There is no actual thing as horsepower. Horsepower can NOT be measured!!! Torque is the only thing that can be measured. ALL HP figures are a MATHEMATICAL PRODUCT of torque compared to RPM (in the case of engines)
For a better understanding, consider the mathematical constant in the HP formula, and the fact that ALL torque/ HP curves have a common marker at that constant which is 5252
HP = torque X RPM / 5252
There is, therefore, only two ways by which an engine can make more horsepower. That is A: make more torque, and/or B: make torque at a higher RPM