Vacuum Gauge Experiment

[Millermagic]

At 70 MPH, I get 34 mpg - It has been proven on a few different trips. Given that information, I'd like to take a vacuum reading at 70mph and then that would give me an idea of how much fuel I'd be using at whatever the vacuum pressure is.

From that, I could find the ratio between vacuum pressure and fuel consumption per hour and take the vacuum measurement at various speeds and then figure out fuel consumption per hour ... then divice how many gallons used in an hour into how many miles traveled in the hour (80 Miles \ xgallons).

I could use that to figure it out right?

Which brings me on to my next question - how do I install a vacuum gauge? I was looking at a few online and I found one that looks like the trans temp gauge I might buy over winter break. It says it comes with tubing. Does that mean that I have to put a hole in a vacuum line somewhere and then run the tubing to the back of the gauge that's mounted inside the car? How hard is this to do?

[Ian Szgatti]

it isn't hard to do at all... if you have the capacity to come up with some of the thoughts you do, you'll be just fine in coming up with a solution. You might find an unused vac port on the intake manifold, or have to create one.

[GreyGoose006]

i may be wrong, but i think that it will be inacurate at lower speeds if you calibrate it for highway driving. air resistance plays a huge part in the drag.
w/o air resistance it would only take 10 hp or so to maintain 70 on the highway.

not only that, but there are gear changes. if you had a one speed trans, then it would surely work... one engine revolution = x wheel revolutions. there would be a direct relationship.

i'm not sure if the ratio would work out perfectly, but it might not matter if you calibrate it at the speed you will be driving the most often, or at least the speed that most impacts your mileage.
if you are one of these guys that accelerates hard during normal city driving, than it may be best to try to calibrate the thing at a lower speed.

what is the final goal?
to get miles per gallon bsed on vacuum pressure?
i'm not sure if vacuum pressure is a linear progression in an engine. it might be tho, so you will have to do your own experimentation to figure that out.
also, if you are going to get it to work, you would be smart to check every vacuum fitting for leaks.
you would also want to take the measurement as close to the manifold as possible. that way you get a better response.
in other words, dont take your measurement after the vacuum resivoir (if you have one)

sounds like a great project tho.
good luck.

p.s. even if it dosent work as planned, a vacuum gauge still can be used to determine how much throttle you are using. sometimes it gets away from you. it would help keep your mileage up if you tried to keep vacuum above a certain value, for instance.

[razr_88]

i have a vacuum gauge installed on my olds 3.8L and , it will tell you engine load. for example if you go up a hill the gauge will show you less vacuum than if you were to go down the hill. typically the more load an engine has the more fuel it will consume. However, if the engine were at really high rpms (which wastes a lot of fuel neways) and the the load is low the vacuum will be really high. In general it will show the load (which can be attributed almost directly to efficiency) according to the specific rpm.

[razr_88]

oh and for installation i took a t-fitting and spliced into the vacuum hose connecting to the fuel regulator and it works just great. i then feed the hose through the body left body panel and cut a small hole in the wire protecter that goes from the driver door to the door frame. this then goes into the inside of the dashboard. be sure to eliminate all possibilities of pinching the hose, because a leak will make your engine run lean and then the computer will have to dump more fuel into the engine to keep the ratio.

[GreyGoose006]

im sorry razr, but you are completely wrong, but you have the right idea.

vacuum gauges do nothing but show the vacuum present after the throttle.
this can be used to infer many things about an engine, but ultimately, it only shows vacuum.

typical midsize cars with 2-3 litre engines have their highest vacuum reading at idle, or within 100 rpm of it.
however, the bigger and torquier your engine, the more you will notice that the highest vacuum reading occurs somewhere between your torque peak, and idle, more favoring the torque peak.
this area is known as peak BSFC, or brake specific fuel consumption.
a BSFC map shows how much fuel an engine needs to make x hp at x rpm.
you are looking for the place where the engine uses the least fuel per horsepower.
this point in the rev-band is usually located at the vacuum peak.

so...

if you install a vacuum guage and drive around in top gear on the highway, you will notice a point at which the vacuum gauge reads maximum vacuum.
this is your ideal cruising speed.




as for the load on the engine, it is hard to explain, but think of an engine like a washing machine.
you wouldnt run a wash if all you wanted was to wash a single sock.
you wait until you have a full load and do a wash.
likewise, with an engine, if you want the most mpg, dont rev your engine past its torque peak (max BSFC)

this means that you keep your revs low and your throttle opening high.

actually, the ideal configuration is to run the engine at the peak BSFC at 85% of full throttle, and use gearing to modulate your speed.