Originally Posted by tractorman
The diameter doesn't affect the output, but the extra weight of the tires does. Also, higher gears tend to be slightly less efficient than lower gear ratio's in the rear end. So those two together lower the numbers.
The dyno doesn't gather TQ, it's actually reading the HP output. If you ran it without an RPM input, you would still have a HP reading, but not a TQ reading. It then figures the TQ from an engine RPM input.
Originally Posted by Murphy
Everything I have read indicates the opposite: That horsepower is derived from torque via a mathematical equation.
I think you guys are talking about two different types of dynamometer.
An inertia dyno measures the acceleration of a mass, often a weighted drum or drums, and infers a horsepower curve from the acceleration curve. A torque curve can be derived from RPM and the inferred HP curve.
A brake dyno measures actual brake torque vs RPM to create a torque curve. The horsepower curve is calculated from the torque curve.