How to use vacuum gauge for FE?

[theholycow]

Ok, I've finally added a vacuum gauge to my 1980 Buick.

In a short drive (16 miles round trip) I found it to be very steady. I was expecting much more action. It mostly stayed at 20, 21, or 22, but got as low as 18 once (4th gear, 45mph, up a mild grade). At 1400RPM idle it's 21. Generally it just didn't move.

Is there a known vacuum level where my Quadrajet's power piston deploys (if I even understand that concept correctly)?

[pgfpro]

Originally Posted by theholycow

Ok, I've finally added a vacuum gauge to my 1980 Buick.

In a short drive (16 miles round trip) I found it to be very steady. I was expecting much more action. It mostly stayed at 20, 21, or 22, but got as low as 18 once (4th gear, 45mph, up a mild grade). At 1400RPM idle it's 21. Generally it just didn't move.

Is there a known vacuum level where my Quadrajet's power piston deploys (if I even understand that concept correctly)?

Usually they will have
5 in./Hg orange spring
6 in./Hg black spring
8 in./Hg yellow spring

Example a 4 in./Hg gold spring will over come the engines 4" or less of vacuum and add fuel through the enrichment system.

[theholycow]

I'll have to keep driving and see if I ever get down to 8 or less. I didn't try a lot of different conditions in my short drive this evening, just 45-50mph the whole way, no hard acceleration, and mild grades.

[GasSavers_BEEF]

I thought the more throttle you gave it, the less vacuum you will have. more vacuum would be better? I am a fuel injected guy and always have been so this may be wrong due to sheer ignorance but that is what I thought.

well did the gauge come with instructions? there are old mpg meters for older cars that are just vacuum gauges. you can cross reference what level means good mileage to what you are getting yours.

you'll figure it out.

[theholycow]

Squeezing FE out of modern fuel injection is pretty much the opposite of a carburetor. Efficiency goes way down when you lose vacuum. With a carburetor there's no way to get better FE by getting rid of pumping losses.

I think older fuel injection systems that don't have a MAF sensor and depend on the MAP sensor operate the same way, and some MAF-controlled ones with lazy people writing their programs...

[Ford Man]

Those are extremely high numbers with very little fluctuation. My '88 Escort will run 0-10 in/hg on take off climbing larger hills and harder acceleration and usually runs somewhere 10-15 in/hg at highway speed, about 20 in/hg at idle and about 23-24 in/hg on deceleration.

[theholycow]

If it continues behaving with the high, non-fluctuating numbers, what would that mean? Something wrong with engine, or it's just a design that runs abnormally high vacuum?

Could I potentially have hooked up to the wrong vacuum source? Is there any vacuum source that would do that? It comes out of the intake manifold so it must be manifold vacuum, right? It's the same vacuum my distributor advance is hooked up to.

[GasSavers_Erik]

Does the needle drop to zero instantly when you turn the engine off?

Does vacuum drop if you gun the engine while stopped?

The only explanation I can think of is that there is a one way valve between your gauge and the manifold (like the valve in the hose between the brake booster and the manifold). Or some sort of factory restriction in the line which lets vacuum bleed off slowly rather than quickly (Honda used these on my civic and crx).

Here's a pretty good site with animations of what problems a vac gauge can diagnose: http://www.secondchancegarage.com/public/186.cfm

[theholycow]

No, it stays up when the engine is off. I expected it to drop immediately. Even after five minutes it's not totally down.

I haven't tried free-revving with the gauge.

I'll have to get a closer look, I thought it was direct to the manifold but there might be a check valve. Why would there be a check valve between the manifold and the distributor? I removed what I assumed is a 2-way check valve when I went straight manifold instead of manifold+port to the distributor advance. Perhaps a different source of vacuum would be better for the distributor as well as the gauge.

There's a big vacuum fitting on the rear of the engine that was for the automatic transmission's modulator that I capped off when I put in the T5. Would that be a good one to use?

Edit: Great link! Looks like there might be similarly great info about other subjects elsewhere on that site.

[JanGeo]

Sounds like you re on the power brake booster side of the check valve which is why the vacuum is nokt going down low. You should be on the manifold sice of that valve.