I installed an aftermarket gauge in my Suburban (5.7L) because I was planning to pull a trailer for a few years and I love it. Now my gas mileage sucks in general but at $4.50/gal in southern Ontario today every little way to save fuel counts. Cruising at 65-68mph with 15 to 17 inches of vacuum is as good as it gets for my truck and that still translates into no better than 16mpg. When I was trailering I was ecstatic if I averaged 10 however a tailwind and/or flat road surface was pretty much essential to achieve these numbers!
Using a vacuum gauge for engine diagnosis is different than using one as an engine monitoring instrument. The indash gauge is like the manifold pressure gauge on a constant speed prop air plane engine. Since you cannot use the propellor RPM as an indicator of power output, the MP gauge lets the pilot know how much power the engine is producing and as a proformance setting guide i.e. cruise power at 20 inches.
For your camper the higher the vacuum reading the better the fuel economy. One specification to learn is at what manifold vacuum power enrichment occurs. With carburated engines, power enrichment occurs in the 8-10 inch range. Some carbs (Holly) use a power valve while others us an enrichment needle that is lifted out of main jet as the manifold vacuum drops. So you would want to stay away from manifold vacuums below 10 or 8 depending on you rig. You can adjust the vacuum by slowing down or down shifting. If the 318 is fuel injected, you also might be able to learn the manifold vacuum at which power enrichment is programed. However, that setting may be keyed to the Manifold Absolute Pressure sensor reading which you cannot access.
If the manifold vacuum gets into the 0-5 inch range, it is definitely time to down shift and get the engine RPM close to the maximum power point (again get the specs on your engine and cam arrangement). At the 0-5 range even with power enrichment there may be detonation occuring even though you do not hear it so watch the coolant temperature gauge and back off to forestall overheat.
The vacuum gauge is a great tool to manage the operation of an engine. IMHO it is more useful than a tachometer unless you are racing.
Hope this helps.
Thanks, Researcher, very interesting. Most of my miles will be highway cruising at 55, where I’m pulling 20+ inches, so it doesn’t look like I need to worry about enrichment. I can’t get the needle to drop below 10 except by accelerating hard. But there’s no way at any steady speed to get below 20, so I guess I’ll count myself lucky on this score.
22" of vacuum sounds quite high for this type vehicle even at cruising speeds. The vacuum on my '88 Escort at cruising speed is usually in the 13-16" range and I’d think with a truck this size and a 318 you’d have to have a pretty wide throttle opening which would decrease vacuum. The vacuum on my Escort is only about 20" at idle with the throttle completely closed and about 23-24" on deceleration. I would come nearer believing 10-14" of vacuum at highway speed. Regardless of whether the gauge is calibrated correctly or not, for best fuel mileage keep it as high as possible in the highest gear that will get the job done. Also when you’re taking off or under high throttle conditions and have the throttle pretty much wide open and the engine under a load the gauge should probably read 0-5" vacuum and increase with speed and lighter throttle positions. If it never drops to 0 or near 0 under extremely heavy acceleration the gauge is not calibrated correctly.
Does the gauge needle rest on the ZERO stop pin when the engine is off? It is a Bourdon tube meter and unless the needle has slipped on the shaft it should be as accurate as it ever was.
Yup, needle goes to zero when not running.
I am curious, jesmed. If you ever confirm the gauges accuracy please post it here. I looked at my vacuum gauges and my recollection remains unchanged. But there are some cob webs and dust bunnies drifting around my head and the Cobbs and bunnies have become permanent guests so I will just live with it.
Thanks Rod, I will try to have my mechanic throw a vacuum gauge on the engine tomorrow when I take the van in for service. Will post back with results.
My old Caddy with a giant V8 used to hit 20-21" when cruising, no problem, and this was an old, worn motor. Do V8s maybe generate a little more vacuum at cruise? I haven’t put a vacuum gauge on a smaller engine at cruise. Maybe just a healthy motor?
I like the in-dash vacuum gauge because I never have to find it. It works without electricity and I’m sure that it must have a reason to exist.
Did you decide that the U-joints caused the rumble you talked about in the other thread?
Elly, the rumble disappeared mysteriously, so I can’t pin it down, but I believe it was torque convert clutch shudder.
I opine that a great place for the manifold vacuum guage would be in the center of the tachometer gauge. In one glance you could ascertain the performance of the engine i.e. the tach needle would swing around on the outer scale and the vacuum gauge swing around on the inner scale optimizing dash space.
The 1968 Hemi Charge had a clock in the center location. IMHO that is not the best place for a clock. Let the Christmas tree and trap lights do the timing.
My B250 has a small hourglass in the center of the dash for timing the quarter mile… 
Update:
I took the van in for repairs back in May, and since then, my vacuum readings have dropped to a max of about 15 inches Hg at 55 mph. Before the repairs, I was getting 20+ inches Hg.
The mechanic didn’t put vacuum gauge on the manifold to verify the in-dash vacuum gauge readings, But I have no reason to doubt the gauge’s accuracy. He did replace the PCV valve at my request, and that’s the only thing he did to the engine that I can imagine might have affected the vacuum readings.
So now my question is, is it reasonable to assume that the reduced vacuum readings (from 20+ inches down to 15 inches) at 55 mph is due to the new PCV valve? Does that mean the old PCV valve was plugged up?
By the way, I’m getting about 13 mpg on the highway with this beast…the 318 engine in a B250 camper van with a ginormous fiberglass camper top…the size of a rolling barn, so I’m not surprised. I was hoping for 15 mpg, but it is what it is.