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Gtech pro (cigarette lighter ie. 12v source) how do they work

  1. Nov 2, 2007 #1
    http://forums.bimmerforums.com/forum/showthread.php?t=863426

    hope that that link works cause if not you prob have to log in
    but let me give you the gist of it.

    you know those like gtech pro dyno's
    that you put in your cig lighter that measure hp and tq and such
    how do they work.

    because the 12v source wouldnt show anything that would indicate hp
    im guessing gyro's

    if you relaly want ill post what the others have said but pretty much the same
    some guy says gps
    but i say that would be sold as a feature and would expensive just for that alone without selling that as another feature.


    some other guy said its the 12v source that varies

    but that bull**** already. 12v source would only get 12 v nothing less or more and the fluctuations in volt or amp wouldnt either or it would show your windows power in hp
     
  2. jcsd
  3. Nov 5, 2007 #2

    Stingray

    User Avatar
    Science Advisor

    Several people on that site got it right. Accelerometers are used to calculate 0-60 etc. This is also sufficient to compute the instantaneous power to weight ratio (P = F v = m a v).

    For other calculations that require knowledge of engine speed, this is taken from the cigarette lighter. While the signal there is nominally a constant ~14 V or so, it's rarely regulated. Each time a spark plug fires (assuming you're not driving a diesel), the current draw causes a slight dip in voltage at the lighter. Knowing the number of cylinders in the car, the frequency of these little dips can be used to calculate rpm. This requires a fair bit of amplification and low frequency filtering to get right, but should be pretty reliable in most cars.
     
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