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    SageBrush Senior Member

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    I have always liked the idea of optimizing fuel economy by keeping manifold pressure as least negative as possible. I imagine it as a surrogate of air throttle constriction.

    So now that I have a new toy (the CTh) and am hell bent on recovering if possible my G2 Prius mpg, I plugged in my old scan gauge and tried to drive with MAP as low as possible. Then I remembered that my G3 equivalent drivetrain has EGR, and i wondered why the SG was not showing negative numbers.

    In fact, the MAP readings vary only by about 3 units when the ICE is combusting, from 8 - 11.

    I have to conclude that I really have no clue what I am doing, what the numbers mean, or what is best.

    Insights, please :)
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    fuzzy1 Senior Member

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    I believe MAP is not a vacuum gauge, but instead a positive absolute pressure gauge. It cannot show negative numbers because absolute pressure doesn't go negative. The ECU finds absolute pressure more useful than vacuum.

    If you are accustomed to minimizing vacuum, then you want to maximize MAP. And beware that the scale changes when climbing mountain passes. Top of scale would be the local atmospheric pressures, typically about 14.7 at sea level, but less than 10 on the top of Pikes Peak.
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    SageBrush Senior Member

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    Thanks, fuzzy1
    Please explain, I am not following.
    I'm glad to hear I was using the information backwards. My drive to work was almost perfect conditions, and I was bummed the SG only read 69 MPG. My G2 has hit 80 MPG on similar days. Just goes to show how good the HSD really is: the driver is an idiot, and *still* gets pretty damned good fuel economy :)

    Ah, that would seem to correlate with my reading of ~ 12 in EV

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