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How It Works
Horsepower measures the rate at which an engine does work. The equation HP = (Torque × RPM) / 5252 ties together rotational force, speed, and power. The constant 5252 comes from 33,000 ft·lbf/min (one HP) divided by 2π. On any dyno chart, the torque and HP curves always cross at exactly 5,252 RPM.
Example Problem
An engine produces 350 ft-lbs of torque at 4,000 RPM. What is the horsepower?
- HP = (350 × 4,000) / 5,252
- HP = 1,400,000 / 5,252 = 266.6 HP
Frequently Asked Questions
What is the difference between torque and horsepower?
Torque is how hard the engine twists the crankshaft (measured in ft-lbs). Horsepower is how fast that twisting force does work. An engine with lots of torque but low RPM moves heavy loads slowly; one with moderate torque but high RPM achieves the same HP at higher speeds.
Why do torque and HP cross at 5,252 RPM?
Because HP = Torque × RPM / 5,252. When RPM equals 5,252, the formula simplifies to HP = Torque. This mathematical identity holds for every engine regardless of design.
How is brake horsepower different from wheel horsepower?
Brake horsepower (BHP) is measured at the crankshaft on a dynamometer. Wheel horsepower (WHP) is measured at the tires and is typically 10–20% lower due to drivetrain losses in the transmission, driveshaft, and differential.
Related Calculators
- Engine Equations Calculator — displacement and volumetric efficiency.
- Gear Equations Calculator — speed from RPM, tire size, and gear ratio.
- HP from Elapsed Time — estimate HP from quarter-mile time.
- HP from Trap Speed — estimate HP using quarter-mile trap speed.
- Torque Calculator — calculate torque from force and lever arm.
- Power Unit Converter — convert between HP, watts, kW, and BTU/hr.
Reference: Cengel, Yunus A., and Michael A. Boles. 2014. Thermodynamics: An Engineering Approach. McGraw-Hill Education. 8th ed.