G-Tech Hp Meter Update

[Bruce Bowling]

> I have made some an initial evaluation of the 
> G-Tech acceleration meter in my Audi.  I have
> found that it is relatively easy to use,
> and is probably worth the money, but there are
> some drawbacks. I will discuss these below.
> 
> Here are the hitches.  All power/speed tests must begin
> from a stop, and be performed on level roads for good 

The reason for the runs from a dead-stop position is that this
system uses an accelerometer (I assume), and one must have
initial conditions to perform the calculation:

Velocity = Velocity(@t=0) + Acceleration * Time
Position = Position(@t=0) + Velocity * Time

Zero velocity make everything easy since one does not have to enter
a MPH into the box (it assumes zero) and you do not have to turn on
the unit when you reach some certain speed.


> accuracey. In addition, the horsepower measurement, which 
> is my main interest, is difficult (underestimates) in
> anything other than low gears.  The effect of wind
> resistance is included in the sensed figure, and 

I assume that this unit will yield the horsepower at the road wheels
and not flywheel horsepower.

Can you enter in your coefficient of drag and frontal area into
the unit for wind resistance?  If they have some buried constant for
these, then all bets are off on what you will get.

FYI, one can use the following to determine HP loss from aerodynamic
effects:

  AirDensity=0.0228645 * barometer(In Hg)/(0.5555* (temp - 32) + 273.15)
  AeroDragFactor = 1.07556 * AirDensity * Coeff_Drag * Frontal Area

See: http://devserve.cebaf.gov/~bowling/aero.html - which does the above.

and see: http://devserve.cebaf.gov/~bowling/airdragchart.html - for a
chart of drag coefficients and frontal area calculation.


Is there any environmental corrections for horsepower available in this unit?
I assume that there is not, and any horsepower number you get needs to
be corrected to a standard or SAE environment.  This can have quite
an effect



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               Bruce A. Bowling
  Staff Scientist - Instrumentation and Controls
 The Continuous Electron Beam Accelerator Facility
    12000 Jefferson Ave - Newport News, VA 23602
                 (804) 249-7240
                bowling at cebaf.gov  
        http://devserve.cebaf.gov/~bowling
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