Once again I read about transmissions being "rated" for horsepower. That is meaningless. The only limitations on horsepower would be when the frictional losses in the tranny got so large that the transmission overheated.

The transmission is affected by TORQUE. Torque is what twists the shafts and puts pressure on the gear teeth making them want to slide apart or break off.

I believe that the hp "rating" is the manufacturer assuming a generic street rodded engine and estimating the hp and torque.
the manufacturer assumes that torque and horsepower will maintain the same ratio over the rpm curve. Say the factor used by the manufacturers is 1.2 lb-ft per 1 horsepower. I'd like to think that gearbox designers designed to a target torque for a particular box. So say the transmission is designed and tested for max torque of 600 lb-ft of torque. The manufacturer then applies the fudge factor and sees that 600 lb-ft "converts" to 500hp. Advertising geeks put it in print and there you have it. The transmission is good for 500hp.

Never mind that the transmission is mated to a 500hp Diesel engine running turbos and that engine puts out 450 lb-ft at idle and maxes out at 1100 lb-ft at 2400 rpm. The transmission never saw more than 500hp yet it is in pieces on the track.

R.