Engine Horsepower from Elapsed Time
Solution
Horsepower vs. Elapsed Time
Estimated HP for a 3500 lb vehicle across quarter-mile elapsed times.
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How It Works
This calculator estimates engine horsepower from your quarter-mile elapsed time (ET) and vehicle weight using the empirical formula HP = W / (ET/5.825)³. The constant 5.825 was derived from thousands of observed drag strip runs relating weight, time, and power.
Example Problem
A 3,200 lb car runs a 12.5 second quarter-mile. Estimated horsepower:
- ET / 5.825 = 12.5 / 5.825 = 2.146
- 2.146³ = 9.889
- HP = 3,200 / 9.889 = 323.6 HP
Frequently Asked Questions
How accurate is the elapsed time HP estimate?
It is an approximation within about ±5–10%. Track conditions, driver skill, traction, and aerodynamic drag all affect real-world results.
Should I use curb weight or race weight?
Use the total weight as it crosses the finish line, including the driver and fuel. For most street cars, add about 180 lb to the curb weight.
Does this estimate wheel HP or crank HP?
The formula estimates wheel horsepower (WHP), since elapsed time reflects power at the wheels after drivetrain losses. Crank HP is typically 15–20% higher.
Related Calculators
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- Gear Equations Calculator — calculate vehicle speed from RPM and gear ratio.
- Engine Equations Calculator — displacement and volumetric efficiency.
- Power Unit Converter — convert between HP, watts, kW, and BTU/hr.