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      07-22-2015, 03:26 PM   #1
rps
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Can someone explain boost gauge readings?

Ever since I put in a boost gauge, I have noticed at idle or when stopping the boost gauge will show a negative reading ie. -0.05. I don't understand how a turbo can be in the negative. Can some of the experts here explain it for me.
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      07-22-2015, 05:32 PM   #2
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Come on man. Vacuum. In/Mercury

-.05 could be in Bar but you didn't say. Please look up bar if you are unclear.

Last edited by X1Denver; 07-22-2015 at 07:31 PM..
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      07-22-2015, 07:22 PM   #3
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Sorry you said explain.

No load means the throttle body is closed. Charge Pressure ( Pre-Throttlebody) is relieved through the diverter/recirc/BOV valve because it ALWAYS sees manifold pressure (in this case vacuum). Waste gate goes closed because vacuum is now the signal. And any pressure being made by the turbo is still relieved since the dv is open. --edited sorry.

Engines makes vacuum. (Please see internal combustion engine.)

Soooo where did you tap your line? Or did you use an electronic gauge. And where does that get its signal?

I worked for a turbocharger company and have built lots of boosted cars.

Last edited by X1Denver; 07-23-2015 at 11:43 AM.. Reason: Clarified a bit more
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      07-23-2015, 07:23 AM   #4
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I plugged in Scangauge II into OBD plug, and get boost on my s28i.

It runs about -0.7 up to 19 psi.

I helps to watch when driving fast on hwy.

If wind is behind I can drive up to 80mph without boost pushing, but if wind is in front boost is busy.

You can tell pretty quick if you'll get 34 mpg on the hwy or my worst 23 mpg @ 80 mph with AC and headwind.

Of course the Scangauge has lots of different mpg readings by tank or trip.
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      07-23-2015, 07:57 AM   #5
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Quote:
Originally Posted by X1Denver View Post
Sorry you said explain.

No load means The throttle body is closed. Plenum (intake manifold) Pressure is relieved through the dv because it sees manifold pressure. (Vacuum) Waste gate goes closed because vacuum is now the signal. And any pressure being made by the turbo is still relieved since the dv is open. --edited sorry.

Engine still makes vacuum. ( Please see internal combustion engine. )

Soooo where did you tap your line? Or did you use an electronic gauge. And where does that get its signal?

I worked for a turbocharger company and have built lots of boosted cars.
I bought the multi gauge from Bimmian that plugs into the OBDII port. It displays boost, intake temp, exhaust temp, water temp, and voltage.

I did do a write up on this forum complete with pictures. Search Bimian 5 in 1 gauge.
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      07-23-2015, 07:58 AM   #6
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Thanks for the replies, now I understand the readings
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      07-23-2015, 11:36 AM   #7
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I know its weird but there are BOOKS on this stuff.

I recommend "Maximum Boost" by Corky Bell for beginners and intermediates.

Also, Looks like that thing reads bar and temps in C.

I'm guessing its using the MAP (manifold absolute pressure) sensor signal on the chargepipe and not the MAP on the intake manifold, because your bar for idle vacuum is higher than what it would be in the manifold. ( -.33bar or so depending on your altitude...)

OR it could be reading requested boost figures from the ECU. Which would be useless especially if you have a piggyback system (JB4 or something)

A Mechanical Boost Gauge is the only way you can read what's in there...Digital gauges can always be manipulated or shown incorrect values. BUT on a newer Bimmer its hard because there is no vacuum port to use on the intake manifold
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      07-23-2015, 12:03 PM   #8
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OH maybe on the N20 there is a port if you have that engine.
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