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JohnJL's Avatar
 
Join Date: Jan 2005
Location: Toronto, Ontario
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Boost/vacuum gauge & sensor recommendation

I am finishing up my first supercharging project, in addition to the ECU logging/having to look at the laptop while I drive I'd like to have a boost gauge on the dash/rollbar.

I am going to run 14-18 lbs of boost. I have a 3-bar MSD MAP sensor. The supercharger is an Autorotor 417 positive displacement twin-rotor.

Firstly, what range of gauge do I need? Is both vacuum necessary or will the engine always run in positive atmospheric pressure? My guess is if, as the gauge recalibrates at startup, it will show vacuum at times during idle, but increasing to boost as the supercharger spins up? I have read and been told the engine still runs at a vacuum, which is why brake boosters and other things running off vacuum

Second, is an electronic or mechanical version the preference for a must-be-reliable environment?

My ECU can put out a variable, configurable signal (as passed on from the MAP sensor to a gauge) so I believe I can adjust it to drive most electronic boost gauges.

Or perhaps I can splice the MSD sensor to send the same signal to both the ECU (for its fuel and spark calcs) and to the gauge? I would think since both are voltage-driven sharing the same signal wouldnt' cause a problem?


MSD has these:


Looks like a decent broad-range, electronic gauge although no information is given on its instruction about calibration.

Here are some more...

Autometer 3659



VDO Mechanical 0-30 gauge



Stewart Warner Mechanical


Help with the range I need and recommendations of electric vs mech are much appreciated.
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1982 911 3.2 Indiash Rot Track Supercharged track toy
1978 911 3.0 Lichtbau toy "Gretchen"
1971 911 Targa S backroad toy
Old 08-18-2008, 02:33 AM
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I went with one from North Holleywood speedo and depending on cams and timing you might need more vacuum reading. I went with the -.5 bar to 1.5 bar guage (around $220) and am happy with looks and its reputation of how it performs.

You definately want a guage that shows the -vacuum so if you get a leak you'll know about it. I run a mechanical gauge and have been told its the way to go.

Here's a pic of mine:
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Old 08-18-2008, 03:34 AM
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I've got the "old skool" boost gauge with extra the tattle tale needle (tattle tale no longer works), but I've been thinking about sending it to NHS to have them refurbish it...I wonder if they could calibrate it to show vacuum?
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Old 08-18-2008, 05:55 AM
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Boost



Boost + AFR

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Old 08-18-2008, 06:25 AM
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Nice setup, like the color
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Old 08-18-2008, 07:07 AM
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Garage
Vac gage is manditory for tuning and for continual monitoring of the state of tune.

Love the LED gage in the tack.
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Old 08-18-2008, 08:45 AM
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Thanks guys.

So to be very specific, will a supercharged engine running at 14 lbs boost ever be running at vacuum?

My primary goal is to have an indicator on the dash to indicate a problem with the motor while on the track. All the tuning is accomplished through dyno sessions and logging of ~20 variables. The gauge is more of a warning system on the track to know when there is a problem.

Also, any other comments on mechanical vs electric? The brake booster hose passes right under the dash so it seems that would be a convenient source to tap into for a mechanical gauge. Interested if there is an appreciable difference in accuracy and reliability between mechanical and electrical.
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2007 911 Turbo - Not a toy
1985 911 Cab - Wife's toy
1982 911 3.2 Indiash Rot Track Supercharged track toy
1978 911 3.0 Lichtbau toy "Gretchen"
1971 911 Targa S backroad toy
Old 08-18-2008, 01:26 PM
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Mechanical gages are more accurate than electric.
You need to make sure you tap into the intake manifold after the throttle body to pick up vacuum and boost as the valves see it. Upstream of the throttle body you will see no vacuum.
Brake booster is good spot.
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Old 08-18-2008, 02:30 PM
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Quote:
Originally Posted by RarlyL8 View Post
Vac gage is manditory for tuning and for continual monitoring of the state of tune.
Brian, please explain.
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Old 08-18-2008, 02:41 PM
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