Vacuum Gauge

[Devin]

Check out my new vacuum gauge. I got a plastic pillar cover from the junk yard and fiberglassed a 90 pvc elbow to it. I happened to have some paint in the exact color of the plastic. I should have gone in with some bondo to fill in after i sanded back the fiberglass but oh well.
Its awesome driving with this thing. I haven't finished a tank with it yet but I'm sure it's going to help economy. Its amazing how the tiniest movement in throttle affects vacuum. Any one else put one of these in?

[Mark]

I've always wanted to install one of those, but I'd probably be looking at it so often I'd get into an accident.

[dlb]

devin, i've been mulling over a vacuum gauge vs an MPGuino (a simple fuel economy computer for OB-I cars) for my 4wd corolla wagon. the MPGuino is about 3x as much as a vacuum gauge but gives tons of real time info so i'm just talking with some guys on the ecomodder forum to see if it is worth the additional cost.

i'm dying to hear how the gauge helps you temper your driving to improve economy. have you changed any habits already?

[Devin]

Dlb, like I said I haven't gotten a full tank on it yet but I have definitely changed some habits. There are places in my normal commute where before I had been keeping the rpms high so as to not bog down, but with the vacuum i see its under very little load. The reverse has been true too. Some familiar hills I had been taking in 3rd when really that brings the vacuum way down to like 5. It's also cool how you can up your vacuum by backing off the throttle and still keep basically the same speed. Its just nice to have one more input to your motor. Plus I like how old school it is. And people think you have a mad turbo making 18psi of boost in your t4.

[Petros]

manifold vacuum is a good indicator, but not the complete picture. High vacuum means low density fuel/air charge to the cylinders= low fuel consumption. Low manifold vacuum mean high density fuel/air mixture=high fuel consumption. so it servers as a kind of a way to determine the load on the engine, vacuum low going up a big hill for instance, and very high when going down the hill.

Usually the MPG meters measure distance traveled and actual fuel consumption, so it is a more direct measurement. most of them will also give you miles traveled, average over a long distance, remaining range until next fuel stop, etc. all useful on a trip, but nothing you could not work out by other means as you drive. You would not be able to get instant MPG at any given speed with a vac gauge or by measure mileage and fuel used at each fill up. But with either a vac gauge, or an MPG gauge that gives instant MPG at any given time, both can be used to tailor your driving habits to get better overall fuel economy (which is really what matters).

[xirdneh]

where on the maniflod did you hook it up?

[Devin]

There's a little plug with two outputs on the manifold. One hose goes to the tvsv tree and the other goes into the metal piping. I just teed into one of those.

[garyfish]

here's a link to an excellent website that explains "How to Use and Interpret a Vacuum Gauge"
http://www.secondchancegarage.com/public/186.cfm

found the link right here on the Forum, originally posted in 2010:
https://tercel4wd.com/forums/viewtopic.php?f=10&t=5629

[dlb]

gary, the graphic at the bottom of that link is awesome. so informative! who knew how useful a vacuum gauge could be!

at this point, i'm leaning towards a vac gauge for my 4wd corolla (because i'm curious about them, the cheaper price, and the fact i don't think i drive my car enough to justify a $70 fuel economy tool), and an MPGuino for my wife's civic since we put most of our kms on it. who knows, it might even help temper her lead foot...yeah right.

[dlb]

devin, where did you get your gauge and how much was it? i bought one on ebay that turned out to be typical chinese junk, brand new and no good. i hooked it up and it read 11" at idle. that seemed way too low so i grabbed the vacuum/pressure gauge i bought for testing last year but had yet to use, hooked it up, and got 19.5". i'm getting a full refund on the junk gauge and am now i'm looking for one that is made here but still inexpensive. i'm thinking about this one.

http://www.ebay.com/itm/201062811463?it ... 23&vxp=mtr

how is driving by vacuum going, btw?

[Devin]

I got mine from rockauto, its from Sun. I think it was like 14 or 15 bucks. Its funny when I hooked it up I drove a full tank trying to keep the vacuum as high as possible. This resulted in the lowest mpg i have ever had, lol. I think like 25. I have learned that high vacuum doesn't equal mpg. The whole concept of pulse and glide is huge. They say the engine is most efficient around 5 psi. So you pulse using your efficient vacuum and then glide it out with some good high vacuum. If you think about it, driving with high vacuum would mean motoring around in 1st gear at high rpm and going 7 mph...
It's all about finding the balance of the big three; accelerator position, vacuum and rpm. Since I have adjusted to driving with the vacuum gauge, I have definitely seen and increase. Way better than 25, and a lot better than before the gauge. After a month of driving with it, you will know exactly what I'm talking about.

[dlb]

good timing. i received my second vacuum gauge in the mail yesterday, tested it (this one works well, it's USA-made by accurate instruments, part #15603), and installed it immediately. went for a short test drive and noticed that downshifting even while cruising happily increased the vacuum. this wasn't surprising since the lower gear means higher rpm which means more sucking coming from the pistons, but it made me think that high vacuum may not the be all, end all for fuel economy.

i've done some searching online for instructions on how to drive with a vacuum gauge to improve fuel economy but mostly just found other people asking the same question, and never getting a complete answer. the installation instructions that came with my gauge have a section on this though, so i'm going to transcribe it here. most of it sounds good to me, except for the last paragraph which i totally disagree with based on your findings devin, and other similar accounts i've found online.


how to use the manifold vacuum gauge to increase gasoline mileage

when traveling on upgrades or even hills, the gauge will indicate lower readings as you accelerate in order to maintain your speed. when the points falls to 7"-9" of vacuum, you should shift to a lower gear to prevent overloading or lugging of the engine.

on decelerating, or traveling down hill the gauge will operate above 21". it can be dangerous to lubricated engine parts if you drive with the pointer in this section for long periods of time, as this indicates the engine is sucking up oil from the crankcase. touching the accelerator occasionally, or slowing down and shifting to a lower gear will avoid this situation.

due to the fact that there is a limit to the amount of gasoline that the engine can use effectively, it is good economy not to accelerate so fast as to cause the point of the manifold vacuum gauge to operate below 5" of vacuum. this indicates that a surplus of gasoline is being supplied to the engine, and this excessive amount of fuel is actually going out the exhaust, or running down the cylinder walls and diluting the engine oil.

on fast pick-up speeds, the gauge will normally operate between 5"-10". however this is uneconomical, and after experimenting you will find that you can pull away from stops and keep the pointer above 10" and therefore save on gasoline.

always remember that the higher the reading on your manifold vacuum gauge, the more you increase your gasoline mileage.

[Devin]

Yea I drove for a whole tank just trying to keep high vacuum and got horrible mileage. Seeing that needle high makes you think that your motor isn't working hard. My new approach is to not pretend that there is no work to be done by your engine. Like say you are going from 0 to 35 miles an hour; it's crazy to think that your motor won't have to work to get you there. A proper amount of engine load ( around 5 to 7 from what I read ) for a short amount of time is more efficient than high vacuum for a long time. On the other hand, It's better to drive up hill with as high vacuum as allows for safe speed.
Post your mpg after you run some tanks!

[dlb]

just finished my first full tank with the vacuum gauge. it was about 80% city driving and took me 3 weeks to use the whole tank yet it is the 4th best tank i've had since owning the car for almost a year and a half, at 13.8 km/L (32.5 mpg). all the better tanks i've achieved were entirely hwy driving. my average since march has been fairly steady at 12.5 km/L (29.5 mpg) so that's about an 11% increase.

all that being said, i realize this is just one tank and it will take a few months before i have enough info to really compare average economy with this time last year. still, this is a great start so i'm hopeful.

[dlb]

finished two more tanks and i am consistently getting 10% better than usual for the same weather and driving conditions. my previous best had been 14.1 km/L (33 mpg) and that was all hwy, no passengers, all windows done up, over one mountain pass. i just smashed that and got 15.7 km/L (37 mpg) and it was all hwy, 3 passengers and the back full of our camping stuff, windows down, over two mountain passes. i definitely drove quite a bit slower than normal but i'm ok with that.