dyno stuff

[Eric Bryant]

> Let's say the engine outputs 200 ftlbs torque at the 
> crankshaft at a given
> RPM. Why would you see a similar number at the wheels when the
> "differential" multiplies the torque ? You shouldn't right ?? I've had
> numbers as high as 1774 ft/lbs on a low 11 sec car, but HP 
> numbers around
> 420. How did they do their calcs ? At this point, I'm the 
> only one in Canada
> with this dyno...and nobody can give me answers.

Well, if you have a motor putting out X ft-lbs of torque at Y RPM, and a
final drive ratio of Z, then you'd measure XZ torque at the rear wheel at
Y/Z RPM.  So, assume the engine makes 200 ft-lbs at 5000 RPM, and you're
testing it in a gear that results in a 10:1 final drive ratio.  I'd expect
to see something less than 2000 ft-lbs of torque (to account for drivetrain
losses) at 500 RPMs at the rear wheel.  This is probably what your dyno is
doing - it's taking a "raw" number and not accounting for gear reduction.
HP ends up being a calculation (power * torque * some constant), so the HP
number will be the same whether the dyno spits out "raw" or "corrected"
torque. 

A Dynojet dyno uses the roller speed and the engine speed (through an
inductive spark wire lead), and then comes up with "rear wheel torque" -
which is actually "crankshaft torque as measured at the rear wheel". 

Is that clear enough, or did I just muddy up the waters?

Hope this helps,

Eric Bryant
mailto:bryante at ghsp.com
http://www.novagate.com/~bryante 
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