Use your vehicle's weight and its quarter mile time or finishing speed to estimate the horsepower of the engine.
Horsepower = weight × (5.825 ÷ ET)³, with the weight in pounds and the time in seconds.
Horsepower = weight × (speed ÷ 234)³, with the weight in pounds and the speed in miles per hour.
A car that is more powerful for its weight finishes a quarter mile, 402.3 meters, in less time and at a higher speed. The elapsed time method uses the time: horsepower = weight × (5.825 ÷ ET)³. The trap-speed method uses the speed at the end of the run: horsepower = weight × (speed ÷ 234)³. In both, the weight is in pounds, the time in seconds and the speed in miles per hour, and the calculator converts the other units for you.
Use the weight of the vehicle as it ran, which includes the driver, any passengers, fuel and anything else of significant weight. The trap speed is the speed at the quarter-mile mark, not the average speed over the run. To estimate peak horsepower, apply the maximum work output of the engine from start to finish.
These are estimates. They assume a good launch and a vehicle in good condition, and the result depends on gearing, traction, aerodynamics and the weather. Power measured at the wheels is lower than at the engine because of drivetrain losses, and the formulas aim at engine power. Treat the number as a rough guide, and compare the two methods, since the trap speed tends to be the steadier of the two.
James Watt defined horsepower in the 18th century to compare steam engines to the horses they replaced. One horsepower is 33,000 foot-pounds of work a minute, which is about 745.7 watts. Engines are measured on a dynamometer, and every engine has a peak horsepower at a certain engine speed, often given as something like 320 HP at 6,500 rpm.
Use horsepower = weight in pounds × (5.825 ÷ ET in seconds)³. A 3,200 lb car with a 12.5 second ET makes about 324 horsepower.
Use horsepower = weight in pounds × (speed in mph ÷ 234)³. A 3,200 lb car with a trap speed of 110 mph makes about 332 horsepower.
One mechanical horsepower is about 745.7 watts.
The formulas estimate engine horsepower. Wheel horsepower is lower because power is lost in the transmission and drivetrain.
They use different parts of the run, and each rests on averages from many cars. The trap speed is usually the better indicator, since it is less affected by the launch.
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