Re: Accelerometers

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   Date: Thu, 8 Oct 1998 14:54:33 -0400 
   From: Mike Chadwick <mchadwick@domain.elided>
   Subject: Accelerometer?

   I recently borrowed an accelerometer from a friend (I don't have the model
   with me, I know it is supposed to be a very good one, about $800) and made a
   few runs in my 635csi.  It calculated (among several other things) 210
   horsepower at the wheels, simply by entering in vehicle weight (plus 60lbs
   for various trunk junk and 220lbs. for my oversized butt)  How accurate can
   this be?  Wouldn't other factors play in, like aerodynamics, gear ratios,
   diff ratios, shift points and/or driver skill, etc.  Does anyone have
   experience with these devices?

   Mike Chadwick
   84' 633csi ('91 3.5L engine, sharked)

An accelerometer will attempt to measure HP via sampling 
mass*accel*speed (which gives you power) and recording the peak.  

gear ratios, shifting, etc won't make a difference, all that matters is
that you've got the pedal all the way down through the meaty part of
the rev range.  the shifts won't be a maximum at all, unless your 
accellerometer isn't calibratted to throw awake the spikes that would
come from no-lift shifting.  (but that would give you twice that HP)

We know the mass is pretty accurate because you know what the mass is.

accel can be innacurate due to a non-flat surface
speed is derived from accel and can be further innacurate.

aerodynamics also will reduce the apparent power, because the motor is
presented with a load that is greater than the pure acceleration one.

	ksh

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