Tach & Vacuum Gauge Installed
 

Well, after reading about how poeple are improving their driving skills by literally installing a piece of instrumentation on their dash, I took it upon myself to try this method and now see why many are opting for this route.

I had absolutely no idea the amount of fuel you could save alone by just installing a vacuum gauge to monitor how heavy your foot is on the pedal.

For the TBI (throttle body injection) GM guys, instead of drilling into your intake and tapping the vacuum gauge in through that route, or running t-fittings into vacuum lines, there is a 9/16" bolt just below the air cleaner directly on the TBI. I used a male compressor coupler and bolted that into this area and hooked my vacuum line directly to that. No drilling, coupler was a direct bolt-in and literally took 5 minutes.

I am noticing that at idle my truck runs at 20 on the vacuum gauge and under load depending on what I am doing and how I am driving it averages about 8-12.

At 60 km/h, I am running at about 2200 rpm and at idel 800-900.

This is a cube van with a 14' box and has a 350 5.7 liter with a 3spd tranny. Before these two installs, I was averaging 27 liters per 100/km or 11MPG. I have just installed these two pieces and switched out the old fuel filter and cleaned the TBI. I will advise on what sort of fuel savings I am getting into within the next couple of fill ups.

Nice. I too hope to install a vac gauge on my truck to fine-tune my gas saving driving skills.

Excellent. Let us know how it goes. I was thinking of installing one in my '96 Ranger with the 4-speed auto, but I'd like to see how well it works for somebody else with a large engine and automatic.

800-900 idle rpm might be a smidgen high. If you've got AC, check again with heater control set to off. If that reduces idle rpms a bit, it's fine; you're done. If not, double check manufacturer's specifications.

Engines differ, but if traffic allows 55 km/h @ ~2,000 rpm may give significantly better FE. A bsfc map/chart would be ideal for an initial guess at best speed/rpm for FE, but they are harder to find than hen's teeth. If you can find a dyno chart of torque, power & rpm for a stock 1989 GM 3.7L TBI, that would also be useful

Is 800-900 rpm just an approximation for a steady idle at 8xx +-5 rpm or does it lope? Loping idle indicates a vacuum leak or some other problem.

FWIW Reading a vacuum gauge give a pretty good summary of various other things a vacuum gauge can tell you.

Couple things I noticed today.....

Today compared to yesterday was lot cooler +11 Yesterday compared to -6 today. My vacuum gauge yesterday was at 20 and today it is at 16. Is cold a factor when it comes to reading vacuum gauges?

Second.

When I am at 60km/h my trucks RPM's are at 2200 and at 100km/h they are at 3000 RPM's. This is a 5.7 TBI 1989 350 with a 3 spd tranny. Seems a little high if you ask me although this truck is a cube van and is meant to haul.

Is there anyway I can lower the RPM's drastically without putting in a 4spd or swapping gears yet maintain driveability and increase fuel mileage?

I am not sure what the factory gears are in the rear-end on this truck. Something I will have to look into

I think cold is a factor because the engine is actually working that much harder. On my Honda, my normal idle is 21-22, but when I start the engine on a cold morning (<32 degrees), it's 16-17, and then when I let the clutch out in neutral, the extra drag of the cold transmission lube drops the idle to 14-15. Also, higher altitudes seem to reduce vacuum as well, probably because the engine needs to work harder to get the same amount of oxygen in.

The reason you're seeing the difference in vacuum in colder weather is because it is putting more load on the engine therefore lowering vacuum.

One thing that can be done to change the gearing and is very simple is go to a larger wheel and tire.

Haha, I can hear it now
"Cube van sitting on 23's,
I'll be getting 17 mpg's.
Drivin' all over picking up homies.
Deliverin' packages for mad money!"

Your lyrics are tyte yo!

Switching to a bigger tire wheel is going to add rotatioal weight thus decreasing FE and lowering mpg.

There should be an RPO code in the glove box that tells you the gearing.

I know s-10 are these numbers:

GU4=3.08
GU5=3.23
GU6=3.42
GT4=3.73
GT5=4.10
HC4=4.56

G80 - locking differential