Stupid question: Indicated NA vacuum at WOT

[Meatball]

Can anyone with an NA coyote tell me what their vacuum gauge reads at wot? Right at 0 or how much below?

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[K4fxd]

It should read slightly above zero.

 

[Meatball]

It should read slightly above zero.

I think zero is engine off (atmospheric pressure), so it should read slightly below I would think. Just curious how much below. Might drop as the engine revs up…

 

[Cory S]

Vacuum/pressure is zero at WOT during NA operation.

 

[Meatball]

Vacuum/pressure is zero at WOT during NA operation.

Seems like it should be.

but the cylinders are drawing in air so creating a vacuum in the intake system, so it has to be something under ambient pressure. I’m guessing that a larger draw and more restrictive intake tract (and maybe higher rpm) would cause an even higher vacuum, which is the main reason I’m asking. You’d think a large diameter intake/filter/manifold plenum cross-section would show less restriction by greater (but still under atmospheric) pressure.

im curious if anybody with an n/a coyote has paid attention to their vacuum gauge at WOT and remembers the reading…how far below zero.

 

[ice445]

Seems like it should be.

but the cylinders are drawing in air so creating a vacuum in the intake system, so it has to be something under ambient pressure. I’m guessing that a larger draw and more restrictive intake tract (and maybe higher rpm) would cause an even higher vacuum, which is the main reason I’m asking. You’d think a large diameter intake/filter/manifold plenum cross-section would show less restriction by greater (but still under atmospheric) pressure.

im curious if anybody with an n/a coyote has paid attention to their vacuum gauge at WOT and remembers the reading…how far below zero.

As far as I know, vacuum is higher when there's more restriction, like at idle. WOT with everything opened up is close to atmospheric, so near 0 vacuum.

 

[markmurfie]

~ 1inHg or 0.5PSI near 1750ft elevation.

 

[W.O.T. Stang]

Seems like it should be.

but the cylinders are drawing in air so creating a vacuum in the intake system, so it has to be something under ambient pressure. I’m guessing that a larger draw and more restrictive intake tract (and maybe higher rpm) would cause an even higher vacuum, which is the main reason I’m asking. You’d think a large diameter intake/filter/manifold plenum cross-section would show less restriction by greater (but still under atmospheric) pressure.

im curious if anybody with an n/a coyote has paid attention to their vacuum gauge at WOT and remembers the reading…how far below zero.

The digital dash reads as low as 0.8 inHG during full throttle and the gauge sits *just* above 0 on mine.

 

[Meatball]

~ 1inHg or 0.5PSI near 1750ft elevation.

Thank you!!!!

 

[Meatball]

The digital dash reads as low as 0.8 inHG during full throttle and the gauge sits *just* above 0 on mine.

Thanks!!

 

[NASteve]

On a N/A application anything above 0 means there is a restriction somewhere in the system.

There is a trick to determine if you have a restriction and it works for EFI and carbs, just plumb a vacuum hose into the manifold off one of the ports, run the hose to a gauge tapped to the windshield and then do a WOT pull, if it reads 0 then you are good, if not then you have a restriction somewhere. It seems ghetto, but it is the simplest form of testing, also helps determine if you gauge is accurate.

It should never read less than 0 since during a WOT pull the because atmospheric pressure and manifold pressure should be equal. BTW there is no such thing as vacuum, just pressure less than atmospheric.

 

[WildHorse]

.7 psi at WOT. 900ft asl.

 

[drive_55_not]

I've got a few in car video shots of the vacumm gauge from my 2018 Gen3, Not sure how accurate it was but it read approx, 5in vac WOT in every vid I looked at.

 

[EFI]

The closer to 0 the better. Anything above that means you have some sort of restriction.

But...only the very best setup N/A engines will get near (or slightly below) 0. A stock mass produced engine won't be getting near 0.

 

[WildHorse]

A stock mass produced engine won't be getting near 0.

Maybe a 70's / 80's V8 won't, or any low compression engine for that matter.
My 575 hp 440 magnum never seen over 90% VE with 9.5:1 cr. WHen we bumped it up to 13.5:1 and it made 606 it was 100-101% VE.

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