Vacuum Gauge?

Discussion in 'Fuel Economy' started by Sabrejet, Jun 25, 2015.

  1. Sabrejet

    Sabrejet Member

    For several reasons, I can't get a Scangauge, Ultragauge, or even an MPGuino right now :(. I would like at least a vacuum gauge until I get one.

    Can it be used as a substitute for the LOD readout on the SG? I am asking because I want to try out pulse and glide with the vacuum gauge, something like this:

    Pulse to speed limit (not more than 50 mph), using max 1.5-2.3k RPM and 0-5" vacuum. Then, FAS to 35-40 mph. Rinse and repeat

    Can a vacuum gauge be used like that? I know that it has some uses in DWL, but I couldn't find much information about P&Ging with one.

    I would really appreciate any input/feedback :)
  2. ALS

    ALS Super Moderator Staff Member

    Yep they are a poor-mans scan gauge. I always used the boost gauge in my Turbo Volvo's as a fuel economy vacuum gauge. The higher the vacuum, the better the fuel economy.

    A quick Youtube video I found to show you how it works for fuel economy.

  3. RedylC94

    RedylC94 Well-Known Member

    Yes, certainly you can do that with a vacuum gauge; I did so for years. What it shows (after inconsistent units are sorted out) is simply the difference between local atmospheric pressure and (absolute) pressure in the intake manifold. I assume the "LOD" figure shown by a ScanGauge is based primarily on manifold pressure, also known as "MAP." Your best engine efficiency might happen at somewhere around 4-5" vacuum, although that will depend on your particular car and conditions. (You won't see exactly 0".)
  4. RedylC94

    RedylC94 Well-Known Member

    No, the higher the vacuum, the greater the throttling loss, and therefore the lower the efficiency. You're correct in that higher vacuum means lower amount of fuel burned per engine revolution, but the problem is that torque output drops off faster with increasing vacuum (i.e., closing throttle) than fuel consumption does, thus efficiency falls.
  5. some_other_dave

    some_other_dave Well-Known Member

    Red, you are confusing efficiency with economy. Efficiency, in this context, is how much fuel is required to add a given amount of speed to the car. Economy is about how much fuel it takes to go a given distance. Not quite the same thing.

    Steady-state, the highest vacuum reading you can get will give you the most economy.

    For P&G, a low vacuum reading gives you the most efficient pulse, and can in the long run help you to get a better economy once you count the glides in.

    So you're both right, but only partly. :)

  6. RedylC94

    RedylC94 Well-Known Member

    In the context of engines, efficiency is the ratio of the amount of useful mechanical work performed to amount of the fuel (or fuel energy) consumed in the same time--- nothing specifically related to "add[ing] a given amount of speed to the car." One can not obtain good economy, as you define it, from a vehicle by operating its engine constantly at high vacuum, when its efficiency is especially poor. If it were true that "the highest vacuum reading you can get will give you the most economy," then we'd save fuel by cruising in the lowest possible gear ratio, allowing the engine to create high vacuum by spinning madly under a light load.

    Your second paragraph is true only in the limited sense that any change that increases vacuum by reducing unnecessary load on the engine (assuming fixed speed and gearing), will indeed reduce fuel consumption---although less than proportionately. A more efficient way to take advantage of that hypothetical reduced power requirement would be to maintain the original vacuum by either "downsizing" the engine or reducing its speed.
  7. Sabrejet

    Sabrejet Member

    From what I have been able to gather, here is how I think vacuum affects fuel consumption:

    -for DWL, cruising in the highest gear with the highest possible vacuum gives the best results. The highest possible reading generally depends on speed, RPM, and environment variables. It may be 10" at 70 mph on my Cultus, or 17" at 50 mph in my Civic (just a guess), both in top gear.
    I guess new cars with FCDs also show the best instant FC at low throttle, high gear combinations (low throttle = high vacuum).

    -for P&G, pulsing at low RPM with the lowest possible vacuum, followed by the longest possible FAS, is ideal.

    I guess the only way to find out is to install a gauge and test it for a few tanks.
  8. some_other_dave

    some_other_dave Well-Known Member

    What mechanical work is performed, then, if not adding speed to the car? (I will claim that "fighting friction" and "fighting gravity" are included in that category of things.)

    That is exactly how you cruise at constant speed efficiently. The highest possible gearing, and then whatever throttle setting is required to maintain your speed. Which is almost always very nearly closed, meaning high vacuum. You don't cruise efficiently by keeping the throttle wide open, nor run in low gears.

    My post was a significant simplification, true.

  9. roothorick

    roothorick Active Member

    An OBDLink LX is like $70 and way more featureful with the right app. Of course, for best convenience, this means leaving a phone or tablet in your car, greatly increasing chance of break-in/theft. Meh. I'm making plans to retire my ScanGauge and rely on this setup.

    Of course, that might still be outside your budget.

Share This Page