Engine Horsepower Calculator

What this calculator does

The Engine Horsepower Calculator estimates how much power an engine produces using data from a quarter-mile run or a dynamometer-style torque reading. It offers three methods: elapsed time (ET), trap speed, and torque multiplied by engine speed.

The ET and trap-speed methods are track rules of thumb. They use vehicle weight plus either the time to finish a quarter mile or the speed at the finish line. The torque method uses the classic relationship between torque, RPM, and horsepower. All three results are estimates, not measured brake horsepower.

How to use this calculator

1. Choose the elapsed-time method, the trap-speed method, or torque and RPM.
2. For ET or trap speed, enter the vehicle weight including the driver and any significant cargo, then choose pounds, tons, or kilograms.
3. For ET, enter the quarter-mile elapsed time and choose the time unit.
4. For trap speed, enter the speed at the quarter-mile finish and choose mph, km/h, or m/s.
5. For the torque method, enter torque and choose pound-feet or newton-meters, then enter engine RPM.
6. Click Calculate.

The quarter-mile distance is 402.3 meters, or 1,320 feet. Use the speed at the finish line, not the average speed over the run.

Formula

ET method:     horsepower = weight (lb) / (ET (seconds) / 5.825)^3
Trap-speed:    horsepower = weight (lb) x (speed (mph) / 234)^3
Torque method: horsepower = torque (lb-ft) x RPM / 5,252
Watts = horsepower x 745.6998716

Weight is converted to pounds and time to seconds before the ET formula. Speed is converted to miles per hour before the trap-speed formula. Torque in newton-meters is divided by 1.3558179483 to obtain pound-feet.

Default example

The default ET example uses 5,000 pounds and 20 seconds. The estimate is about 123.53 horsepower, or roughly 92,115 watts. The default trap-speed example uses 5,000 pounds and 70 mph and returns about 133.85 horsepower, or roughly 99,812 watts.

The torque example uses 300 pound-feet at 5,000 RPM. Horsepower is 300 x 5,000 / 5,252, or about 285.61 horsepower. The same torque entered as 406.745 newton-meters produces the same result.

Assumptions and limitations

• The ET and trap-speed formulas assume the engine is producing maximum power throughout the run and that traction, gearing, and driver skill are ideal.
• Vehicle weight must include the driver, passengers, fuel, and cargo. Understating weight understates the estimate.
• Air density, altitude, temperature, tire grip, launch technique, and shifting all affect real quarter-mile results.
• The trap-speed method uses the speed at the finish, not the average speed.
• The torque method gives an instantaneous estimate at one RPM point; peak horsepower usually occurs at a different RPM.
• These methods estimate flywheel or crank power approximately. They are not a substitute for a calibrated dynamometer.

Frequently asked questions

Why do the ET and trap-speed methods give different answers?

They are empirical formulas fitted to different measurements. ET reflects the whole launch and run, while trap speed reflects the power available near the finish. Small differences are normal, especially for cars with traction problems or unusual gearing.

What weight should I enter?

Use the weight of the vehicle as raced, including the driver and all fluids and cargo. If you only know the curb weight, add the driver's weight and any items removed or added for the run.

Is the torque method more accurate?

The torque method is mathematically exact for the relationship between torque, RPM, and horsepower, but only for the RPM point you enter. It does not tell you peak horsepower unless you enter the RPM where peak power occurs.

Reference table

Vehicle weight units

Use the same weight unit for every vehicle in the comparison. Mixing units will distort the power-to-weight ratio.

Source

Reference: Calculator.net - Engine Horsepower Calculator. The ET and trap-speed formulas are common drag-racing approximations.