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Thread: Calculating MPG from VSS and MAF from OBD2

  1. #121
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    May 2011
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    Minot, ND
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    Quote Originally Posted by lude-de View Post
    I've worked on this for two days now, and I've gotten to the point that I'm going to post my questions.

    My situation is unusual. I am working with the ODB1 Honda ECU that is chipped and has a data port installed. From this I can poll for information that the ECU is using to manage the engine.

    I am getting IAT in Fahrenheit , MAP in PSI, RPM and VSS in MPH.

    Here's an example of the numbers

    IAT = 88.39
    MAP = -16.97
    RPM = 3094
    VSS = 60.89


    IAT F to K = 304.3
    MAP PSI to kPa = -117.06
    RPM = 3094
    VSS = 60.89

    I plug that into the first formula to get IMAP:

    IMAP = RPM * MAP / IAT
    1190.218994413408 = 3094 * -117.06 / 304.3

    Then I plug that into the MAF formula:
    MAF = (IMAP/120)*(VE/100)*(ED)*(MM)/(R)
    MAF = (1190.218994413408/120)*(80/100)*(2.056)*(28.97)/8.314
    MAF = 9.918491620111732 * .8 * 2.056 * 28.97 / 8.314
    MAF = 56.84564559003256

    Then I plug that into the MPG formula with adjustments because I'm in MPH and not KPH:
    MPG = (14.7 * 6.17 * 454 * VSS) / (3600 * MAF / 100)
    MPG = (14.7 * 6.17 * 454 * 60.89) / (3600 * 56.84564559003256 / 100)
    MPG = 2507288.59794 / 2046.443241241172
    MPG = 1225.19332440382

    Now, as much as I'd like this to be true... it sadly isn't. I'm getting around 30 - 31.5 MPG on a tank of gas. I've checked my conversions several times and I'm sure they are correct.

    Can someone please put a second set of eyes on these equations to help me figure out what's wrong here. Unfortunately, I don't have an ODBII car to compare numbers with.

    Thank you in advance.
    Try #2 for posting...don't know where the first reply went...Here's the short version...

    #1 - MAP value figured wrong. You're reading manifold vacuum, not manifold pressure. Subtract your reading from ambient baro pressure (use 29.92 for starters) and work with that instead.

    #2 - F to K is off slightly.

    #3 - There's an extra /100 in the final solution.

    When I do the calculation the right way, I get about 31-ish MPG.

  2. #122
    Newbie lightner's Avatar
    Join Date
    Sep 2007
    Location
    La Jolla, CA
    Posts
    8

    On-Line Gasoline MPG Calculator (Using MAF and/or MAP/IAT/RPM)

    For those that want to calculate MPG using MAF (and/or MAP/IAT/RPM) for gasoline vehicles I've put together a Webpage with a Javascript-based air/fuel flow calculator...

    http://www.lightner.net/obd2guru/IMAP_AFcalc.html

    You may find this useful for "checking your work".

    As many of you have discovered---based on comments here and email that I regularly receive---getting all the units and calculations exactly right can be hard. And, in an embedded system without floating-point support, avoiding integer overflow errors can proved difficult.

    Best regards,

    Bruce D. Lightner
    [email protected]
    http://www.lightner.net/lightner/bruce/

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