Vacuume gauge

[Hot Georgia]

I have a question for you.
I'm too cheap to buy a real-life scan gauge for my other vehicle (Grand Caravan).
We've heard that vacuume gagues were once used for instantanious MPG reference and wounder if they provided accurate engine load information.

I could run a plastic vacuume tube alot easier than messing with wires but if they're not really that useful what's the point.

Thanks in advance
-Steve

[rh77]

Originally Posted by Hot Georgia

I have a question for you.
I'm too cheap to buy a real-life scan gauge for my other vehicle (Grand Caravan).
We've heard that vacuume gagues were once used for instantanious MPG reference and wounder if they provided accurate engine load information.

I could run a plastic vacuume tube alot easier than messing with wires but if they're not really that useful what's the point.

Thanks in advance
-Steve

It's very rudimentary and not too accurate, but it's something, at least, to show air movement and consequently fuel flow into the heads. A turbo boost gauge is pretty good at showing positive and negative pressure (negative when you're in drive and coasting). Most auto parts stores carry them or something similar. The hardest part is drilling through the firewall, followed by tapping into a VAC line and mounting the guage somewhere on the dash. If the set isn't complete, the parts store or hardware store can provide items like a pneumatic "T" and similar. I've only installed a boost guage on a turbocharged vehicle and used this technique; I would assume the process is similar for this application.

Oh and by the way -- I love the Avatar. Moo-hoo-ha-HA-HA (pinky to lip) ... right Mr. Bigglesworth?

edit: the ScanGauge plugs directly into the OBD-II port on every car after '96, so that's easy, but still expensive.

RH77
"Why can't I get a freakin' Hybrid that runs on freakin' Diesel"

[onegammyleg]

Hi Hot Georgia

¨We've heard that vacuume gagues were once used for instantanious MPG reference and wounder if they provided accurate engine load information.¨

Ahh , back in the age of dynosaurs (my era) every economy guy had a vacuum gauge.- me too.
Vac gauges came in either with a calibrated scale or the simple ¨power¨ normal¨economy¨ markings.

While it was the best we had at that time they , even then, gave limited benefit for , well , anyone.

If you forget about the fuel and ignition side of an engine for a moment you will see that and engine is just a big air pump.
When a pump is sucking air through an orifice (a hole) of a suitable small size in relation to the air volume a vacuum (sucking) is created.

A vac gauge will show this and not anything else.
In a petrol engine application it will continue to show just the same thing.

Vac gauge readings typically show high on coasting (engine breaking) a bit less at idle and it goes down untill it reaches almost zero on the gauge when the throttle is pressed to the carpet.

If you think about it this is what should you should expect to see from it.

When trying to use the gauge for FE driving I guess you would try and keep the vacuume reading as high as possible for any given speed load condition.

I have found that this tends to trick you into using too light a throttle and you lose speed , which will require more pedal later on to get back up to speed.

For FE purposes it was of marginal use in the 70's and now days there are better gauge offerings.

[omgwtfbyobbq]

If you have a BSFC map, one of 'em with a tach will tell you how efficiently your engine is operating.

[onegammyleg]

Hi omgwtfbyobbq

A BSFC map . darn it . I left my copy at home.

[omgwtfbyobbq]

Use the search Luke.
I've found two out of the three BSFC maps I've been looking for, but in all honesty, most of that info can probably only be had through SAE articles provided the engine is new enough, which will c0$t. I would think that most manufacturers would build engines that are most efficient at ~1/2-3/4 throttle through a reasonable rpm band for that vehicle, like 1.5-3k, or 2-4k... But, until the BSFC map is found, iyou won't know for sure, and it's a PITA to find out through experimentation.

[lindermant]

Hot Georgia,

I'm thinking of upgrading my SG-I to an SG-II, so mine might be for sale... you interested in an 'as new' SG-I?

[Coyote X]

I use a vacuum gauge on my car. If you are in a OBD1 car then a scangauge isn't quite as much use you have to get something like a supermid.

I think a vacuum gauge is a pretty handy thing to have on a car. It can tell you when something isn't running quite right. It can also tell you at what point when you are pushing the gas pedal down you are at 0 manifold vacuum so pushing the pedal down farther just makes the engine start moving to the wide open 11:1 max power ratio instead of the normal 14:1

It will not give you an exact mpg reading but if you have it somewhere you can see it all the time then it will give you a lot of extra information beyond what you can tell by seat of the pants. I think even when I put a mpg gauge in my car I will still keep the vacuum gauge because it is still the best way to tell early engine problems. That is how I noticed I have a exhaust valve starting to go down. It started bouncing at idle slightly. It is now bouncing a lot worse and the valve is starting to mess up my mileage but I have known about it for weeks already, just to lazy to fix it.



You can see I stuck a standard diagnostic vacuum gauge right on the top of my dash. It was the biggest one I could find as well as pretty cheap.