Probably a dumb question

toyman

10 Year Member

Jul 19, 2007

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Vernon BC

Sep 3, 2007

#1

• Sep 3, 2007

• #1

I've noticed that practically all boost gauges show both boost and vacuum. So I'm wondering if it's possible to plumb the system such that if boost is not being produced the gauge would measure vacuum. I'm not certain why one would do this but it may have some value that explains why boost gauges almost always are able to measure vacuum. Perhaps it's a tuning carry over from simpler times before computers.

 

HISSIN50

"How long does it take to get help in here?

15 Year Member

Nov 29, 1999

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Sep 4, 2007

#2

• Sep 4, 2007

• #2

I don't think I understand. You can have it read vac and boost.

 

toyman

10 Year Member

Jul 19, 2007

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Vernon BC

Sep 4, 2007

#3

• Sep 4, 2007

• #3

HISSIN50 said:

I don't think I understand. You can have it read vac and boost.

Click to expand...

Maybe I'm missing something here as well. When I plumb the gauge sending unit into the discharge side of the intake manifold (KB S/C) are you saying that at idle the gauge will be showing vacuum and as rpm increases the gauge will move into the boost range? I hadn't though about that. I was thinking that to measure vacuum in this instance the gauge would have to be tapped into the intake before compression takes place.

 

9

94v6GT

New Member

May 13, 2006

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Sep 4, 2007

#4

• Sep 4, 2007

• #4

toyman said:

Maybe I'm missing something here as well. When I plumb the gauge sending unit into the discharge side of the intake manifold (KB S/C) are you saying that at idle the gauge will be showing vacuum and as rpm increases the gauge will move into the boost range? I hadn't though about that. I was thinking that to measure vacuum in this instance the gauge would have to be tapped into the intake before compression takes place.

Click to expand...

whether your N/A or SC you mount the vacuum/boost gauge to a vacuum point on the manifold. if the manifold is under vacuum at the time the gauge will show vacuum. it the manifold is under pressure, it will show boost, which is just negative vacuum.
-mount it on the manifold.

 

HISSIN50

"How long does it take to get help in here?

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Sep 4, 2007

#5

• Sep 4, 2007

• #5

Toyman, your thinking was right and 94v6GT stated why. You simply need to be on the other side of the 'restriction' for your vac readings.

 

S

Synned

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Mar 31, 2005

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Philly

Sep 4, 2007

#6

• Sep 4, 2007

• #6

I've always wondered about something...why doesn't boost screw with things that take reading from vacuum? Like the EGR, wouldn't boost affect that, the smog pump, etc?

 

99FiveOh

15 Year Member

May 20, 2006

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J-Ville, FL

Sep 4, 2007

#7

• Sep 4, 2007

• #7

The explaination I got was that most of your vacuum referenced devices only work at idle or under light loads, which in turn would mean no boost is being produced. I know the brake booster is vacuum controlled, but as soon as you lift your foot to push in the brake pedal, the boost disappears and you have vacuum again.

And I believe the EGR doesn't operate at WOT either. And, even without a supercharger, when you hit the throttle hard, the gauge reads "zero".