Real HP loss numbers

[Tom Parker]

Daniel Ciobota <dciobota at hiwaay.net> wrote:

> Let me jump in on this discussion with an observation.  Rear wheel hp
> numbers
>are measured by observing the acceleration of two 1000lb drums by the driving
>wheels of the car.  That's how hp is calculated, rotational speed vs. time,
>known weight of 2000lbs and known drum diameter.  From those numbers, and
>final gear ratios (rpm/drum rotation), the chassis dyno calculates
>instantaneous torque required to accelerate the drum at that rate.

Around here, most dynos have brakes. The operator lets off the dyno brake,
puts the car in gear and eases up to the desired engine speed at very light
throttle. The torque and hp gauges on the dyno both read zero as the dyno
is not absorbing any energy (except in its bearings which may be taken
into account).

Then he twiddles a knob and the dyno brakes come on and he puts
his foot down. There comes a time when the engine is at full throttle
and the wheels are no longer accelerating. The BHP and Torque gauges on the
dyno now show how much energy is being dissapated in the dyno brakes.

If the operator increases the dyno brakes, then the assembly will slow down,
but the power dissapated in the brakes will increase for a short time as the
momentum in the rollers and spinning parts of the car is converted into
energy. If the operator decreases the brakes, then the dyno will speed up and
he can investigate what is going on at higher revs.

This type of dyno does not suffer from any momemtum effects. The type of dyno
you describe does indeed have all the affects you describe, but around here
they are the exception, not the norm.

--
Tom Parker - tparker at nznet.gen.nz
           - http://www.geocities.com/MotorCity/Track/8381/