What should vacuum pressure be?

[TeeM403]

Hey im just wondering what the normal vacuum pressure is when hooked up to stock boost controller? Mine says about 16psi is that normal
Seems a
Lil high

 

[herms99gst]

Do you mean stock boost levels are engine vaccum at idle?

 

[And_44]

My idle vacuum is -21psi. Stock internals. Even have the stock air can on still. Tried it off but car ran like ass and was registering under 25mhz of air flow at idle. Put can back on and problem fixed.

 

[cjridert1]

-16mmHg is high (rather less negative than you would expect), your car should be pulling 20+ if everything is stock and healthy. Is it running poorly or were you just checking to check?

 

[zack13277]

Mine at idle is about 17, and in gear rolling to a stop it goes to about 24. The car runs fine.

 

[jtmcinder]

My idle vacuum is -21psi.

sigh

 

[91mits]

around -19.5 to -21mmhg...on mine..

 

[And_44]

sigh

Haha, okie I didn't say mercury. Is -1.5 bar good?

 

[NHerron]

Holy crap no one knows how idle manifold pressure is measured anymore? We need Bill Nye the science guy up in here

Try 480mmHg or 18.9inHg. It's not measured by PSI, however you can convert from mmHg and inHg to psi. 18.9inHg equals 9.28psi. 9.28psi is still a positive pressure so the "-" is incorrect. It's technically considered a lower pressure rather than vacuum.

OP, pretty sure you're talking about 16psi boost, right? Like you can get the engine up to 16psi boost while driving. I would think on a stock setup, fuel cut would occur by that time. Anyway, I couldn't find a spec for that in the FSM. I did however find how much boost there should be with the stock boost controller DISCONNECTED though, which is pictured. This is simply running a hose directly from the turbo discharge pipe nipple, to the wastegate actuator. That should net you 6.0-10.8psi with manual transmission equipped and 5.4-9.9psi automatic transmission equipped.

Most probable, I would suspect the stock boost controller is stuck open somehow or the signal hoses running to the wastegate actuator are leaking.

And just because I was there, I pulled the info on idle vacuum specs. You should be sitting at 480mmHg (18.9inHg) engine warmed up, 750 RPM's, no accessories on. This info was pulled from the 90-91 FSM

Oh look, my post count is counting itself

Wleocem To Tnrues, Mr. TeeM403

 

[boost97gst]

Crower 272s mines at -12hg. Stock cams 8.5:1 fresh engine it was -21. cams have a drastic effect on idle vac. Also, the vac will be slightly lower as your idle speed has to increase. before i could run at 750 to 800 rpm, with the crowers i HAVE to idle at 1k. Too choppy otherwise. Its a quick and dirty way to see if you have any vac leaks too. If you normally pull -19 then one day its -12 then you know either your compression is shit, or more likely, a small leak has developed

 

[cjridert1]

Holy crap no one knows how idle manifold pressure is measured anymore? We need Bill Nye the science guy up in here

Try 480mmHg or 18.9inHg. It's not measured by PSI, however you can convert from mmHg and inHg to psi. 18.9inHg equals 9.28psi. 9.28psi is still a positive pressure so the "-" is incorrect. It's technically considered a lower pressure rather than vacuum.

I guess you could argue 760mmHg (sea level) - 480mmHg (car vacuum) = 280mmHg, which is still positive, but this is absolute #, not a practical gauge value. Since a boost gauge reads in gauge pressure/vacuume, Id think it would have to have a "-" before it to be technically correct, however, it is redundant given you're measuring vacuum. I couldnt find a Bill Nye youtube that discussed these differences further.

 

[And_44]

I just said what my boost gauge says. That's in psi. I'm sure I don't boost to 15 inhg as that would be about 1-2psi. My elevation is 500ft above see level.

 

[jtmcinder]

Haha, okie I didn't say mercury. Is -1.5 bar good?

deeper sigh

 

[vingasoline]

Crower 272s mines at -12hg. Stock cams 8.5:1 fresh engine it was -21. cams have a drastic effect on idle vac. Also, the vac will be slightly lower as your idle speed has to increase. before i could run at 750 to 800 rpm, with the crowers i HAVE to idle at 1k. Too choppy otherwise. Its a quick and dirty way to see if you have any vac leaks too. If you normally pull -19 then one day its -12 then you know either your compression is shit, or more likely, a small leak has developed

With bc272 my vac was -16 @ 1000rpm. I just degreed them and now its -17 @800. Idles like stock

 

[NHerron]

I'm just talking how the industry measures 'manifold vacuum'. You're right, our gauges are measured with respect to ambient pressure, therefore "boost" would actually read, for example, 10psig. G refers to Gauge pressure. psiA is Absolute pressure.
0psiG = 14.7psiA (sea level)
0psiA = -14.7psiG. This is a real vacuum.

Kind of a bummer, but I can't find the Bill Nye thing, maybe it was another science guy

 

[cjridert1]

deeper sigh

hahahah

 

[And_44]

Haha seems we all got off topic besides a few posts. 16psi on stock boost controller is to high. Test your wastegate as described in manual. It "should" open at around 8-14psi. If it opens on the low end then go one step back to the boost control selonoid. You could just run a vacuum line between your positive side of j pipe to the waste gate and run wastegate spring pressure. Bypass the stock boost control selonoid all together.

Or splurge and pick up a manual boost controller.

Also I hope your using an aftermarket boost gauge and not going by the stock one.

It is funny though that the only dsm wiseman thats responded is being a wise butt and offering no input at all.

 

[STI EATER]

18-20 with stock exhaust on my 1g. 17-19 with 3'' exhaust no cats. recently pressure tested and no leaks!

 

[BLOHS7844]

Why is everybody upset with there vacuum that they pull? If you don't have leaks then it should be pulling a good vacuum, right? Or would worn piston rings cause a low vacuum?

 

[ef_junky]

My boost gauge is ready around 14-10 (fluctuating) inhg vacuum and building boost is 10psi max around almost 5k then falls to about 4psi I just cecke the timing and exaust cam is out bout tooth or so I think that's my problem. Would like second opinion and I have O2 housing atm. Dump for wastegate and it's open all the time not just under full boost

 

[NHerron]

Would like second opinion and I have O2 housing atm. Dump for wastegate and it's open all the time not just under full boost

Can you explain a bit better not only on this but what problem you are having.

 

[crazyquik22023]

deeper sigh

So much fail already in this thread.. I'm sighing right here with you.

 

[And_44]

Haha, it's just going and going the original poster hasn't even answered any questions or posted back after the one from the start.

 

[herms99gst]

Yes worn rings will affect your engine vaccum. The vaccum is read at the manifold by the map sensor. If you have worn rings, you will have lower compression. Compression affects vaccum. Vaccum is a good way to find out different problems in a motor.

 

[boost97gst]

With bc272 my vac was -16 @ 1000rpm. I just degreed them and now its -17 @800. Idles like stock

BC272s and Crower 272s are different.