Engine Horsepower from Trap Speed

Horsepower equals weight times trap speed over 234 cubed

Solution

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Horsepower vs. Trap Speed

Estimated HP for a 3500 lb vehicle across trap speeds.

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How It Works

Trap speed is the velocity recorded as a vehicle crosses the quarter-mile finish line. The formula HP = W × (V/234)³ converts that speed and the vehicle's weight into an estimated horsepower. The constant 234 was derived empirically from drag strip data.

Example Problem

A 3,500 lb car traps at 110 mph. Estimated HP:

  1. V / 234 = 110 / 234 = 0.4701
  2. 0.4701³ = 0.1039
  3. HP = 3,500 × 0.1039 = 363.6 HP

Frequently Asked Questions

Is trap speed more accurate than elapsed time for HP?

Trap speed is less affected by traction and driver reaction time, so it often gives a more consistent HP estimate. Elapsed time is more sensitive to launch technique.

What weight should I use?

Use the total race weight: vehicle curb weight plus driver, fuel, and any cargo. For most street cars, add roughly 180 lb to the published curb weight.

Why does the chart curve upward so steeply?

Horsepower scales with the cube of trap speed. A modest increase in speed requires a large increase in power because aerodynamic drag rises with velocity squared and the time to accelerate shortens.

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