What set of variables is required to measure horsepower?

Horsepower is calculated based on the relationship between torque, speed, and the engine's rotational frequency. Torque represents the engine's ability to do work, while speed, typically measured in revolutions per minute (rpm), indicates how quickly the engine can perform that work.

The formula to calculate horsepower is:

[ \text{Horsepower} = \frac{\text{Torque} \times \text{RPM}}{5252} ]

In this formula, torque is measured in pound-feet and RPM indicates the engine speed. The inclusion of "time" in choices does not factor into the standard horsepower calculation since the rpm already provides a time context; it shows how many times the engine completes a full rotation in one minute. Similarly, displacement and pressure are not direct variables used in the calculation of horsepower.

Thus, the combination of torque, time, and rpm is not required, as the direct variables needed to accurately measure horsepower are specifically torque and rpm, allowing for a clear understanding of the power being generated by the engine.