Horsepower from Trap Speed

Turn a quarter mile run into an engine power estimate. Four published formulas side by side, each with its real-world accuracy - not one number pretending to be exact.

Weight with driver

Weight unit

Trap speed

mph

Speed unit

Quarter mile ET (optional)

From trap speed

Formula		R² of fit

From ET

Low confidence: ET is dominated by launch, traction, and the first 60 feet, which carry no information about power. The ET formulas fit real data far worse than their trap speed versions - R² 0.62 to 0.79 for the classic three, 0.91 for the best fit. Use these as a sanity check, never as the answer.

Formula		R² of fit

Accuracy: every constant here is empirical, fitted to runs by typical light passenger cars, so an average drag coefficient and frontal area are baked in. A brick-shaped truck and a slippery coupe with the same weight and power will not both be predicted right, and Hale is documented to overestimate street cars. Wind alone moves trap speed 2 to 4 mph, and these are quarter mile constants - an eighth mile trap speed cannot be used here. Treat the spread between the four formulas as the honest uncertainty band.

Crank vs wheel horsepower

The estimates above are of rated crank horsepower - the best fit was regressed against manufacturer ratings, so a typical car's drivetrain loss is already inside the fit. To compare against a chassis dyno sheet instead, pick an assumed loss. It is an assumption, not a measurement: commonly circulated figures run from about 13% for a front-wheel-drive manual to about 23% for an all-wheel-drive automatic, and individual cars vary widely.

How the estimate works

All four formulas descend from the same physics: over a quarter mile, trap speed scales with roughly the cube root of power to weight. Huntington fitted the constant 224 to drag strip data in 1958, Hale used 234 in 1986, and Fox used 230 in 2001. The Road & Track best fit (published by Lucius in 2008) also freed the exponent and regressed both against 243 magazine road tests, landing on mph = 215.39 (hp/wt)^0.3018 - the strongest fit of the four at R² 0.93. Weight means as-raced weight including the driver, roughly curb weight plus 200 lb. The default run is the 3,900 lb, 320 hp Mitsubishi 3000GT VR4 worked example from that regression - the calculator returns its rated power from its predicted trap speed. To run the same math forward instead - predict ET and trap speed from power and weight before you ever stage - use the quarter mile calculator.

Trap speed is defensible, ET is not

Trap speed is set almost entirely by power and weight, which is why the mph formulas fit real data at R² 0.86 to 0.93. Elapsed time is set largely by the launch: tires, surface preparation, reaction to wheelspin, converter or clutch behaviour. Two identical cars can run half a second apart on ET while trapping within 1 mph. That is why the ET formulas fit far worse and why this calculator never headlines an ET-based number when a trap speed is available. Enter both and the launch check quantifies the gap: it predicts the ET your trap speed is worth, and any difference from the ET you actually ran is time that lives in the launch.

Crank, wheel, and the drivetrain myth

None of the four formulas contains a drivetrain term - front, rear, or all-wheel drive is simply not a variable, and any calculator claiming one of them "accounts for AWD" is mistaken. What is true: because the best fit was regressed against manufacturer-rated crank horsepower of real cars, the typical drivetrain loss of that fleet is already baked into the constant. The number it returns is therefore directly comparable to a factory rating, not to a chassis dyno readout. If you want a wheel figure, apply a loss percentage as the separate, clearly-labelled assumption it is - the drivetrain-loss percentages in circulation are folklore averages, not measurements of your car. And if the estimate has you planning hardware, the injector size calculator works from the same crank-horsepower basis.

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