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Mike_Lettrich's Avatar
 
Join Date: Nov 2006
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Reading a Boost/Vacuum gauge?

This is probably a stupid question, but:

If a vacuum/boost gauge is hooked up to the throttle body below the butterfly, it will show a vacuum at idle. Let's assume, just to keep the math easy, that it shows a 1 bar vacuum at idle. Let's also assume that the turbo system is set up to produce a maximum 1 bar of boost.

What will the vacuum/boost gauge read at maximum boost? 0? Are there other variables at work that would affect the reading?

Thanks

Mike
Old 05-28-2008, 07:08 AM
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Vacuum is negative pressure, boost is positive pressure. Atmosheric pressure is actually 1bar at sea level so 1bar of vac would be 0 pressure and 1bar boost would be 2bar absolute.
Which has nothing to do with your question.
Vac/boost gages are calibrated to take all that into consideration, 1bar vac would register 1 bar vac and 1 bar boost would register at 1bar boost.
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Old 05-28-2008, 08:54 AM
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I have a VDO analog vacuum/boost gauge plumbed from my manifold. At cold idle it reads ~15"hg, or about -7psi. Off-throttle coast-down will reach as high as 20"hg (which is limited by the vacuum limiter, big ugly cadmium plated diaphragm on left side of throttle body). The gauge will swing from vacuum past zero into + boost at anything more than granny-driving, and will read ~15psi (1bar) at WOT. A normally-aspirated car will read highest vacuum at idle and off-throttle and (with a good intake) show zero vacuum at WOT.
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Old 05-28-2008, 08:55 AM
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Mike, you could also look at it in terms of absolute manifold pressure, which is what airplanes use. At dead engine, sea level you'll see one bar. Everything else will be dependent on engine speed, throttle setting, boost and altitude. So on an absolute manifold press gauge my car would read .4 bar at idle and 2 bar at WOT.
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X-1, AFM744, Jon, '92 C2 Turbo
JE 8:1 pistons, 964 cams, 38mm intakes ported/polished/twinplugged, ARP hardware, B&B headers/exhaust, 355whp. Full-blown GT35R 3.2 intake EFI pending...
Where my misspent time and money is currently going.
Old 05-28-2008, 09:01 AM
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Thanks guys. Your explanations were helpful.
Old 05-28-2008, 02:13 PM
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If you hook up the vacuum/boost gauge to a vacuum port in the throttle body that is below the butterfly but on the side of the throttle shaft where the butterfly is going downward as you open it and crossing over the little hole in the TB throat you will get a venturi effect from the extreme velocity of the air rushing by as the butterfly passes over that hole and your vacuum reading will be higher than what is actually in the manifold and inaccurate.

At full throttle there is probably some venturi effect in any port or hole anywhere in the throttle body due to the air velocity rushing over it.

Take your vacuum/boost signal from the intake manifold somewhere away from the throttle body for a more accurate reading.

The brake booster hose is already there and perfect.
T into it, or get one of the plastic brake booster check valves that has a hose connection for a boost/vacuum gauge.

North Hollywood Speedometer "NHS" probably has them.
Old 05-28-2008, 06:18 PM
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