Why trap speed measures power and elapsed time does not
A drag strip gives you two numbers: elapsed time and trap speed. They measure different things, and only one of them is a power measurement.
Elapsed time is dominated by what happens in the first sixty feet. Tyre compound, suspension setup, launch rpm, converter stall and track preparation can swing a sixty-foot time by two tenths, and every one of those tenths carries through to the finish line. Two identical cars can be half a second apart on elapsed time with the same engine.
Trap speed does not care. By the top end of the track the car is at wide-open throttle in a tall gear with no traction problem left; it is simply converting engine power into speed against drag. Terminal speed therefore reflects how much power the engine made and how much mass it had to push, and very little else. A car that spins the tyres off the line and one that hooks perfectly will trap within a mile or two an hour of each other if their engines are the same.
That makes trap speed the honest check on a power claim. If someone's timeslip says 3,600 lb and 118 mph, the physics says their engine is making about 460 hp, whatever the dyno sheet claims. Compare the result with a torque-and-rpm calculation from the dyno trace and see whether the two agree.
Where the cube law comes from
The cube is not a fudge. Take a mass m accelerated by constant power P from rest. Power equals force times velocity, so m · v · dv/dt = P, which integrates to v2 = 2Pt/m. Integrating velocity to get distance and eliminating time gives:
v = (3 · x · P / m)1/3
Terminal velocity over a fixed distance x varies as the cube root of power over mass. Turn it round and you have P ∝ m · v3 — power is proportional to weight times the cube of trap speed, which is exactly the formula on this page. The constant K is what converts the proportionality into an equality in pounds, miles per hour and horsepower.
You can even work out what K would be in a frictionless world. Over 1,320 feet, converting horsepower to foot-pounds per second and pounds to slugs, the ideal relationship comes out as HP = W · MPH3 / 22,210,600, which is K = 281. Real cars need K = 234, and because power scales with the cube, that gap means a real car needs (281/234)3 = 1.73 times the ideal power — roughly 42% of the engine's output goes to aerodynamic drag, rolling resistance, driveline friction and the fact that no engine holds peak power across the whole run.
That is why the constant is empirical and why it is not universal. A slippery modern coupe on sticky radials loses less than a brick-shaped truck on street tyres, so it needs a larger K. The 234 value suits most well-sorted cars; 224 is the more conservative choice for a draggy body, a tall ride height or a roof rack. Nothing else in the formula changes.
Worked example: 3,500 lb, 115 mph trap
A car crosses the quarter-mile stripe at 115.0 mph. On the scales with the driver and half a tank it weighs 3,500 lb.
- Form the speed ratio. 115.0 ÷ 234 = 0.491453.
- Cube it. 0.4914532 = 0.241526, and 0.241526 × 0.491453 = 0.118699.
- Multiply by weight. 3,500 × 0.118699 = 415.4 flywheel horsepower.
- Convert to a dyno-comparable figure. At 15% drivetrain loss, 415.4 × 0.85 = 353.1 wheel horsepower.
- Check the ratio. 3,500 ÷ 415.4 = 8.42 lb per horsepower, which is 237 hp per short ton.
Now see how sharply the cube bites. Add just 5 mph, to 120.0: the ratio becomes 0.512821, cubed 0.134864, and the power required is 3,500 × 0.134864 = 472.0 hp. Five miles an hour cost 57 horsepower — a 13.6% power increase for a 4.3% speed increase, which is the cube law in action (1.0433 = 1.136).
This is also why a trap speed is such a sensitive instrument. A one mile per hour error in the reading is worth about 2.6% in the power estimate, so a timeslip is worth more than most people's guesses. It is also why chasing the last few mph of trap speed gets so expensive.
How much to trust the number
Treat the result as accurate to roughly ±5% on a normal car with a clean run, and as a sanity check rather than a measurement on anything unusual. Four things move it.
Weight accuracy. Power scales linearly with weight, so a 200 lb error on a 3,500 lb car is a 5.7% error in the answer. Weigh the car at the track with you in it; do not use the brochure kerb weight.
Aerodynamics. The constant embeds a typical drag area. A car with a big wing, an open roof, a roof rack or a raised suspension traps slower for the same power, so the formula under-reports its output. Drop to K = 224 for those and the estimate rises by about 14%.
Altitude and air density. The engine makes less power in thin air and the car meets less drag, and the two effects do not cancel. A run at Denver will trap slower than the same car at sea level, and the formula will report the power it actually made that day rather than its corrected sea-level output.
Whether the driver stayed in it. Any lift, any shift missed, any wheelspin at the top end and the trap speed understates the car. Take the best of several clean runs.
Where the estimate disagrees with a chassis dyno by more than about 10%, the dyno is the more suspect of the two. Rollers can be calibrated optimistically, correction factors can be applied twice, and strapping and tyre pressure change the reading. The strip does not have an opinion. Cross-check the ratio you get here against the power to weight calculator using the dyno figure and see which story is consistent.
Flywheel horsepower by trap speed and weight (K = 234)
| Trap speed (mph) | 3,000 lb | 3,500 lb | 4,000 lb |
|---|---|---|---|
| 90 | 170.7 | 199.1 | 227.6 |
| 100 | 234.2 | 273.2 | 312.2 |
| 110 | 311.6 | 363.6 | 415.5 |
| 120 | 404.6 | 472.0 | 539.5 |
| 130 | 514.4 | 600.1 | 685.9 |
| 140 | 642.5 | 749.6 | 856.6 |
| 150 | 790.2 | 921.9 | 1053.6 |
Weights are race weights including the driver. Doubling the weight doubles the power needed for the same trap speed; the speed term is what scales with the cube.
Limits and assumptions
- The constant is empirical. It bundles drag, rolling resistance and driveline loss into one number, so a car with unusual aerodynamics needs a different constant rather than a different formula.
- It estimates the power made on that run, not corrected sea-level power. A hot, high day gives a lower trap speed and a correspondingly lower estimate.
- Eighth-mile conversion is a rule of thumb. The 1.25 factor fits well-hooked cars; a traction-limited car that is still gaining hard at the eighth will beat it.
- Parachutes, wings and tall gearing break it. Very fast cars carry drag the constant never anticipated, so the estimate reads low.
- A partial lift ruins the reading. Trap speed only measures power if the throttle was open all the way to the stripe.
- Wind matters. A tailwind flatters the trap speed and a headwind punishes it, and neither appears anywhere in the formula.
Key terms
- Trap speed
- The average speed through the speed trap, a 66 ft segment ending at the finish line. It is not an instantaneous speed at the line.
- Race weight
- The mass the engine actually accelerated: car, fuel, driver and everything aboard, weighed as it ran.
- Sixty-foot time
- The first 60 ft of the run. It measures launch quality and is the part of elapsed time that trap speed deliberately ignores.
- Cube law
- The relationship P ∝ m·v³ for a fixed distance, which follows from accelerating a mass at constant power.
Using trap speed alongside the other tools
Trap speed is the audit, not the design tool. Use it to check a claim after the fact, and use the airflow and fuelling calculators to decide what to build in the first place. If the estimate comes back lower than your dyno sheet, the usual explanations are an optimistic dyno, a heavier car than you thought, or a run where the engine was not making its rated power — a hot intake charge, a lean top end, or ignition timing pulled by a knock sensor.
Because the formula is a rearrangement of P ∝ W · v3, it also works as a planning tool. Fix the trap speed you want and it tells you the power required at your weight, which you can then take to the power to weight calculator to see whether weight or power is the cheaper path. If the answer is power, the route usually runs through forced induction, with injector sizing and compressor pressure ratio as the next two decisions.
One more use worth knowing: consistency. Because trap speed is insensitive to launch quality, comparing trap speeds run to run is the fastest way to tell whether a tuning change actually did anything. If a new intake gains you two tenths of elapsed time but no trap speed, you improved the launch or the shift points, not the power. If it gains 2 mph at the trap on a 3,500 lb car trapping 115, that is 3,500 × [(117/234)3 − (115/234)3] = about 22 hp — and that is a real gain you can take to the bank.