum Fitting

General engine tech -- Drag Racing to Circle Track

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Vacuum Fitting

Post by Bad67 » Sat Oct 05, 2019 8:13 am

I want to add a vacuum fitting to this manifold (see photo) to supply vacuum for PCV and a transmission vacuum modulator. It is a multiple carb, dual plane manifold that does not have plenum common to all 4 carburetors. However it does have a crossover that connects the center two carbs. I was planning on putting the fitting there. Is that going to causes tuning issues? Neither a PCV nor a modulator would require a huge volume of vacuum so, I am thinking it would be a non-issue. I don't see any other way of doing it. Opinions?
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Re: Vacuum Fitting

Post by Coloradoracer » Sat Oct 05, 2019 8:49 am

You'll need two fittings. You cannot connect a pcv and vacuum accessory together, as the pcv is just a controlled leak...I've tried it in the past and failed miserably. You need a fitting for the pcv and one for the vacuum. How about instead of drilling the intake, adding a 1/2" spacer under the carbs and install the fittings into those? The last intake I used that didn't have a provision for that, so I added a spacer and put the fittings there. Worked like a champ.
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Re: Vacuum Fitting

Post by cardo0 » Sat Oct 05, 2019 3:10 pm

Well I suggest using a vacuum gauge connected to a plate under each carb and measure what kind of vacuum is there. Maybe any carb location will work or the two carb shared plenum will be have enough vacuum. Ya know so many owners complain about their PCV system but rarely does any of them bother to measure the vacuum on each side of the PCV to see what's going on. Problem they have could be many other things like excessive blow by but they don't have the patience to test and find the real problem they have - they prefer to keep changing their PCV and location (heck many of those types don't even know how correctly plumb in a PCV and catch can). Vacuum gauge and hose/fitting cost pocket change.

Now I know a lot of the owners using independent runner EFI induction use tubing to connect each tiny runner of all eight runners and some plumb in a small volume can to steady the vacuum signal. But I would try the under the carb plate first to see if that's adequate.

Hope this can help.
74 corvette: 350 4 speed
94 Z28: Gen II 350 auto

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