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I am cobbling a Mitsu? throttle body to an EA81 inlet manifold. The TB has two vacuum? port brass tubes to connect stuff up to. I understand manifold vacuum and what and when it works and can be used for (at times of study, never remember though ]. These two tubes port through to above the throttle blade (both of them!) side by side almost.

I recall old Fords of late 60's early 70's Stromberg BV2 single barrel had a vacuum source above OR below throttle plate depending on manual or auto, one was above the other below, neither had both.

Anyone able to sprout the pros and cons of above butterfly, below butterfly and manifold vacuum and the differences of, uses of etc?

ta

i know it sucks

lmao

dibs

Steptoe I think you may need to do a search on "ported vacuum" .

As I understand it it works like this .

A port drilling on the atmosphere won't do anything while the throttle is closed , with the throttle partially open the air speed past it will be high and what happens at the drilling depends upon the localised pressure at its mouth . With the throttle valve open in I imaging the part throttle cruise position I'd expect the pressure there to be lower than atmospheric or to use the very unscientific term - a partial vacuum.

On limited time here but .
A total vacuum is absolute zero or 0 psia - a for absolute . You need to understand that 0 psig - g for gauge means atmospheric pressure at sea level . You can't have a negative pressure because absolute zero is as low as its possible to get . Anything above absolute zero is positive pressure including atmospheric pressure at sea level ie ~ 14.7 psia/100 kpa/1 Bar/1 atmosphere .

If your drilling is downstream of the throttle plate then at closed throttle the pressure will be lower than atmospheric and rise progressively as the throttle opens towards atmospheric pressure or above it if the engine has some form of forced induction .

The uses for a manifold vacuum signal are many and the location of the hole is there to fine tune what the manufacturer needs to device to see pressure wise . Good examples are your fuel tanks carbon canister purge valve/s and the EGR valves actuator . The manufacturer needs to have a low enough pressure signal at the sorts of engine revs that they need the EGR valve to open so experiment with the drilling's location till it works reliably .
Early MPFI dizzy actuator supply is another , if you check your WSM you will see there are two drilling's (one ahead of the other) for the dizzy signal supply . Obviously one hole didn't work over a wide enough throttle range so they played about until they got it close enough .

Cheers A .

Thanks Adrian, and like Dibs says, it sucks so it wll be a case of suck it and see

An update. It does help if things are clean. I now discover the port labeled in casting as E has a hole above and below the throttle plate. Port D just has one hole above the plate.

Has anybody experience in identifying these ports as D and E ? Think it is Mitsubishi