Geekin' again

[George Roffe]

OK, calling all engineers and fellow geeks.

I read Rob's thoughts on how the Dynojet measures hp and torque and a
related discussion came up on the G20 Forum.  An argument of sorts ensued
and I went back and pondered (as well as did a little research on the
subject using links that were provided on the G20 Forum.

So, to recap, Rob thought that the Dynojet measured wheel hp by itself and
needed the speed sensor to calculate torque.  What follows is my
ponders.  Please feel free to shoot 90mm Howitzer rounds through it. :-)

George Roffe
Turd Rex

Regarding the Dynojet dyno, I think I may have been right in the first
place. I have to do some more research, but I think I have it.

Where T = torque, I = rotational inertia, and a = angular acceleration,

T=Ia

(a should be a lower case alpha, but since I don't know this digithead
stuff enough to reproduce it here, so I'll settle for a lower case a)

Since I for the drum is known, and a is measured with data acquisition, T
can be readily calculated. So far so good. The question is how we get to hp.

Well, since hp=TxN/5252 (where N = rpm), we simply plug in the figures for
T and N. T was calculated as above. RPM is the rpm of the drum (not the
engine of the car spinning the drum through the drivetrain). Therefore,
since we know N and T at the drum through data acquisition, we can
calculate the hp at the drum as well as the torque. Pretty clever, don't
you think?

So, now we know the torque (and thus hp) being applied to the drum. What we
cannot do is relate the torque and hp at the drum (and thus the wheels) to
the rpm of the engine. So, we need to also collect the engine rpm data to
plot the torque and hp as a function of engine rpm rather than the drum rpm.

So, there you are. My take on this after pondering for a day. I'm open for
shooting holes in this. I think this theory will hold up, but we'll see. If
not, I'm sure we'll come away with additional insight.