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Vacuum Question

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Old 12-12-2004, 11:44 AM
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Charlotte944
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Default 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.
Old 12-12-2004, 12:51 PM
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badass951
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I believe 16" Hg to 20" Hg is generally the range you want to be in. If you have a modded cam, at low rpm you will have less vacuum. Also, if you are at a higher elevation you will not be able to obtain as much vacuum (seeing as you are in North Carolina I doubt that is the case).

EDIT: do a search, as this topic has been discussed many times
Old 12-12-2004, 01:51 PM
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theedge
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951s should be running very high teens to very low 20s (20+ is usually on rebuilt engines from what ive seen posted here). When you replaced the vacuum lines, did you replace EVERYTHING? Like the ones going over to the fuel purge, brake booster, etc? And have you replaced all the crush washers for the banjo bolts on the intake manifold?

And lastly, how are you measuring your vacuum?

Edit: My car is over 212k miles, and stays between 18-20" when warmed at idle. Cold starts it goes as low as 10-12". When I was up in the mountains it was around 15 or 16 when warm. I think one morning up there it started below 10", but considering that morning was -12 degree C... Ive replaced all the vacuum lines, deleted that damn venturi, and ziptied all the connections. Only lines I havent replaced are the ones going into the cabin, and the big hose that goes to the brake booster.

Last edited by theedge; 12-12-2004 at 02:21 PM.
Old 12-12-2004, 02:01 PM
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Mike1982
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I have noticed that my car puts out about 14 vacuum when hot, only about 10 when COLD. I hit about 22 when off the gas when in gear.
Old 12-12-2004, 03:54 PM
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Charlotte944
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When I had the engine torn down for the head work (I busted an exhaust stud so I had to take the head to a machine shop.), I inspected all of the vacuum lines and fittings. I replaced several of the major hoses under the intake, deleted the venturi, and replaced the Blow-Off Valve. I did not replace the crush washers for the banjo bolts. I have verified that the brake booster is not leaking.

So far the only vacuum fitting I haven't touched is the fitting on the KLR.
Old 12-12-2004, 04:27 PM
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Charlotte944
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BTW: Do you get new crush washers in a head gasket kit? If so I just may have some.....
Old 12-12-2004, 05:48 PM
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Originally Posted by Charlotte944
BTW: Do you get new crush washers in a head gasket kit? If so I just may have some.....
No idea about that.



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