vacuum Gauge confusion
 

So I installed a vacuum Gauge in my 89 vw fox sedan 1.8 8v 4speed with cis-e mechanical fuel injection. When the car is fully warmed up and idling at 1k rpm the vacuum gauge shows 22 units (I don't know what vacuum is measured in). When driving I try to accelerate only letting the gauge drop to 10 at the lowest and my cruising goal is 20 but sometimes the car loses power to overcome a hill so my cruise will drop to 15 till I reach my desired speed.

Something funky is that in 4th gear the car can hit 20units at both 32mph/1500rpm and 45mph/2100rpm, of course the car is more capable of taking the hills at 45mph/2100rpm, and at the low rpm I have to drop the vacuum to 10 just to overcome the smallest inclines. My question is which speed is more fuel saving? Any tips you guys have as well?

27mpg before gauge and techniques, 30mpg after gauge, I need to hit 35mpg to make my daily commute cost $10 exact. There are a lot of unavoidable hills and stops in my commute as well.

I would guess there are a few sliding scales of drag and efficiency going on inside the engine, and you have two different scales experiencing their best values at those two speeds in that gear.

I can't remember where I read it but I've seen it stated that ~1200fpm is the optimum cylinder travel speed inside an engine (can't find the resource now), but maybe you're getting better airflow at the other speed - whichever one it is. Different factors combine here and there, sometimes to add to each other, or cancel out or subtract from each other.

And those units are inches of mercury.

The units are inches of mercury, usually abbreviated In Hg.

It's possible that the timing is significantly retarded at 1500 RPM compared to 2100 RPM for emissions purposes. In that case, the torque COULD be less for the same fuel burned. If so, the higher speed should get better mileage. Maybe. You would need to drive one way for a tank, the other way for the next tank, and repeat until you saw a clear pattern to know for sure.

The largest benefit of a vacuum gauge is as a guide to minimize the range from the lowest to highest vacuum that you use when driving a particular route. The steadier you control the gas pedal, the better your gas mileage. Except for accelerating, sometimes you are better off to just step on it and go.

If you can borrow a portable fuel air ratio meter, find the fuel air ratio vs vacuum. Rich mixtures give a little more power by burning a lot more fuel. Then try to stay below the fuel enrichment vacuum.

The premise behind watching manifold pressure/vacuum is to increase engine efficiency. That's not the same as the absolute quantity of fuel being used.

That manifold vacuum drops when climbing a hill, at least without getting into enrichment, is a good thing. Low(er) engine speed and low manifold vacuum/high manifold pressure reduces two of the sources of wasted work within the engine.

For insight, find a BSFC map that also has lines of constant power on it and you can trace the effects of engine rpm - determined by gearing for a given road speed - vs manifold pressure for the given power output required to maintain that road speed.

Sometimes it's possible to trade off the increased vehicle drag associated with a higher road speed against the benefit of the engine operating at a higher efficiency combination of load (manifold pressure) and rpm. That can mean even though the power requirement is higher, the quantity of fuel used is similar or lower.

Knowing if you are in enrichment or not is really useful. There are any number of simple LM3914 based gauges that will show you that at very low cost. The circuit I described and provided a schematic for, for indicating TPS or MAP voltage on a string of LEDs is based on one. For an O2 sensor application, merely delete the 330k resistor that is part of the voltage divider for the TPS or MAP input.

The components for that are ~$20. Alternatively find one of the pre-packaged kits for only a little more or an assembled version for about twice that.

With the vacuum gauge and O2 LED meter, aim for the lowest possible vacuum achievable without losing the closed loop oscillation on the LEDs i.e. without entering into the enrichment. Just when that occurs will vary with rpm and load.

Your resource is the MIA Frank Lee.

regards
mech

I believe the CIS VWs have a secondary throttle and that maybe opening causing the vacuum drop.

Frank's page is live, he's just not participating at this time. I've been keeping my eyes peeled.

Is this the Frank Lee page that you are thinking about?

http://ecomodder.com/forum/showthrea...peed-1477.html

^^Yup.

Tele-man's search-fu is the best. I didn't realize that I was in fact repeating Frank's own question. <facepalm> But RH77 came back with a reference to put some foundation under that house of cards, so it's all good.

Now, must strain non-engineer brain to understand all the big words...

BIG words are just lots of little words all-strung-together (ala' German language)...ha,ha!