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Re: Vacuum gauge - what does it normally mean?

30hg is a perfect vacuum so, No, it is not correct for an idle vacuum. Idle vacuum will vary according to engine modifications such as camshaft replacement. I believe that a reading of 17-20" is pretty much normal at idle. If I'm incorrect then someone correct me please. The vacuum gauge isn't used much anymore for engine diagnosis except if you consider MAP sensor voltage a vacuum reading and a way to determine engine condition.

David Short

Hi all... > > I just installed a vacuum/boost gauge on a 2002 GT... > > Vacuum measures 30hg at idle, then lowers on throttle open (as it > should), and sits around 30hg during normal driving. > > My questions: > > Is 30hg the correct reading normally for this car? > > Can I infer that keeping the vacuum around 30hg during acceleration is > the most efficient, and that anything less is just dumping gas down the > throttle?
Reply to
Acura God
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Thanks for the replies...

Okay, so used in a relative manner, is it valid to infer that keeping the vacuum Hg higher during acceleration is the most efficient?

I notice that if I open the throttle 100% then the vacuum drops, then rises as the car accelerates... but if I slowly accelerate (without pedal to the metal) then the vacuum does not drop as much....

Reply to
Craig Plesco

Right. It sounds like the originator may have his gauge hooked up to the incorrect or shared vacuum line.

The vacuum gauge (manifold pressure) shows a function of engine loading; sits around 17" Hg at idle and will go to near zero at very heavy loading like going up a hill in top gear; may approach 27" or so coasting. I have a VDO in my 5.0L and is interesting to watch when the cruise control is engaged.

Something else to take one's eyes off the road and rearview mirror.

Don

Reply to
donrm

Craig Plesco opined in news: snipped-for-privacy@news.covad.net:

Yes... but not as much as in carbed models. Saab did a study years ago that showed "brisk"* acceleration (using same basic port efi as Ford) actually was more economical than slow acceleration.

Please bear in mind that:

  1. You are driving a Mustang, fukringoutloud.

  1. "Economical driving" that causes OTHERS to slow is NOT economy. They should have to use up their gas so you can save yours?

  2. Cars, like any other living breathing being, need anaerobic exercise.

I suggest you run a full tank using brisk then one using slow accel... see how much difference there is, I have -several times in several cars- and there just isnt that much difference. All the way from my SVO to 2L Probe (well, the Probe and SVO I used WOT a lot) to 3.8L TBird.. no more than .5 to

1 mpg.

*Brisk is that which does not cause ECM to go to openloop mode. About 2/3 pedal

Reply to
Backyard Mechanic

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