Vacuum Question
12-12-2004, 11:42 AM
#1
Three Wheelin'
Thread Starter
Vacuum Question
I have searced everywhere I can think of (Work Shop Manuals, Hayne's Manual, Renn List Archives, and several other sources), and I cannot find any definatitive numbers for vacuum.
When I bought my '84 several years ago, I started with vacuum (as read at the FPR) of about 14" Hg. After doing some checking, cleaning, and "tweaking" I got vacuum up to 18" Hg, or there abouts. Same thing for my '87. Vacuum was below 15" and now runs 18" to 19".
My '86 951's vacuum is even lower than that. When I bought the car last March, vacuum (as measured between the brake booster and the intake manifold) was running about 10" Hg or so. After doing a head gasket, new valve guides, new valve stem seals, new injector seals, new throttle body O-ring, tie-wrapping all vacuum fittings, and deleting the venturi, vacuum is running about 15" Hg.
All vacuum readings were taken at idle, with the engine "hot". Idle RPM and fuel/air mixture are "spot on", and both the '87 NA and the 951 have passed emissions well below the limits for North Carolina.
My '87 is stock, but my 951 has Guru 15# chips, a 3.0 BAR FPR, and a ReliaBoost MBC.
From my previous experience with other cars, I would think that vacuum should be in the high teens to low 20's, but I have not been able to verify what the spec is for these engines.
In light of that, it would be greatly appreciated, and (IMHO) highly beneficial to other owners if we could come up with some better information. To that end, I would like to ask those of you who do their own work to take a minute and get a vacuum reading and let me know what numbers you have.
When I bought my '84 several years ago, I started with vacuum (as read at the FPR) of about 14" Hg. After doing some checking, cleaning, and "tweaking" I got vacuum up to 18" Hg, or there abouts. Same thing for my '87. Vacuum was below 15" and now runs 18" to 19".
My '86 951's vacuum is even lower than that. When I bought the car last March, vacuum (as measured between the brake booster and the intake manifold) was running about 10" Hg or so. After doing a head gasket, new valve guides, new valve stem seals, new injector seals, new throttle body O-ring, tie-wrapping all vacuum fittings, and deleting the venturi, vacuum is running about 15" Hg.
All vacuum readings were taken at idle, with the engine "hot". Idle RPM and fuel/air mixture are "spot on", and both the '87 NA and the 951 have passed emissions well below the limits for North Carolina.
My '87 is stock, but my 951 has Guru 15# chips, a 3.0 BAR FPR, and a ReliaBoost MBC.
From my previous experience with other cars, I would think that vacuum should be in the high teens to low 20's, but I have not been able to verify what the spec is for these engines.
In light of that, it would be greatly appreciated, and (IMHO) highly beneficial to other owners if we could come up with some better information. To that end, I would like to ask those of you who do their own work to take a minute and get a vacuum reading and let me know what numbers you have.
12-12-2004, 11:56 AM
#2
Pro
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I use an Autometer Boost Gauge with vacuum readings in my Turbo. The line is teed right near the KLR box. Cold reading is normally around 15" Hg, warm idle seems to be around 18" Hg. I can only assume these numbers are close to normal.
12-12-2004, 11:59 AM
#3
FPR and MBC will not have any effect on your idle vacuum reading. Chips *might*, but I doubt it. Only way I could see that happening is if they drastically changed your idle ignition timing hence I don't see it happening.
That said, a warm idle reads 15" Hg in my stock 951 but with 101k miles & original vacuum lines I'm willing to bet some under hood maintenance could get me 18+" Hg.
From what I understand you should see 18-20" Hg with a warm idle.
That said, a warm idle reads 15" Hg in my stock 951 but with 101k miles & original vacuum lines I'm willing to bet some under hood maintenance could get me 18+" Hg.
From what I understand you should see 18-20" Hg with a warm idle.
12-12-2004, 04:41 PM
#4
Drifting
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I have a 15 when warm at idle with newer hoses in SOME places, not all. I only get about 10 at idle freezing cold out. With no gas I see 20-22 while in gear slowing down.
12-12-2004, 04:48 PM
#5
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At cold I get ~13hg, at warm I get ~18-19hg. This is on the ZT-2. I used to get better numbers until I replaced all the vaccum lines with Lindsey's. Go figure.
I've double-checked everything. I'm replacing the Lindsey lines with higher quality (thicker) lines to see if there is a difference.
I've double-checked everything. I'm replacing the Lindsey lines with higher quality (thicker) lines to see if there is a difference.