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Thread: Calculating Instantaneous Net Power

  1. #1
    Joe (Moderator) joecar's Avatar
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    Default Calculating Instantaneous Net Power

    Edit: This power calculation comes already provided by the EFILive V7 and V6 Scan Tools as the built-in pid "{CALC.POWER_RW}".
    Edit:
    This was written a long time ago when EFILive V5 was current, which is why it contains references to "{SAE.J1979.VSS}".


    Using EFILive's calculated PID feature, you can calculate the "instantaneous net road power" at all recorded sample points using the following equations:

    (for sake of example, I'm using my vehicle's mass (3750 LB = 1701 kg))

    Using Imperial units (mass in LB, VSS in MPH):
    3750 * {SAE.J1979.VSS} * dx({SAE.J1979.VSS}, 6) / 8226.63

    Using Metric units (mass in kg, VSS in km/h):
    1701 * {SAE.J1979.VSS} * dx({SAE.J1979.VSS}, 6) / 12960.0

    Notes:

    a. Vehicle total mass must include driver and all payload mass (to be accurate, you must account for all the weight).

    b. The constant number 6 is the dt factor in the derivative function; you may want to read up the EFILive help on this function and play around with some different dt values.

    c. The constants 8226.63 and 12960.0 are due to the various conversion factors between Metric and Imperial units and between non-mks and mks units (m, kg, s: the default units of physics).

    d. The above equations are arrived at from the standard physics definitions of velocity, acceleration, force, work, etc. (i.e. they do not include any empirical influence or corrections).

    If the units used were the classic physics units (kg, m, s) then the
    instantaneous net power at each sample point would simply be:

    P = m . v . dv/dt

    (m is in kg, v is in m/s, and dv/dt is the slope of the VSS "curve")

    These equations calculate the instantaneous (at each sample point) net road power that's left over to accelerate the vehicle (i.e. after mechanical friction, road friction, air resistance, and anything else you can think of, has been overcome by engine power).

    :?
    Last edited by joecar; February 4th, 2006 at 03:01 PM.

  2. #2
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    cool! But math makes my head hurt.. :roll:

    Maybe I'll write that into some script and post it
    *Quality post-whoring since 9:27pm

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  3. #3
    Joe (Moderator) joecar's Avatar
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    Quote Originally Posted by RoboGeek
    cool! But math makes my head hurt.. :roll:

    Maybe I'll write that into some script and post it
    All you have to do is create a calculated pid containing either of the following equations (depending on if you want Metric or Imperial) which you enter by typing or pasting:

    Using Imperial units (mass in LB, VSS in MPH):
    3750 * {SAE.J1979.VSS} * dx({SAE.J1979.VSS}, 6) / 8226.63

    Using Metric units (mass in kg, VSS in km/h):
    1701 * {SAE.J1979.VSS} * dx({SAE.J1979.VSS}, 6) / 12960.0

    Of course, you use your own vehicle weight instead of 3570 LB or 1701 kg.

    This is like a "road dyno" using EFILive, if you get my drift...

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    These calcs will get me a HP reading???

    any testimonials to its accuracy?

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    Joe (Moderator) joecar's Avatar
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    I'm hoping that someone with access to a dyno will post their results.

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    Default Dyno results

    Dyno results are nowhere near....

    Although the graph smooths out on the dyno, it shows around 30rwhp more than on the dyno graph...

    And another thing the graph on the street is very very wavy.. anyway to correct that?
    "All that is needed for the triumph of evil is that good men do nothing..."

  7. #7
    EFILive Developer Site Admin Blacky's Avatar
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    Select "Generic" scan mode and only have a single PID (SAE.J1979.VSS} or {SAE.VSS} selected.
    You should get a frame rate of about 20 per second in V5, and up to 40 per second in V6,- the faster the better.

    (Note the speed difference in V5 to V6 is due to V6 supporting 115200 baud and V5 only supporting 19200 baud).

    Also try changing the formulas to increase the time frame over which the power calculation is made. i.e. increase the second parameter of the dx() function.
    i.e. increase it to say, about 10 or 12.

    The higher the value the "smoother" the curve. But the less detail will be shown because the calculation is made over a longer time frame.

    Paul

  8. #8
    Joe (Moderator) joecar's Avatar
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    The Highlander,

    Okay, there's a 30rwhp difference (I was looking to see feedback like this, thanks I do appreciate this);

    Is this difference throughout the rpm range, or is it localized...?

    Also keep in mind, the formula calculates HP using the laws of motion (i.e. it uses physics theory) and does not account for some things (e.g. tire deflection, and others...?), and does not do any corrections (for altitude or whatever) like some dynos do.

    So, 30 rwhp is about 10% (or less maybe) of max, so it's not too far off.

    But actually, could you post the pictures of both graphs somewhere...?

    (It is usable enough to see an hp increase after doing a mod).

    Thanks again for your feedback.

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    That is what I am trying to prove or something...

    Its 30 in its max point... but throughout the whole band, another thing.. I used the weight of the drum and not the weight of the car... I hope I can smooth out the curve..

    The curve is smooth on the dyno but on the street is very very ERRATIC...
    "All that is needed for the triumph of evil is that good men do nothing..."

  10. #10
    Joe (Moderator) joecar's Avatar
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    The curve is smooth on the dyno but on the street is very very ERRATIC...
    Are you saying that the calculated curve is smooth on the dyno and erratic on the street, or are you saying that the dyno's curve is smooth and the calculated curve is erratic...?

    (... I do apologize... I am suffering from jetlag... we flew SYD to AKL on Saturday, and AKL to LAX on Sunday... )

    Is there some place where you can post images of both the dyno curve and the calculated curve that you got...?

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