Estimate engine horsepower using quarter-mile trap speed, elapsed time, dyno correction factors, or fuel consumption (BSFC).
Uses quarter-mile trap speed to estimate peak horsepower. Formula: HP = Weight × (Trap Speed / 234)³
| Method | HP | kW |
|---|---|---|
| Trap Speed | 234.14 | 174.6 |
| Elapsed Time | 382.25 | 285.04 |
| Dyno (corrected) | 1,096.33 | 817.53 |
| BSFC | 60 | 44.74 |
| HP | kW |
|---|---|
| 100 | 74.57 |
| 150 | 111.86 |
| 200 | 149.14 |
| 250 | 186.43 |
| 300 | 223.71 |
| 350 | 261 |
| 400 | 298.28 |
| 500 | 372.85 |
| 600 | 447.42 |
| 700 | 521.99 |
| 800 | 596.56 |
| 1000 | 745.7 |
A stopwatch cannot read your engine's rating. Quarter-mile formulas are often sold as horsepower detectors, but what they actually estimate is the effective power that moved one particular car, at one weight, through its tires, on one day. Add 200 lb of ballast or a poor launch and the figure moves, even though the engine did not change. Use the four methods on this page to cross-check each other, and treat none of them as a dyno sheet.
Start with a 3,000 lb car, driver included, that trapped at 100 mph. The Trap Speed tab returns 3,000 × (100 ÷ 234)³ = 234.14 hp, shown as 234.14 HP and 174.6 kW.
Now the Elapsed Time tab with the same 3,000 lb and the page's default 12.5 seconds: HP = 3,000 × (6.29 ÷ 12.5)³ = 382.25 hp. That is a long way from 234, and the reason is that the defaults are not from one run.
The two formulas are rearranged cube-root fits to old drag-strip data: trap speed ≈ 234 × (hp ÷ weight)^(1/3) and ET ≈ 6.29 × (weight ÷ hp)^(1/3). A car that makes 382 hp at 3,000 lb should trap about 117.8 mph, and in general the two agree only when ET × trap speed is near 6.29 × 234 = 1,471.9. Because of the cube, a 5% error in speed becomes a 15.8% error in horsepower: 105 mph gives 271.05 hp against 234.14 at 100.
The BSFC tab uses HP = fuel flow (lb/hr) ÷ BSFC. With the default 30 lb/hr and 0.50 lb/hp·hr that is 60 hp; at 0.40 it would be 75 hp. The page lists 0.40 to 0.55 as typical for gasoline.
The Dyno Correction tab multiplies your measured figure by a correction factor built from a reference pressure compared with dry-air pressure at your altitude and humidity, and from your air temperature compared with 77°F, each under a square root. The factor is printed under the result. Results show two decimals, and kW uses 0.7457 kW per hp.
Trap speed is generally treated as the better indicator, because ET also depends on launch, traction and shift quality. Use values from the same pass for both tabs, or the two answers will disagree.
Yes. Use race weight with the driver, fuel and anything else in the car. Entering the curb weight from a brochure makes the horsepower estimate look lower than it should.
Not necessarily. Gearing, tire slip, aerodynamics, density altitude and drivetrain loss all sit inside the quarter-mile figure. Treat it as a ballpark that can be off by 10% or more, and a dyno as the more reliable measurement.
On a mild day near sea level it should land close to 1.00, typically within a few percent. If the factor printed under the result is far from that, recheck the units (altitude in feet, humidity as 0 to 100) and trust your dyno sheet's own corrected number over this estimate.
Not directly. The box is in pounds, so multiply kilograms by 2.2046 first. A 1,360 kg car is about 2,998 lb. Speed can be entered in km/h: 160 km/h becomes 99.4 mph, which gives 230.08 hp at 3,000 lb.
Multiply gallons per hour by the fuel's density, which is roughly 6 lb per US gallon for gasoline. Fuel flow also has to come from the same load point where BSFC was measured, or the horsepower is meaningless.