Originally Posted by HybridSS
Depends on wether its a dynojet or something like a mustang dyno. Mustang dynos and Land & Sea dynoes have some type of load cell that does DIRECTLY MEASURE TQ. Then it uses the rpm pickup to calculate the HP figure.
Dynojet claims on thier website that they are measuring HP. But this isnt really true. All they have done is calculated a standard back at the factory based on the weight and diameter of thier drum. They probably have some type of system that applies a known TQ to each drum to accelerate it and then they can calculate the software calibration for that system.
I am refering to an inertial dyno (dynojet) not a load dyno (mustang dyno).
You can't just measure the torque at the wheels and use the engine RPM for the calculation. The numbers would change dramatically with gear ratio differences, and they don't. What is really measured is horsepower at the rear wheels and that then is translated into engine torque via the normal calculation based on engine rpm.
Here is a great explanation that I found (I would claim it as my own, but everyone here knows I ain't no damn mathmagician!):
"A combination of two laws of physics, force equals mass times acceleration and work equals force times distance, gives us this equation: W=m X a X d. "W" is the work, in pounds-feet, the rear wheels are doing, "m" is mass equivalent (the drums), "a" is acceleration (increasing drive wheel speed) and "d" is distance (drum circumference). Once we have the work, we can find horsepower. One horsepower is 550 pounds-feet of work done in one second so, we divide the work number by the length of time measured, then divide the number we get from that by 550. To simplify: we get horsepower by multiplying the mass, acceleration and the distance, then dividing that product by time multiplied by 550. This can be expressed by: hp = (m X a X d) ÷ (t X 550).
Torque can be figured by multiplying the horsepower by a constant, 5252, then dividing that product by the speed at which the thrust force was measured. Generally, with rear wheel numbers, axle ratio is not considered in the torque computation. For comparison purposes, this makes more sense. The computer factors out the axle ratio by using engine speed data in the torque derivation."
I know that is sorta hard to follow but if you really think about what is saying you can see that rear wheel horsepower is measured before engine horepower OR torque is calculated...thus eliminating the gear ratio factor.