The Stovebolt Site Forums The Shop Area Engine Shop vacuum gauge inside cab

HI...we have seen some trucks with a vacuum gauge installed in the gauge cluster. Does this cause one to drive differently? What are the merits? We use a vacuum gauge at tune-up/timing on the 235 but never have thought of having one in the dash. Were they ever a standard item on any cars/trucks? I would like to hear from anyone that has one and what are its merits. We are thinking of buying one. Thanks, Steve

A vacuum gauge is standard equipment on my 66 barracuda as a "Performance Indicator". By paying attention to it I can get better mileage. It also gives me a clue when something isn't right with the engine.

The vacuum gauge can diagnose a variety of engine malfunctions if you take the time to learn what the readings might indicate.

Thanks Tim, that link is great. I'm printing one for my tool box.

If anyone is still following this thread, I have to disagree with some of the information contained in the link provided in the earlier post. So, taking each condition separately and following with the illustrations and information provided in that link:

Normal engine: Information given is good- no question there.

Steady low: Ok, but there are other causes for that condition. With this, and the other "... low vacuum" readings, the gauge connection itself should always be a suspect.

Steady very low: Ok, but other indications will be far more prevalent than just low vacuum- sluggishness, backfiring (intake or exhaust), hard starting, etc. If I saw that indication in the shop, I would use it only to substantiate the suspicion of valve timing- but if valve timing is off enough to cause that low level of vacuum, most often the ignition timing will be off as well- distributors are camshaft-driven.

Steady, extremely low: If there is an intake leak large enough to cause that level of manifold vacuum, the fuel mixture will be too lean for the engine to continue running. If by chance an engine were still running with that large an intake leak, no matter where, the speed would be so erratic that the vacuum gauge would fluctuating severely- half-scale or so. Not good information.

Needle rhythmically drops: Ok, but you will seldom see more than 1 1/2 inches of mercury drop with a compression misfire, and the drop depends on the compression loss and engine speed. An ignition misfire causes the condition described, but the drop is seldom more than 1 "Hg. An intake manifold leak affecting one cylinder more than others will give a similar vacuum indication, but will diminish as engine speed increases. Ok with the sticking valve idea.

Drops to 0, then rises: The indication is correctly described, but the stated causes are pretty shaky. With a sudden full-throttle acceleration, the vacuum shown by a gauge will reach zero, if for no other reasons than momentum of the gauge mechanism, spring tension of the internal tube, and slow acceleration of the engine- some accelerate faster than others when the throttle is snapped fully open. Also, some gauge movements are heavily damped to prevent immediate responses to large changes, so gauges from two manufacturers may give different readings. The possibilities given are technically valid, but you will not see the results with an engine that is not loaded. If you run an unloaded engine long enough at full throttle to reach 4-5 "Hg, you risk catastrophic failure. Those indications would be valid if the engine/vehicle is loaded with a dynamometer, but don't try that in your driveway with no load on the engine. An exhaust restriction doesn't show up with that check.

Needle wavers rapidly, 10-20 "Hg: Not likely for the causes given; that level of fluctuation is showing the "death thoes" of and engine that is only a few revolutions from stalling. The possibilities cited will cause the same indications as compression loss/compression misfire, although the indications may be intermittent or erratic. The slow valve opening cited is incorrect- remember valves open under camshaft/lifter/rocker arm force, then close under the lesser spring force, so slow closing, not slow opening. The irregular wavering described with that same indication is somewhat all right, but higher speed above 1200-1400 RPM will diminish the indications to the point of being invisible.

Drifts at idle, stabilizes at high speed: No. Drifting at idle speed represents a fuel mixture situation- rich or lean are similar. The burned valve causes a compression misfire that does, in fact, diminish as speed increases; at low speed, compression misfire will appear as the regular, rythmic drop. See the condition described in "Needle rhythmically drops." It can't be both ways- and it isn't.

Needle jumps 2-5...: That is a normal indication. The return to "normal" is dependent on the speed of deceleration- a loose engine decelerates slower than does a tight engine. An exhaust restriction does in fact cause back pressure. In that state, air doesn't exit the engine properly, so air doesn't enter the engine in the proper quantity, therefore, lower manifold vacuum rather than higher vacuum. And under deceleration, there is less air going into the engine, so there is less air exiting the engine, so exhaust restriction becomes less of an effect. High air flow, large effect from restriction; low air flow, low effect from restriction.

Needle does not jump...: Technically correct, but if piston rings are worn to that extent, the likely indication will be a heavy cloud of exhaust smoke that is a much better indicator of ring/cylinder wear than is a vacuum reading. But this is technically correct, just don't overhaul your engine based on that indication. A wet compression check is a much better indication of ring/cylinder wear.

Not mentioned: Exhaust restriction will show up as a declining vacuum level as engine speed increases. Holding the engine at a given speed will stabilize the vacuum at what ever level exists for the throttle position and the degree of restriction: wider throttle, more restriction, lower vacuum level.

There can be more said about vacuum checks.

Harvester

I have one in the cab of my 37. I permantly mounted it there because I like the gauge and it does help me keep my foot out of "it".
The older Mercedes gasoline engine vehicles had them in the gauge cluster.
One thing that throws a vacuum gauge off is mulitple carburetion.

Does this vacuume range apply to all stock Inline 6 and V8 Chevy Engines? I realize that some hotter Cams will mess up your low speed Vacuume, but with a stock motor is this pretty much acros the board?

I believe the vacuum range will apply to all throttle managed engines regardless of the number of cylinders but the ones with few cylinders might show a pulsating vacuum if the revs were low enough.
The engines with radical valve timing show lower vacuum at reduced revs because of leakage out the exhaust valve during the increased/longer overlap period when both valves are open.As the revs come up this leakage doesn't have time to effect much on the faster flowing inlet charge.
Vacuum readings are higher when the throttle plate restricts the engine from getting all the mixture it wants so the same throttle plate position will give increasingly higher vacuum readings as the rpms come up.
The carb power circuits are designed to richen up as the vacuum decreases/throttle opens so a vacuum guage visable and used by the driver can help increase fuel economy.
Basically we can say that driving to always give the highest vacuum reading will give the highest economy---and drive your passenger and motorists around you crazy.You will be driving like "old folks"-crawling away from stops-dropping speed on hills,etc.But seriously it is a useful driving tool probably more effective for the average driver than a tach. and adds a new challenge in driving.
One thing more before the arguement starts.I am considering lower vacuum a lower number on the normal vacuum guage but actually the higher number is a lower vacuum.
Vacuum is the condition when pressure is reduced below zero on the pressure guage.So zero would be no pressure-no vacuum and 10 on the vacuum guage would be a minus on the pressure guage.

When you hook up the vac gauge, do you use the floor or the carpet attachment?

**snicker**

Most engines in the emission-control era won't idle at a vacuum level much above about 18"Hg, mostly because of retarded ignition timing. But even with lower idling manifold vacuum, vacuum gauge indications are still valid and useful. A difficulty with both after-market and OEM dash-mounted gauges is that they are usually heavily damped, and as a result they don't show some of the useful information that a test instrument will show.

Adding some to previous information, "vacuum" as normally used refers to absolute pressure that is below atmospheric pressure and is measured in either inches of mercury (in hg, or "Hg) or millimeters of mercury (mm Hg). Standard atmospheric pressure at sea level in pounds per square inch is 14.7, which translates to 29.92 "Hg and 760 mm Hg. The full-scale vacuum gauge reading for those two measures in nice round numbers is 30 "Hg or 760 mm hg.

So when you open the throttle in a running engine, the manifold pressure rises (toward atmospheric pressure) and manifold vacuum lowers proportionally.

Harvester

I hope I'm not opening a can of worms here, in a nutshell, how would you use the vacuum gauge to get correct timing?

Slowly rotate the distributor (in both directions) at idle until you get maximum vacuum.

WOW...a lot of great information...now to find a nice looking small vacuum gauge that will fit under the dash.

SD66Burb brought up a good point answered by Tim. I was told that one must also deduct from the vacuum reading for each 1,000ft. of altitude from the highest reading. I think it was 1HG for every 1,000 ft. (I'm probably wrong) what is a correct deduction ratio? My uncle has always timed his engines with a vacuum gauge and I use the BB timing light method which we learned in high school....we are always at odds on this subject is one way really better for these old sixes?

Thanks, Steve

Who,me you say you would like to find a small vacuumm guage to fit for a driving aid.Might I say I have found that a large face guage is more useful than a tiny one that strains you to see small changes.
I once had a 67 Impala w/327 that a PO had installed a commercial guage in a pod about the size of a Sun tach on the upper left corner of the dash.That was one that I could use because location and size made changes visable without having to "look for it".One factory installed on my turbocharged F85 olds was a compound guage showing boost and vacuum but was mounted on the console and was less usful.Bigger is better.
Care and a good feel is called for when using a vacuum guage to set timing because of different timing setups from the factory.Static (idle) timing changes effect the timing over the whole Rpm range and when the advance mechanisms come in action the timing can be over advanced for the conditions.Remember what Jerry said about the detrimental effects of over advancing when looking for max power and RPM.It WILL limit you.
In my opinion using a vacuum guage to set timing is an approximation and not as good as a timing light if the engine is near stock.If modified with cam timing change or compression change then all the factory standards are just a reference point to work from as you arrive at a spark management curve that will give you the best performance.For sure if you just advance the static timing after a cam change to restore proper throttle response you will be over advanced when the advances are all in.

You may be thinking of a barometer. Barometric pressure is affected by altitude at approximately 1HG for every 1,000 ft. The pressure at sea level is pretty close to 30hg, here at 6,200 feet it is pretty close to 24hg, even though the weathermen report it adjusted to sea level.

I could be wrong, but I don't believe barometric pressure will have a significant affect on the operation of an internal combustion engine which has been tuned for best performance in its current environment.

FGT, You ought to hear the tales we here in the lowland hear from our tourists returning from your area after tackling pikes peak etc. with their motor home or hauler.Baro pressure has everything to do w/charging an IC engine and pumps and people.here's the way the people in Hydraulics school taught us to think concerning fluid flow(did we know that gases are considered a fluid);as Harvester was saying:a vacuum is not---anything, just a lack of air pressure stacked up above us in the atmosphere and totalling approx 14.7 PSI @ sea level.Higher elev=taller stack/higher pressure and lowr elev=lower stack/lower pressure.A pump/engine/human are pumps-something in them in operation makes a void wanting to be filled by fluid and the needed fluid (liquid or gas) is pushed in by the available baro pressure.
The most urgent need for the supercharger was not only for power nuts but to increase the power/efficiency of aircraft which in normal operation have drastic pressure changes and the need for fuel/air mixture compensation.
One common major use for the supercharger/turbosupercharger was in diesel engines used at higher elev and Cat,Cummins,Murphy,and all the big players had kits or original equipment to make their engine produce the SAME power as the naturally aspirated one @ lower elevations.And their advertising performance charts usually gave power ratings based on an elevation standard.After all it wouldn't be good for the reputation if a product didn't do the job because of an overlooked detail.Even Breaks&Scrapum, in their small engines list and probably originally furnish different jetting for different elevation operation--has anyone else cussed and mourned that we don't have adjustable mainjets for our small engines now?I had a relative who on moving to the Colo area had to retune their lawnmower(carried from here) with a different carb jet before it would "do right".
Sorry for the long post.Now lets hear other testamony.

You are correct that a sea-level car can't breath up here but you missed the last eight words of my post.

The barometric pressure does not change the aspiration needs of the engine. The engine must be tuned for the current environment in order to operate at optimum efficiency. Once tuned, local minute fluctuations in barometric pressure have very little affect.

You are right I didn't read for meaning and there was that word significant.
It doesn't change the aspiration NEEDS but the ability to provide the needed air does change---a minute amount--probably not noticable to my coarsely tuned seat of the pants indicator.But the serious performance people get intensely intrested in local immediate conditions both temp and BPress and GOD only knows what else.

Regarding setting timing with a vacuum gauge, I have found that adjusting timing to the highest vacuum level results in overly advanced timing. I have found, after much experimentation, that using a tachometer and vacuum gauge together will give a better result using the following procedure:

1. Disconnect the vacuum connection from the distributor.

2. With the tach and vacuum gauge connected, advance the distributor until either the RPM or vacuum stops increasing.
It doesn't matter which one peaks first; stop advancing the timing when one reaches a peak.

3. Retard the timing to reduce the vacuum 1 "Hg.

When using this procedure, make sure the vacuum gauge is connected to manifold vacuum, not a ported source.

I have read, but not tried, other procedures that advance timing to the highest manifold vacuum, then retard to lose 1.5 "Hg. While I haven't tried that procedure, I think it will result in retarded timing, especially if the engine is operating above about 3000 feet elevation.

Regarding the change in atmospheric pressure/barometric pressure, that pressure does, indeed, change at about 1 "Hg per 1000 feet of altitue above sea level. The change is fairly consistant up to about 10,000 feet altitude.

Because of that change, the power developed by any internal combustion engine diminishes as its operating altiude increases above mean sea level. The change is not especially evident below 3000-3500 feet elevation. The reason power diminishes with an increase in altitude, assuming proper fuel mixture and ignition timing, is that the cylinder compression pressure diminishes as atmospheric pressure/manifold pressure decreases. That in turn results in reduced combustion pressure (from fuel burn) with resulting lower thrust on the crankshaft (lower torque).

A rule of thumb that is often used to determine compression pressure is: Atmospheric Pressure X Compression Ratio. Applying this approximation to an engine at 6000 feet elevation, 7:1 compression ratio and atmospheric pressure of about 9 PSI (15-6), gives a compression pressure of about 60 PSI at normal engine cranking speed.

There is much more that can be said on this topic; compression pressure is not as simple as the approximation impies, but this is enough for now.

Harvester

Great information! This is like taking a college course. Harvester, you have made my uncles day with your response to timing. He said you are spot on using RPM and vacuum gauge. Also he agrees with Hg per thousand foot reduction. He was a mechanic in WW2 on bombers in the pacific theater and pressure as you stated has a lot to do with the internal combustion engine. I'm ready to learn a new timing method in this I will understand both methods. I will also purchase a vacuum gauge that has a big number face...Thanks, Steve