The Stovebolt Site Forums The Shop Area Engine Shop Driving Vacuum Gauge

Not sure if this is the right place to post this but I'll try it.

I read somewhere on here about some older vehciles having an in the dash vacuum gauge. Evidently you can modify your driving style by watching the gauge in order to get the best mileage. I'm intrigued by this idea. I need to get a new vacuum gauge because the last one I got from HF isn't working right. I thought maybe I'd run a line from the intake back into the cab and hang the gauge from my GPS mount.

If I do this, what am I looking for? Should I try to keep the vacuum above a certain level? What does right look like while you're driving?

If the concept proves useful, I'll have to find an actual vacuum gauge that can be properly mounted. Anyone have any ideas on a source for one of those?

Hey Larry, I saw under dash,vacuum gauges on sale at Wally World ( they just opened a new one near us). Just like the Econ gauge in some of the new vehicles,you want to try to keep the highest vacuum reading to get the best fuel economy. The more load you put on an engine the lower the vacuum.A steady reading of 23 inches at idle with no load is great.On a hill climb your vacuum will drop to 2-3 inches.Old timers would say to drive like an egg is between your foot and the gas pedal.Waste of good eggs if you ask me !! Hope this helps.

Plymouth had one of these on the drivers Ed. car I used in High School. The instructor loved to tell when we ignored it. Yes it was just after horses were phasing out. ")

I can remember seeing mounted in the console ahead of the gear shift on some cars in the late 60's(?). I think it might have been Pontiac. They had a green zone for economy.

These units are always on ebay, lots of cool vintage pieces and they are great for setting timing and adjusting carb.

Larry,
You should be able to pick up a gauge at Auto Zone, Advance Auto, NAPA etc.
Vacuum gauges show when the engine is working in the most efficient manner. The driver can adjust the gear and throttle position to provide the optimum fuel economy by monitoring the engine vacuum.

Identify a vacuum line on your vehicle's engine. Vacuum lines are typically small, soft black rubber hoses. They are located on your vehicle's intake manifold. Once a vacuum line is identified, cut it in half with a razor blade knife and insert the plastic "T" from the gauge kit.
Install the vacuum gauge tubing. Insert tubing from gauge kit into the free end of the plastic tee that you have installed into the vehicle's vacuum line. Run this new tubing from the "T" fitting to the firewall of the vehicle making sure to route it away from moving parts and use zip ties to secure it. Run the new tubing through an access hole in your vehicle's firewall and into the interior of the vehicle.
Mount the gauge. Find a location where the new vacuum gauge will fit without interfering with any of the vehicle's equipment and is in the driver's sight. Place a strip of Velcro (hook and loop tape) on the flange of the gauge's mounting bracket and another strip onto the vehicle's dash. Insert the vacuum tubing into the rear of the gauge following the manufacturer's directions on the package. Once the tubing is attached, push the gauge against the Velcro on the dash.
Start the engine. Check the gauge for movement to ensure it's working. Remove the hood prop rod and close the hood.

A quick & fairly easy job,
Harold

It can be used to test your cruising mixture.
Find a slight grade that requires light throttle to maintain speed, and record your highest vacuum setting.
Any jet or metering rod change that brings the vacuum up means better mileage = more efficient.
Same can be done with spark timing but remember you're dealing with the vacuum advance (not the initial or centrifugal).

Gentlemen;

All good input. Thanks.

That leads me to a related question. My burb has a 78 250 with the integrated head. It has a Delco Remy HEI distributor. There is a vacuum advance on the distributor with a line running to the manifold below the carb. My question is, how do I know if the vacuum advance is working or not? I've seen videos of the "old fashioned" distributors visibly rotating a bit when the vacuum changes. Mine doesn't do anything that I can see. Is there a test to check it out?

I'm curious because when I punch the throttle the vacuum drops instantly to 0 and the engine sputters a bit before catching back up. If I smoothly but quickly push the throttle it drops a little but the engine doesn't sputter. Its on the initial "stomp" that the vacuum goes to 0. Is this normal?

I found a "guide to the vacuum gauge" on line that talked about all the abnormal readings/issues with the gauge. My engine doesn't seem to have any of those issues. The only thing of concern is the depletion of vacuum on hard acceleration.

pull the end of the hose off at the dist, with the dist cap off
put a vacuum source on the dist and see if the dist plate moves.

Vacum source could be a hand held vacuum devise, or the one most people use a piece of hose and your mouth.

Here is an interesting interactive vacuum gauge link. Scroll to the bottom of the page and run through the various scenarios.. good stuff : http://www.secondchancegarage.com/public/186.cfm

One of the gauges I installed along with volts, oil press. and water temp. gauges in my '65 is a vacuum gage. In regard to tuning, a vacuum gage is essential.
http://i645.photobucket.com/albums/...nt%20Jan%202010/InteriorRepaint003-1.jpg

Thanks for the link to the secondchancegarage.com site John. Very interesting material there that we can all benefit from.

I had a 1975 plymouth VALIANT that had a light on the front fender that would lite up when you pushed down on the gas to much (DRIVER-SIDE).I asked the dealer at the time about the lite and his reply was (GET THIS)If you are using to much gas to drive the car it comes on to make you back off on the gas pedal to conserve fuel.That lite drove my wife crazy.She believed if a lite comes on there is something wrong.I cut and taped the wire that went to it.

understanding the vacuum readings video

What most of those vacuum gauge tutorials omit is that at idle you might see a 1-2in drop on the guage. Its worse with a 4 cylinder. I was chasing the needle on my L134 in the jeep and it turns out that, at idle, a 4 cylinder will bounce 1-2 points.. is a valve timing issue. 6's and 8s don't bounce much, if at all at idle.

Thanks for the continued ideas. I'm on the road this week. I'll check out the tutorial video and play around on the truck this weekend. I'll keep you posted.

The position of the vacuum tap matters.
A tap too close to an individual throttle disc makes the gauge "nervous" at low speed.
The more buffer (plenum) volume between the tap and the disc the steadier the reading will be.
Be sure to get the spark right before making mixture adjustments - more initial advance has a big effect on idle vacuum!

A lot of the time you don't get a choice. My L134 had two taps by my GMC270 only has one.

The at idle bounce is not an indication of an issue (although a lot of the tutorials will say that a bounce at idle is an issue) but if its only 5-10in drops, not 1-2in ticks.

Taking the motor even slightly off idle stops the bounce and does not interfere with other inductors.

I finally got a chance to play around with the vacuum gauge this weekend. The first thing I did was zero the gauge. I figured out that the rubber cover and plastic face plate on the gauge were removable. There is a small flat head screw on the face of the gauge that can be turned to adjust the needle. Out of the box from HF, mine bottomed out at 10 instead of 0. I saved myself having to buy a new gauge.

I hooked up the gauge to a port on the intake manifold and set about checking the timing. I was using a cheapo HF timing light so it was a bit challenging. I unhooked the vacuum line from the advance on the distributor and plugged the line. I had the timing right where I thought it was good. The problem arose when I hooked the vacuum line back up to the advance. As soon as I got the line even partway on the nipple the engine would instantly die. Hooking the vacuum line to the advance killed the engine every time. I had to call my man Joe H to have him help me figure out the issue. I've got it running pretty good now with the advance hooked back up, but I'm not totally convinced I solved my issue.

I got a long piece of vacuum line from my FLAPS and ran it through the firewall to the cab. I hung my vacuum gauge from my GPS mount and was able to read it while driving. It made for some very interesting driving. It's amazing how little throttle will make the vacuum drop. I found that if I could hold the vacuum above 10 inches while driving around the local area it seemed pretty economical. I then took it out on the interstate for a test drive. The first thing I discovered was how non-aerodynamic my Burb is. There was about a 20 mph wind that was blowing west to east parallel to the highway. I tried to keep the vacuum at or above 10 inches while driving. On the flats or downhill it would go as high as 15-17. I had to baby it going uphill to keep it above 10. Running with the wind I was able to maintain 60-65mph consistently without going below 10. Running into the wind it was more like 50-55 without busting 10. It was amazing how much difference the headwind made.

I welcome any thoughts/suggestions on my vacuum advance-timing issue and any interpretations on my driving gauge readings. Should I try a scientific experiment on timing settings. I'm think of finding a long hill that I can fun up and down. If I pick a speed and document my vacuum readings can I play with the timing in order to try and get the vacuum as high as possible at the given speed on the same hill??

Mechanical timing is only going to get you so close... and as things wear the timing will shift yet again so I would really aim for the best average of all speeds.

The advance setting that works on flat ground may ping the motor up a hill... that's why the vacuum advance helps so much... on flat ground, high vacuum, lots of advance. Up a hill low vacuum, a little less advance.

You can get a 'fuel economy' vacuum gauge and drive that.. its pretty much what you want to do but in a nice color coded gauge.

You *could* look at the old Model T' and A's... they had manual spark advance so you could dial in the motor.... but with vacuum advance and mechanical/spring loaded dist weights much of the 'easy stuff' is already taken care of.

Pfarber;

Hooah. Thanks. I don't want to make this super complicated. I'll look for a fuel economy vacuum gauge.