Originally Posted by
Epstein
Nice try. Dynojets directly measure power at the wheels. They then back calculate against the RPM of the motor. On something like a dynapack that measures torque at the wheels directly, this is true. That's why they make you punch in the overall gear ratio so the computer can divide it out, then back calculate HP.
Horsepower at the wheels is independant of gear ratio or tire diameter. The pitfall of your logic is the 3rd term in the tq-hp equation. That's rpm. If you jack up the wheel torque with a higher final, you're also spinning that wheel faster per engine rpm. The 2 factors are equal and cancel out.
The last statement holds true on a manual transmission. However, when dealing with a vehicle with a high stalled automatic (say 3500-4000 and above), the curves and results are very skewed. Unless the dyno can pre-load the vehicle, the stall can and will continue to slip aggressively causing the actual power numbers to read lower than they actually are.
Might want to ask the LS1 F-body guys about this one, it happens to them quite frequently.