How this instrument works
Elapsed time and trap speed are the two numbers a drag strip's timing lights actually produce: how many seconds a car takes to cover the standing quarter mile from a dead stop, and how fast it is moving when it crosses the far end. Engine builders and hot rodders lean on this same weight-to-power scaling law — a rule of thumb that has circulated in drag racing circles since the 1960s — to bench-race a swap or a power adder before the car ever makes a pass down the strip.
The cube-root shape is not arbitrary. If an engine delivers roughly constant power P over the run, the work it does in time t is close to P·t, and that work becomes the car's kinetic energy, about one-half its weight times the square of its speed. Average speed over the run is also close to the quarter-mile distance divided by t. Combine those two relations and the elapsed time falls out proportional to the cube root of weight over power, with trap speed proportional to the cube root of power over weight — the same shape the formula's constants, 6.290 and 224, were tuned to match against real cars.
The estimate assumes every one of those horsepower actually reaches the pavement, which is where it breaks down. It ignores gear ratio, tire compound, launch technique, and aerodynamic drag, so a car with poor traction or a lazy shift point will run slower than predicted, while a well-prepped car on slicks will beat it. The other common error is feeding in wheel horsepower straight off a dyno sheet: the constants were calibrated to flywheel power on a typical street car, so a wheel-hp figure — already discounted for drivetrain loss — quietly makes the prediction too optimistic.
- Enter the car's total weight in the Vehicle weight, lb field — curb weight plus driver, fuel, and any ballast on board.
- Enter the engine's rated output in the Engine power, hp field, using flywheel horsepower rather than a wheel-hp figure from a dyno.
- Read the Estimated quarter-mile ET, seconds field for the predicted elapsed time over the standing quarter mile.
- Read the Estimated trap speed, mph field for the predicted speed at the finish line.
- Adjust either field to compare builds — a lighter interior against a bigger cam — and watch both outputs update together.
Worked example — 3,200 lb car, 300 hp
A 3,200 lb street car with a 300 hp engine has a weight-to-power ratio of 3,200 ⁄ 300, or 10.667 lb per horsepower. Cube-rooting that ratio gives 2.20105, and multiplying by 6.290 gives an estimated elapsed time of 13.846 seconds for the standing quarter mile — the exact figure this instrument returns for those two inputs.
The trap-speed side of the same pass inverts the ratio: 300 ⁄ 3,200 is 0.09375, whose cube root is 0.454257. Multiplying by 224 gives an estimated trap speed of 101.759 mph as the car crosses the finish line. Treat both numbers as a bench-racing estimate, not a guarantee — real gearing, tire grip, driver skill, and aerodynamics all push the actual result away from this weight-and-power-only prediction.
Questions
What do ET and trap speed actually measure?
Elapsed time is the number of seconds from the starting line to the far end of the standing quarter mile, and trap speed is how fast the car is moving at that far end. Both come straight off the drag strip's timing lights on a real run; this calculator predicts them ahead of time from nothing but weight and power.
Why does the formula use a cube root of weight over power?
Because for a roughly constant-power engine, the work done over the run scales with power times time, and that work becomes kinetic energy tied to weight and speed. Working through those two relations together leaves elapsed time proportional to a cube root of weight over power, which is exactly the shape drag-strip data has long supported.
How accurate is this weight-to-power estimate?
It is a rough estimate, not a guarantee — the formula ignores gear ratio, tire grip, launch technique, and aerodynamic drag, all of which shift a real pass away from the prediction. Treat the output as a bench-racing figure for comparing builds, not as a number to bet a race on.
Does the power figure mean flywheel or wheel horsepower?
Flywheel horsepower. The constants 6.290 and 224 were tuned against real cars rated by their advertised, flywheel output, which already implicitly bakes in typical drivetrain loss. Entering a wheel-hp number from a dyno sheet — already discounted for that loss — will make the predicted ET and trap speed too optimistic.
Why do two cars with the same weight-to-power ratio run different times at the track?
Because the formula only sees weight and power; it has no idea whether the tires hook, the gearing suits the engine's powerband, or the driver leaves cleanly. Two cars can share an identical ratio and still be tenths apart once traction, gearing, and driver skill enter the real run.
Does the formula assume a specific quarter-mile surface or distance?
Yes — a standard 1,320-foot standing-start quarter mile on a prepared drag strip surface, which is what the underlying data was fit to. Running the same car on a loose or damp surface, or over a different distance, will produce a real result the formula was never built to predict.