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Just to add more confusion.. and a little history Hp has been quoted over the years in different ways. As far as I know, the Nebraska tests have been pretty consistent in using "belt"(or "PTO") Hp for comparison. Then they have used a consistent measurement of drawbar Hp in their published results to indicate how much you can get to the ground while pulling. If you look at a bunch of tests you start to get the picture. Unless it is a heavy well balanced tractor, there is usually some unballasted test that has relatively poor results at the drawbar, then the tractor manufacturer adds ballast (probably pre-determined earlier by their own testing) to produce a more favorable result. Adding too much weight, particularly in the front just makes matters worse, but the optimum weight distributed correctly will optimize this drawbar horsepower. But engine manufacturers also quote Hp meaning "brake horsepower". This is measured speed and torque (as you suggested) with just the engine in a test stand. But this too can be very confusing and misleading wihout knowing what accessories were included. During the muscle car years when horsepower was king, it was often quoted with high numbers that not only did not include alternator, but might not include the fan, or the water and fuel pumps either. Later the standards groups, like SAE, put the kabosh on this unrealistic method and the Hp numbers tumbled. Then add smog pumps and other smog detuning, the numbers looked even worse when they weren't necessarily that bad when it came to real working conditions. Jim in OH
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