Calculate engine horsepower from torque and RPM, convert between power units, and analyze engine performance characteristics.
HP = (Torque × RPM) ÷ 5,252
Where torque is in lb-ft and RPM is revolutions per minute
| Unit | Value | Conversion Factor |
|---|---|---|
| Horsepower (HP) | 286.1 | 1.000 |
| Kilowatts (kW) | 213.4 | 0.7457 × HP |
| Metric HP (PS) | 290.2 | 1.0139 × HP |
| BTU/hour | 976,248 | 2544.43 × HP |
Horsepower is a unit of measurement for engine power, originally defined by James Watt in the 18th century. It represents the rate at which work is performed, specifically the power needed to lift 550 pounds one foot in one second, or 33,000 foot-pounds per minute.
Understanding the relationship between horsepower and torque is crucial for engine performance analysis:
The standard formula for calculating horsepower from torque and RPM is:
Horsepower = (Torque × RPM) ÷ 5,252
Where 5,252 is a constant derived from the definition of horsepower (33,000 ft-lb/min ÷ 2π radians per revolution).
The actual power output of an engine measured at the crankshaft, without losses from transmission, differential, or other drivetrain components.
The power actually delivered to the wheels after drivetrain losses, typically 10-20% less than brake horsepower.
The theoretical power produced by the combustion process inside the cylinders, before mechanical losses.
1 HP = 0.7457 kW
Kilowatts are the standard SI unit for power, commonly used in electric motors and international specifications.
1 HP = 1.0139 PS
Used primarily in Europe, metric horsepower is slightly different from the imperial horsepower standard.
Engine dynos measure power output by loading the engine and measuring torque at various RPM levels. This provides accurate power curves showing how horsepower varies with engine speed.
Chassis dynos measure wheel horsepower by loading the vehicle's drive wheels, providing real-world power output including drivetrain losses.
Understanding the torque-RPM-horsepower relationship helps in engine tuning for optimal performance at specific RPM ranges.
Power-to-weight ratio is crucial for acceleration performance, while peak horsepower affects top speed capability.
There's generally an inverse relationship between horsepower and fuel economy:
Electric motors provide instant torque and different power characteristics:
Whether you're analyzing engine performance, comparing vehicles, or planning modifications, understanding horsepower calculations helps you make informed decisions about automotive power and performance characteristics.