How to use vacuum gauge for FE?
 

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)?

Re: How to use vacuum gauge for FE?
 

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.

Re: How to use vacuum gauge for FE?
 

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.

Re: How to use vacuum gauge for FE?
 

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.

Re: How to use vacuum gauge for FE?
 

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...

Re: How to use vacuum gauge for FE?
 

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.

Re: How to use vacuum gauge for FE?
 

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.

Re: How to use vacuum gauge for FE?
 

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: https://www.secondchancegarage.com/public/186.cfm

Re: How to use vacuum gauge for FE?
 

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.

Re: How to use vacuum gauge for FE?
 

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.

Re: How to use vacuum gauge for FE?
 

Yes- the trans modulator vacuum fitting should be a good one to use for the distributor and for your gauge.

Perhaps that funky manifold/ported vacuum mixing valve bled off manifold vacuum through the line that went to the carb and since you rerouted it, the vacuum is no longer bled off.

This explains why your car is spark knocking under heavy load- the vacuum advance is present even when load is high.

Re: How to use vacuum gauge for FE?
 

I'll have to get some fittings and hook that all up to the transmission modulator fitting.

Re: How to use vacuum gauge for FE?
 

This is where it's currently hooked up. This thing has 3 nipples and a rubber boot that fits all 3, combines the outer two into one, and sends the middle one to the distributor (formerly, to the thingy that went to the distributor). Doesn't look like all the other vacuum check valves but could be.
https://lh6.googleusercontent.com/_o...0/IMG_6427.JPG

Re: How to use vacuum gauge for FE?
 

THey may be venting some air flow thought the lines to prevent fuel from backing up the line into the vacuum diagram on the distributor and moisture too. In my Geo there was a recall for the map sensor line to allow some air flow through it to prevent water from backing up and freezing in it.

Re: How to use vacuum gauge for FE?
 

I run vacuum gauges connected to the distributor vacuum advance line on both my cars.
For me, vacuum reads zero at idle, rising to between 2/3 - 3/4 manifold vacuum at light engine loads, then falling as manifold vacuum falls (engine load increasing).
I find cruising at the highest vacuum reading I can hold on the freeway gives me my best FE. This is about -42 Kpa in the 1.8L Mitsubishi and about -60 Kpa in the 2.8L Datsun.
Pete:)

Re: How to use vacuum gauge for FE?
 

Your distributor isn't hooked up to manifold vacuum? What is it hooked up to instead?

Re: How to use vacuum gauge for FE?
 

Normally the distributor vacuum advance is connected to the throttle body at the butterfly so at ide there is very little or no vacuum so the timing is retarded at idie. Keeps it from stalling when cold and burns more gas for the rpm. Then as you apply throttle the vacuum advance gets some as the butterfly valve is opening.

Re: How to use vacuum gauge for FE?
 

I think you're on the right track by looking for another vacuum source. I've seen one way valves that look like what you have pictured above. If it's not a one way valve I think you have something restricting the vacuum line going to the gauge.

Re: How to use vacuum gauge for FE?
 

I don't think it's a restriction, it jumps up quickly it just doesn't go down quickly.

Re: How to use vacuum gauge for FE?
 

[QUOTE=theholycow;160422]Your distributor isn't hooked up to manifold vacuum? What is it hooked up to instead?[/QUOTE]

Although I haven't seen an exploded diagram of this model Mikuni carby, I believe this line comes from a small port just above the throttle butterfly (air cleaner side).
I've heard that some vacuum advance actuators do run on manifold vacuum, but I don't know how your Buick is meant to be connected.

Re: How to use vacuum gauge for FE?
 

My Buick is meant to be connected to two vacuum sources, through a modulator device. This guy:
https://www.corvetteactioncenter.com/...m-advance.html
makes a great case for connecting it directly to manifold vacuum.

Re: How to use vacuum gauge for FE?
 

thc, that's a really interesting article.
I know nothing about American cars, so I have to accept that this guy knows what he's talking about.
The big plus here, is that it's an easy mod to make - and easily changed back if it isn't to your liking (that is a performance orientated site and they have different objectives than we do).

Good luck,
Pete.:thumbup:

Re: How to use vacuum gauge for FE?
 

My vote is for manifold vacuum advance for fuel economy. The 85-87 Honda CRX HF's were carbed and all used manifold vacuum for advance.

Re: How to use vacuum gauge for FE?
 

I'm going to try both the one that I think is ported vacuum (which was previously one of the two simultaneously in use for it) and good manifold vacuum.

I may do some A-B-A if they both drive well but I suspect that ported is going to run badly.

Re: How to use vacuum gauge for FE?
 

I might be new here, but I have run Vac gauges for years, on my bikes, and muscle cars. The vac gauge needs to be on the manifold, below the butterfly to get the right readings. I know my old 67 Pontiac would idle @ 17, and as soon as you get into it, it would drop. Whether injected or Carbed, it will do the same thing.
Now with your car, it sound like the is a check valve or diaphram where you have it hooked up no. You need to have it hooked direct to the manifold to get the right readings. I use them on my Motorcycles, to make sure the carbs were Sync'ed. Good luck
The FNG

Re: How to use vacuum gauge for FE?
 

Good news, everyone!

I moved the distributor advance to ported vacuum and the vacuum gauge to manifold vacuum (as reported in more detail in my spark plug thread).

Now:
- the vacuum gauge works right :)
- idle is partially better (down to 1000RPM) :D
- no spark knock! :D

My driving style was pretty close to good. I need to go easier immediately after shifting gears, and there's a couple hills I maybe should climb in a lower gear.

Re: How to use vacuum gauge for FE?
 

Yahoo!

Re: How to use vacuum gauge for FE?
 

As long as you're not lugging the engine on the hills you'll be more efficient climbing them in the higher gear at lower vacuum readings. I've verified this with the use of the Scan Gauge in my '97 Escort. I've also noticed in my '88 Escort that after I get to highway speed I can back out of the accelerator slightly raising engine vacuum and not losing speed for a pretty good distance.

Re: How to use vacuum gauge for FE?
 

Congratulations!
I suppose I'll have to add this to my "must try" list.;)

Re: How to use vacuum gauge for FE?
 

Thanks, I was going to ask about that...there are situations where I'm not sure whether low RPM at low vacuum and presumably rich mixture is worse than higher RPM with higher vacuum.

Turns out there's also a lot of times when the engine is struggling and bogging and vacuum is marginal, but downshifting makes vacuum drop even more...you gotta really lay into it to get this engine to higher RPM.

Re: How to use vacuum gauge for FE?
 

Sometimes, or maybe all the time, my needle shakes like the movement demonstrated in scenario 3 (but not at that level). Does that mean anything?

Re: How to use vacuum gauge for FE?
 

It might be a cylinder missing. Purposely pull off a spark plug wire and see if the gauge acts the same way/gets worse.

Re: How to use vacuum gauge for FE?
 

So I've been reading up on Quadrajets. I've got more material from different sources to read yet, but I did come up with this bit:
"On some Q-jets, two springs are used beneath the power piston. The smaller (primary) spring seats in the center of the piston. A larger diamater (secondary) spring surrounds the smaller spring. It exerts added force upward on the power piston. It starts the first stage of power enrichment at 8-10 in.Hg manifold vacuum. The center spring gradually continues enrichment until it is completed at 5-7in.Hg manifold vacuum."

I'll eventually find out if mine is that type. All the reading is because I'm studying to rebuild mine.

Re: How to use vacuum gauge for FE?
 

I just found this, from another manual, in my notes:
"On M4ME models a remote vacuum source is used to operate the spring-loaded power piston instead of the conventional manifold vacuum source. A tube in the front of the throttle body (center) connects the remote vacuum source (power enrichment control valve) to a passage to the power piston vacuum channel."

I'm not so sure about that, I don't think I've seen it in any vacuum hose diagrams.

Re: How to use vacuum gauge for FE?
 

Hello,

This is very interesting discussion for me since I also have installed vacuum gauge in my car (I have the old ODB port in my car so I can not use SGII) about month ago.

I do know BSFC chart for my car but I'm unable to tell - using vacuum gauge - when (during acceleration) I hit 75%-80% load.
Does it a constant value? I mean it will be valuable if someone with SGII (where is displayed load) AND vacuum gauge tell me that 75% load on SGII is when vacuum gauge shows eg. 7 or 8 In/Hg?
If not then how to determine 75-80% of Load? Any ideas?

How am I doing?
 

So, here's the strategy I've worked out and the information I know. It seems to work well.

https://i51.tinypic.com/2ql8wmp.jpg

At 10-7 in/hg, often I can back off just enough to reach 11 and still maintain enough speed. Occasionally I can downshift but usually that just makes it worse, as I have to get deep into the throttle to get the higher RPM of the lower gear.

By the time I hit 5 I'm sure I'm as deep in power piston territory as possible and backing off won't suffice, so I might as well go wide open and pulse until I am going fast enough to back off to 11. Sometimes in the 10-7 range I may need to just give up and open it up for a pulse too.

The power piston section assumes that I have the two stage power piston that was in some Quadrajets. I'm not sure of that. I'll find out, if I can ever get my hands on a core to rebuild (or if I give up and accept some downtime while I rush through a rebuild with the car parked).

Any thoughts on potential improvements?

Re: How to use vacuum gauge for FE?
 

I see 75% LOD on the Scangauge matches up to 12-12.5 PSI MAP. Since 14.7 is ambient that means the throttle is mostly open and free-flowing and causing very little vacuum due to restriction.

That converts (wikipedia) to 25 in HG. Since this is a vacuum reading, subtract from 29.92 = 1 atmosphere to get ~ 5 in HG.

This site confirms that "about 5" number.

Re: How to use vacuum gauge for FE?
 

Hmm...so, do you think I might do better to stay at 5in/hg than to open it up the rest of the way? There is a definite increase in power, and power piston level is the same, but I can't say for sure whether efficiency increases or decreases.

I imagine that the AFR stays pretty steady, possibly leaning slightly, going from 5 to 0, putting the most rich mixture at 5, hence why I have been avoiding 5.

Re: How to use vacuum gauge for FE?
 

I know little-to-nothing about carburetors. What I do know is you want to stay as close to ideal air-fuel ratio as you can. Whatever conditions take you lean or rich are to be avoided.

Re: How to use vacuum gauge for FE?
 

I ought to add O2 sensors...otherwise it's all just guesswork.