This paper reports on the development of a digital horsepower limiter for a variable volume piston pump. This is used to limit the pump's operation to a window bounded by flow and pressure limits, as well as the theoretical constant torque curve representing the prime mover's horsepower capability. A microprocessor based controller was designed, built, and tested. The digital horsepower controller continuously monitors pump flow feedback, and uses preset flow, pressure and horsepower limits. It then generates a conditioned pressure signal which is sent to an existing pressure limit control system, allowing the prime mover to be operated at its rated capacity. Testing of the digital horsepower controller indicated a stable system which maintained a constant input torque. This combined control system allows reduction in the size of the prime mover, lowering capital cost, improving efficiency, and reducing noise levels.