How Accurate is the Vacuum Gauge for Helping to Maximize Gas Mileage?

[mazama]

Sent: 3/27/2005 10:53 AM

I recently acquired an '84 Chieftain.  By watching the vacuum gauge closely for the first few hundred miles I was able to get 8 mpg which was a mile per gallon more than the previous owner.  My question is this...When I am going over mountain passes the gauge indicates  better fuel performance in 2nd gear and 40 mph.  Is this an accurate reflection of gas mileage or is it a function of the operation of the vacuum gauge?.  To what degree is the vacuum gauge an accurate indicator of fuel mileage.

[HeavyHaulTrucker]

Sent: 3/28/2005 1:46 AM

Used properly, the vacuum guage is the best "window" into your mpg.  When vacuum is high, you're getting your best gas mileage because the engine is in either "cruise" or "coast" mode and not using much fuel.  When vacuum is low, you are in "power" mode and the engine is sucking gas pretty heavy.

If you have swapped over to the Edelbrock carbs, then you may be able to obtain a slight increase in gas mileage by swapping the step-up piston springs from the stock orange ones to the blue ones from the spring assortment kit; this sets your "break-over" point for cruise to power mode transition down around 3 inches of vacuum.  This means that the carburetor won't stage rich until vacuum drops below 3 inches; if you keep vacuum up above that, you are in cruise mode and getting good gas efficiency.

John

[DaveVA78Chieftain]

Sent: 3/28/2005 10:58 AM


The vacuum gauge is not a indicator of MPG.  It is more an indicator of engine load (stress).  As John said, the higher the vacuum, the better the milage.  While the following is from the observations I have made on my 77 Dodge 440 with a thermoquad, the basic principles are the same for your 84 GM 454 engine (as well as other engines).When runnig along on fairly level ground against a headwind, the engine has to work harder than if there is no wind.  Because you have to give it more throttle to overcome this additional stress (headwind) the vacuum gauge will show a lower reading if there was no headwind. With a tailwind, the engine does not have to work as hard therefore the vacuum gauge will show a higher vacuum reading.  When going up a hill, it has to work harder.  You compensate by pushing the throttle harder which results in a lower vacuum.  Simply put, the throttle plate is not restricting the air flow such that a higher vacuum can build up in the intake manifold.  The rear "power" barrels in a 4 barrel carb are usually operated based on engine load.  This allows them to open gradually and not bog the engine down.  This, I believe, is what John was reffering to as "power mode".  Depending on the design, when the vacuum drops to around 5 to 3 inches (accellerator pushed harder than normal; uphill, passing, etc.) the rear 2 barrels start opening up allowing a larger volume of air/fuel to flow.  Sometimes you need it (interstate on-ramp access) usually you do not need that much extra effort.  So, basically, keeping the vacuum as high as possible (easy on the throttle) insures you only asking the engine to work as hard as is needed for the given situation.  The other vacuum related device used for fuel economy, is the vacuum advance.  While it is engine vacuum operated, the actual tap off point is above the primary throttle plate (ported vacuum) not below it (manifold vacuum the gauge is reading).  On the stock 440 with a thermoquad, vacuum advance does not start to kick in until the intake manifold vacuum reaches 9 inches (my observations on my rig).  It is very, very sensitive to throttle fluctuations and engine loading.  The vacuum advance will increase to around 15 inches (max vacuum advance and maximum gas milage) when the intake manifold vacuum reaches around 20 inches.  In a smooth situation, vacuum advace will typically run around  5-10 inches.  Because the port for vacuum advance is above the throttle plate, it will be 0 inches when you take you foot off the throttle (intake manifold goes high but tap off point does not see it being above the throttal plate).  If you depress the throttle hard (getting up to speed from a red light, passing, interstate on-ramp access, hill climbing, etc.) vacuum advance will also go to 0 inches.  I watch both intake manifold and vacuum advance because of the hilly area I am in.  I don't get to be in the vacuum advance sweet spot a lot so I have to ensure I do not put my foot in it enough to open the rear 2 barrells and waste a lot of gas.As far as the better MPG in 2nd at 40mph, that is actually telling you that the engine output is more matched to the load your asking of it.  Your not getting better gas milage than in 3rd becasue the rear wheels are not turning as many revolutions for each revolution of the motor.  Additionally, it also means your not asking the transmission to work as hard which keeps the transmission temps down.  In hilly areas I watch transmission temp close even though I only have a stock gauge.  Transmission temp can climb fast on hills.  When it does, slow down and down gear.  The Dodge manual itself says to drop speed and downshift if the grade is longer than 1/2 mile.  The Dodge transmission is a good one as long as you keep it within the operating parameters they recommend or a little lower temp if possible.  I have had to drop into 1st at 20mph with some of the 9-10% grades in WV.


Dave 

[lockman]

Sent: 4/2/2005 11:42 PM

I use my tachometer and vacuum guage as much if not more than my speedometer. I know there is no direct relation between inches of vacuum and mpg, but there sure is in relation to fuel economy. My understanding of my 440-3 and thermoquad carb is that the secondaries and power circuit become active at approx. 8" or less. If by using a lighter foot I can keep the guage at 10 or above I use less fuel because I am using only two barrels and a leaner mix. The old girl will cruise and hold 15 up to just over 55 mph then drop steadly till my comfortable max of 65mph and a guage reading of 6. There are numerous times in the day when I could apply more throttle and tank the guage to maintain my set speed or hold steedy on the foot and above 10 and lose a few mph on slight hills. I much prefer dropping to say 54 from 58 mph than dropping from 9.5 mpg to say 6 or 7mpg.     

[engineer bill]

I recently bought an '89 Winnebago with 454 chevy (carb). The vacuum gauge & vacuum hose were missing and there was no tach. This older thread seems to be about using the combination of the Tach and the Vacuum Gauge to operate the rv more efficiently and I'm all for that. The Tach connections we have addressed separately and I will also post on the whole gauge project seperately.

Here's the thing: I have a new vacuum gauge and I'm trying to discover the best place to tap into the vacuum system for the gauge. Attached is a picture of my vacuum diagram from the side of my air cleaner. Is it important where I tee in? How about the simplest, under the hood at the round vacuum reservoir?

Any suggestions? Thanks.

[cncsparky]

Obviously, you would want to tie into full manifold vacuum.  There are many 'timed' vacuum signals in that diagram to avoid.  Using a hose connector tee, I tied mine in near the vacuum bulb mounted in front of the radiator.  Seems like one of the easier places to get to.  However, the closer to the manifold, the better.

[Lefty]

On your diagram above, left side, in between the distributer and carb, there is a Manifold Vacuum Source. this is where you want to tee off of. This provides true manifold vacuum that is not timed or restricted.

[circleD]

Mine is on the driver side teed into the line going to the ball out front.

[Rickf1985]

That diagram is only showing the emissions connections. It does not show the connections to the heater control canister which you found earlier or several others like the transmission modulator connection. These lines I just mentioned all come from the manifold connection between the carb and distributor. The transmission will come out the top and the heater line will come out the drivers side of that tree. You can tee into the heater line for the vacuum gauge as was mentioned above.

[engineer bill]

Great information to add to what's on the diagram. I will tap into the HVAC canister line for the vacuum gauge. That will be simple. I'll report back, probably next week on how it goes.

[engineer bill]

To add to what Rickf1985 said about the manifold vacuum connector ... a picture.