Vacuum gauge in a classic?

Discussion in 'Fuel Economy' started by xcel, Feb 21, 2016.

  1. xcel

    xcel PZEV, there's nothing like it :) Staff Member

    Hi All:

    It has been over 2 decades since I drove a vehicle with a Vacuum gauge and am wondering what is the best method to control consumption with the feedback provided?

    Anyone remember?

  2. RedylC94

    RedylC94 Well-Known Member

    You can do that if you know "the best method to control consumption" using a MAP (manifold absolute pressure) readout. Just keep in mind that MAP=atmospheric pressure minus "vacuum" (assuming of course the traditionally inconsistent units are translated into consistent units). So, if local barometric pressure is 30" and you want MAP of 12 psia, which equals about 24.5", shoot for a vacuum of 30"-24.5"= 5.5". The exact load for best BSFC would depend on where the specific car's fuel system starts to enrich, I suppose.
  3. worthywads

    worthywads Don't Feel Like Satan, I am to AAA

    Keep it out of Power, stick with Economy.

    xcel likes this.
  4. RedylC94

    RedylC94 Well-Known Member

    I don't believe that will work so well. Gauges with those nonsense labels typically had "Economy" at the high-vacuum (i.e., low MAP) end of the range. In other words, light load, where the engine is not at its best efficiency. That's the opposite of "DWL."
    xcel likes this.
  5. EdwinTheMagnificent

    EdwinTheMagnificent Legend In His Mind

    I guess it depends on whether you use the gauge for pulsing or for gliding. I've never used one , but old enough to have seen them.
    xcel likes this.
  6. RedylC94

    RedylC94 Well-Known Member

    Set your ScanGauge for MAP and watch where that number tends to hang out in your Prius, which of course automatically adjusts it to maintain best efficiency. Optimum MAP (and hence "vacuum") probably wouldn't be radically different in a carbureted engine, although that would vary somewhat by model (and altitude). Honda's CVCC system may have introduced some unusual quirks in the engine performance map, though.

    I used a vacuum gauge many years in my GLC.
    xcel likes this.
  7. basjoos

    basjoos Well-Known Member

    They react somewhat like a throttle position sensor. In my car I mainly use it know where the maximum throttle setting I can use and still keep my car running in lean burn. It basically mirrors the usec output on the SuperMID.
    xcel likes this.
  8. RedylC94

    RedylC94 Well-Known Member

    At a fixed engine speed, yes. However, vacuum (which is merely the difference between atmospheric pressure and absolute pressure in the intake manifold) depends on engine speed as well as throttle position.
    xcel likes this.

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