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ok, got it.

what about this wide range of vacuum readings.

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Old 06-16-2013, 03:17 PM
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Brian, those numbers from your red rocket sounds a lot like my car, except I still keep boost limited to 0.8.
Old 06-16-2013, 04:38 PM
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this morning at 80mph it was still -.5bar.
i bumped my timing back up yesterday. i was not there long.

what kind of info can we get from vacuum readings at say 80?
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86 930 42kmiles [__] RUNNING:[__] NOT RUNNING: ____77 911S widebody: SOLD
88 BMW 325is 200K+ SOLD
05 BMW 330CI 130K:: [__] RUNNING: [__] NOT RUNNING:
08 VOLVO V70 190K:: [__] RUNNING: [__] NOT RUNNING:
90 B2200[__] RUNNING:[] NOT RUNNING:__2000 MER E320 WAGON [] WRECKED:[]RUNNING:
Old 06-17-2013, 01:54 AM
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The vacuum readings reflect the overall health of the engine itself, air recirculating devices, engine tune, and modifications. Results will vary at 80mph due to air drag, boost characteristics, and how the engine is modifed or set up. Also the vac reading needs to be taken below the throttle plate to see what the engine sees and not what the intercooler and turbo are seeing. Those who have cut into the brake booster line are seeing manifold vacuum below the throttle plate.
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Old 06-17-2013, 04:07 AM
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i bet mine is spliced into the brake booster. i have not looked into where it is plumbed but i know it is below the plate.
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Old 06-17-2013, 06:16 AM
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The brake booster line is the thick hose going into the side of the 930 intake manifold up above the #2 cylinder.
A reading from it anywhere up to the brake booster checkvalve is exactly the same as intake manifold vacuum.
Do it's relatively large inside diameter compared to the average vacuum line it reacts to manifold vacuum changes instantly.
Old 06-17-2013, 06:26 AM
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Quote:
Originally Posted by T77911S View Post
ok, got it.

what about this wide range of vacuum readings.
At idle my vacuum is 6.5-7.0 psi absolute read off the MAP sensor I installed.

Dont know at cruise but I guess from my VDO gage its pegged usually right above the negative 0.5bar to neg 0.4bar mark depending upon the levelness of the road I am on. This translates to approx 7.5 to 9psi absolute. Of course very light throttle applied.
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Old 06-17-2013, 06:40 AM
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Paid attention on a little road trip because of this conversation. I can't figure out what it means to have -.05 bar at 80mph. Is it because of a big turbo or less efficient turbo? I can't quite wrap my head around it.

Here is mine last night.

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Old 06-17-2013, 06:52 AM
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i/we have been looking at the boost gauge at a steady speed. i think 80 is now the reference.
thats hard for me to do around here and stay steady for very long. 70 might be better.
as the steady speed goes up, the vacuum gets less, to the point that at some steady speed, you are in boost. i wonder what speed is "0" .
i set the cruise on my old volvo 760 turbo at 120 for around 5 or 6 miles one time. vacuum gauge was steady in the boost.

i orignal thought or hope was that now i am seeing less vacuum at a certain speed, does that mean i have fixed all my vacuum leaks. i was thinking if i am sucking in air, my vacuum will be less.?

the old days guys tuned a motor by vacuum. i dont know how though.
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88 BMW 325is 200K+ SOLD
05 BMW 330CI 130K:: [__] RUNNING: [__] NOT RUNNING:
08 VOLVO V70 190K:: [__] RUNNING: [__] NOT RUNNING:
90 B2200[__] RUNNING:[] NOT RUNNING:__2000 MER E320 WAGON [] WRECKED:[]RUNNING:
Old 06-17-2013, 08:16 AM
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Some of us old guys still tune by vacuum.
Vacuum levels at high speeds (over 70 mph) become meaningless due to drag which put load on the engine = boost.
Turbo type makes little difference in engine vac at cruise as the turbo is not under load.
Old 06-17-2013, 09:50 AM
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Using Brian's definition of boost threshold: mine tips from negative to positive at 1,600 rpm's in 4th gear...

and, a bit of a clarification at 80 mph: 0.15 BAR at 80mph steady cruise on level ground in 4th gear...

i put 100 miles on it this weekend with little 45 mile trip. finally got the new wheels and tires mounted and road-force balanced. No vibrations and smooth as a baby's bottom at 100mph!!
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Old 06-17-2013, 09:54 AM
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3rd gear floored from about 1200rpm, i hit .1bar at 1800rpm.

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86 930 42kmiles [__] RUNNING:[__] NOT RUNNING: ____77 911S widebody: SOLD
88 BMW 325is 200K+ SOLD
05 BMW 330CI 130K:: [__] RUNNING: [__] NOT RUNNING:
08 VOLVO V70 190K:: [__] RUNNING: [__] NOT RUNNING:
90 B2200[__] RUNNING:[] NOT RUNNING:__2000 MER E320 WAGON [] WRECKED:[]RUNNING:
Old 06-18-2013, 05:36 AM
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