Automotive manufacturers and car enthusiasts measure a car's performance by two common benchmarks: horsepower and torque. Maximum horsepower consists of the amount of power that a car's engine can generate at a given engine speed, measured in rotations-per-minute (rpm). Maximum torque equals the amount of a car's force along a vector direction; for cars, this means the vehicle's forward thrust at a given engine speed. Scientists measure a car's power in horsepower and the torque in foot-pounds (ft-lbs). An equation that relates the two will let you determine a car's horsepower at the rpm where it achieves a given torque rating.
Write the torque-horsepower equation out on a piece of paper with the pencil. The equation for this is: (Torque x Engine speed) / 5,252 = Horsepower.
Plug the engine's torque and engine speed ratings into the equation by rewriting the equation to include the number of the engine's torque and the engine speed. For example, the 2011 Honda Accord LX produces 161 ft-lb of torque at 4,300 rpm. Plugging those numbers into the torque-horsepower equation would yield: (161 x 4,300) / 5,252 = Horsepower.
Multiply the torque by the engine speed. Use a calculator or do the calculations manually on a piece of paper. In the example of the 2011 Honda Accord LX, this would result in the equation:
692,300 / 5,252 = Horsepower.
Divide the number from Step 3 by 5,252 to get the car's horsepower. For the 2011 Honda Accord LX, this would solve the equation to yield that the car achieves 132 horsepower at 4,300 rpm.
