Vacuum increases as rpm increases

[lil corb]

Something I haven't seen before with any turbo car. Finally got my car started and still working out the bugs (idle issues, waiting on Link). But anyways, we noticed something really strange with my boost gauge (Old school Stewart warner) At idle, if you give the engine some rpms the vacuum increases but when the car is driven the vacuum acts in a correct manor. (vacuum decreases as more throttle is given)

I removed a lot of vacuum lines and boost gauge is a mechanical one with source T'eed into FPR vacuum line leading to Intake manifold. All the lines look good including boost line.

BTW. before this major work done (about 3 years ago ) the gauge was working good.

Is it possible gauge itself is going bad? Everything works with it including power, lights etc... Haven't hit boost yet as I'm still fixing bugs.

Any ideas?

Thanks

 

[Bud92gsx]

Ideling should have vaccume. But as the throttle is applied, you should see the vaccume going away and then into turbo psi.

 

[lil corb]

Ideling should have vaccume. But as the throttle is applied, you should see the vaccume going away and then into turbo psi.

It does have vacuum Bud, what I mean is normally as you rev the vacuum decreases (going closer to 0 then increasing in positive pressure when the turbo starts to spool)

Mine goes further into vacuum (from 14in-hg to 20 in-hg etc...)

Which is not normal.

 

[Bud92gsx]

That is correct and the last part confusing at the same time...


It should be decreasing when throttle is applied, not increasing..

 

[lil corb]

That is correct and the last part confusing at the same time...


It should be decreasing when throttle is applied, not increasing..

That's exactly what I am saying. It is not doing what it is supposed to do. I am trying to see if anyone has had the issue or seen it before? not how the gauge works, I should know that after all the work I've done.

Any ideas?

 

[Bud92gsx]

My boost gauge sounds like it's tee'd into the same line.Unfortunatly mine doesn't even read-, only positive pressure.

Well I can tell you something is fishy, but Unfortunatly,I don't have an answer, so This is where I take a bow and only moniter the thread from here...

 

[ChiTownHussla]

Vacuum at idle should be 20 -21 to begin with not 14

 

[TrevorS]

Something I haven't seen before with any turbo car. Finally got my car started and still working out the bugs (idle issues, waiting on Link). But anyways, we noticed something really strange with my boost gauge (Old school Stewart warner) At idle, if you give the engine some rpms the vacuum increases but when the car is driven the vacuum acts in a correct manor. (vacuum decreases as more throttle is given)

My best guess is you have a vacuum leak. What is the actual vacuum reading at idle (and is the needle stable or moving/jumping around)?

http://imperialclub.com/Repair/Fuel/VacuumGauge/index.htm

 

[lil corb]

My best guess is you have a vacuum leak. What is the actual vacuum reading at idle (and is the needle stable or moving/jumping around)?

Intake Manifold Vacuum Tests

I have Fp2 272 cams and at a steady idle it is vacuum is -14 in hg. Which is normal from what I research with 272. Just when I give it throttle vacuum increases rather than decreases.

Would a vacuum leak cause this? which would make sense as to why I am having idle issues. I can understand idle surges and unstable idling do to vacuum leaks but making the vacuum increase with revs is a first.

any other thoughts?

Thanks

 

[olddsmer]

Your gage is bad. You seem to have it hooked up correctly. Your vacuum is in line at idle with the cam profile you have. Switch gages with one of your buddies and see if it doesn't work correctly.

Regards
Greg

 

[simple jake]

Are you stabbing the throttle or lightly pressing it and slowly increasing rpms? Because the same exact thing happens to my gauge when I slowly increase the rpms. My vacuum increases from 19 inHg to 22 inHg when I slowly press the throttle. I'd assume that's normal.

 

[92nolatist]

Same thing here, if the throttle is slowly pressed, the vacuum will increase. Probably because there is no load on the engine.

 

[BurrowZ]

OP what you are experiencing is normal. It seems your vac is low, probably due to the cams but you COULD have a leak.

Your gauge is fine.

 

[92awddsm]

First off, blt your car and eliminate any possibility of a vac leak. If you are increasing rpms slowly(part throttle) vac will rise. If you go wot, it will should drop to 0 on a free rev.

And a vac leak WILL NOT make vac rise when you rev it.

 

[TrevorS]

First off, blt your car and eliminate any possibility of a vac leak. If you are increasing rpms slowly(part throttle) vac will rise. If you go wot, it will should drop to 0 on a free rev.

And a vac leak WILL NOT make vac rise when you rev it.

Perhaps I'm confused, but I question your statement "a vac leak WILL NOT make vac rise when you rev it". A small fixed idle vac leak will be offset by modest increase in air flow as the throttle is gradually increased, resulting in an apparent increase in vacuum.

 

[92awddsm]

Any leak aft throttle plate will not cause an increase in overall engine vacuum. if it is pulling vacuum, a leak will only decrease vacuum regardless of engine rpm or throttle opening position. Vacuum is nothing more than a measurement of restriction in the intake track. A leak is a void in the restriction.

 

[Alex7747]

Perhaps I'm confused, but I question your statement "a vac leak WILL NOT make vac rise when you rev it". A small fixed idle vac leak will be offset by modest increase in air flow as the throttle is gradually increased, resulting in an apparent increase in vacuum.

Nope you got it backwards, if there is a vacuum leak at idle it will either stay the same or go down.

 

[TrevorS]

Any leak aft throttle plate will not cause an increase in overall engine vacuum. if it is pulling vacuum, a leak will only decrease vacuum regardless of engine rpm or throttle opening position. Vacuum is nothing more than a measurement of restriction in the intake track. A leak is a void in the restriction.

Nope you got it backwards, if there is a vacuum leak at idle it will either stay the same or go down.

OK, I won't try to argue, the more I consider it the more likely it seems the vacuum baseline would shift as a function of a leak, but I'm now wondering why a different cam would result in increasing vacuum with increasing throttle! Any insight?

 

[92awddsm]

OK, I won't try to argue, the more I consider it the more likely it seems the vacuum baseline would shift as a function of a leak, but I'm now wondering why a different cam would result in increasing vacuum with increasing throttle?

Have you gone wot on a free rev to see what vacuum does or do you ease into it and hold it steady and a given rpm? WOT creates zero vacuum and zero boost on an n/a car.

 

[92awddsm]

And for the record, Im not trying to argue either. Im trying to help you out.

 

[Colt4G63 also]

It's all totally normal, and the argumentative wiseman is correct.

The reason 272s have a lower vacum reading at idle, is because of cam overlap.

The reason the vacuum increases with RPM, is because the momentum of the air overcomes the effect of cam overlap. Hence the reason aggressive cams need to be idled higher to keep the engine running.

 

[lil corb]

It's all totally normal, and the argumentative wiseman is correct.

The reason 272s have a lower vacum reading at idle, is because of cam overlap.

The reason the vacuum increases with RPM, is because the momentum of the air overcomes the effect of cam overlap. Hence the reason aggressive cams need to be idled higher to keep the engine running.

Thanks everyone for their help. It only does it when I blip the throttle. So to confirm vacuum increases with rpm in my case because of the 272 cams? My stock Evo vacuum at 18 - 20 in hg and vacuum only decreases with rev. Also my dsm before cams, the 272 cam overlap making vacuum increase on revs makes sense

 

[turboglenn]

Any engine when RPM is raised and held steady will have an increase in vacum (at least when there's no load onthe engine) It's just like when you let off the gas on the road in gear and vacum goes to say 25in/hg instead of the idle of usually 19-21in/hg, it's because the engine is pumping more air and with th torttle plate restricting what it can take in the vacum will rise..

so basically on any engine, if you raise the RPM and the throttleis at a low opening (say holding the ar at 3k RPM in neuttral), you should always have more vacum than at idle.

 

[TrevorS]

Have you gone wot on a free rev to see what vacuum does or do you ease into it and hold it steady and a given rpm? WOT creates zero vacuum and zero boost on an n/a car. (And for the record, Im not trying to argue either. Im trying to help you out.)

I normally ease into it and my vacuum measurements tend to be fairly static rpm. I've blipped carburetor engines in the past and seen the vacuum drop, but never noticed an increase. Sorry, bad phrasing I guess, I was viewing myself as possibly argumentative, not you. I was just thinking that at higher rpm, the engine could potentially draw more vacuum resulting in overshadowing a small leak.

Did run into an interesting behavior recently where I had accidentally swapped the EGR and Evap vacuum lines at the TB. The resulting vacuum was about -15.5 in-Hg at 650 rpm and around -21 in-Hg at 1Krpm. Took me quite awhile to realize my error. My guess is at the lower rpm, excessive exhaust was being metered into the intake manifold (essentially a leak).

It's all totally normal, and the argumentative wiseman is correct.

The reason 272s have a lower vacum reading at idle, is because of cam overlap.

The reason the vacuum increases with RPM, is because the momentum of the air overcomes the effect of cam overlap. Hence the reason aggressive cams need to be idled higher to keep the engine running.

Any engine when RPM is raised and held steady will have an increase in vacum (at least when there's no load onthe engine) It's just like when you let off the gas on the road in gear and vacum goes to say 25in/hg instead of the idle of usually 19-21in/hg, it's because the engine is pumping more air and with th torttle plate restricting what it can take in the vacum will rise..

so basically on any engine, if you raise the RPM and the throttleis at a low opening (say holding the ar at 3k RPM in neuttral), you should always have more vacum than at idle.

Thanks for the inputs guys -- still learning !