Read power and conditions together
The worksheet’s empirical model is MPH = 234 × (HP / weight_lb)^(1/3). Cubing the equation shows why small speed differences can imply larger power differences within the model. Record weight, wind, track and weather before attributing a change to engine output. Air density is one relevant observation, not an automatic correction factor.
Model MPH = 234 × (HP/weight_lb)^(1/3); model HP = weight_lb × (MPH/234)³Follow the worked comparison below for the input units and assumptions.
Work through an example
400 hp at 3,200 lb yields 117 MPH in this model. Increasing model power by 10% raises model speed by the cube root of 1.10, about 3.23%, rather than by 10%.
Understand the cubic inverse relationship
Rearranging the worksheet gives HP = weight_lb × (MPH/234)^3. At unchanged weight, a 1% speed increase implies approximately 3.03% more model power because 1.01 cubed is 1.030301. This sensitivity makes measurement and model assumptions important.
Record weather without inventing a correction
Log station pressure, temperature, humidity and wind alongside a pass. Density altitude summarizes an atmospheric state under the selected model; it does not automatically supply a power correction for every engine or an ET correction for every vehicle.
Inspect how trap speed changes inferred model power
Fix weight at 3,200 lb and compare measured trap-speed inputs within the stated empirical model. Because the inverse is cubic, a small speed difference creates a larger proportional power difference. This sensitivity makes consistent measurements and model scope essential.
Model MPH = 234 × (HP/weight_lb)^(1/3); model HP = weight_lb × (MPH/234)³- Record race-ready weight and the actual trap-speed measurement from the time slip.
- Keep the coefficient and power definition consistent when comparing passes.
- Cube MPH/234, then multiply by pounds for the inverse model horsepower.
- Record wind and atmospheric conditions separately rather than assigning every speed difference to engine output.
| Trap speed input (MPH) | Race weight (lb) | Inferred model hp |
|---|---|---|
| 110 | 3200 | 332.41 |
| 117 | 3200 | 400 |
| 120 | 3200 | 431.56 |
At 117 MPH the model returns 400 hp for 3,200 lb. A 1% speed increase corresponds to a 3.0301% model-power increase at the same weight. The equation cannot distinguish additional engine power from aerodynamic, weight or condition differences that affect the actual pass.
Check the inputs before interpreting a difference
Trap speed is an end-of-run timing measurement, not the average speed of the entire run. Density altitude is a separate atmosphere calculation and does not automatically weather-correct this power estimate.
Before you apply the result
A density-altitude value is estimated from station pressure, temperature and humidity; it is distinct from track elevation. This platform does not automatically correct trap speed or horsepower for weather, induction type or engine management.
Try the calculation
Enter your own values in the related calculators. Compare one input at a time and keep units and operating conditions visible with the result.
Frequently asked questions
Is trap speed the average speed of the run?
No. It is an end-of-run timing measurement, not distance divided by total ET.
Can this estimate replace a dyno result?
No. The coefficient-based model does not isolate all vehicle losses, aerodynamic effects or measurement conditions.
Does entering density altitude correct trap speed?
No. The site provides a separate atmosphere calculation and does not automatically correct the trap-speed model.
Why is horsepower inferred from trap speed so sensitive to a small MPH change?
The model cubes the speed ratio. At unchanged weight, a speed factor of 1.01 becomes a horsepower factor of 1.01³, approximately 1.030301.
Reference and method
Hale coefficient model: ET constant 5.825 and speed constant 234. Lucius documents the formulas and compares them with road-test data. These are empirical relationships, not physical laws or calibrated vehicle predictions.
Jeff Lucius / Stealth 316 — Quarter-mile formulas and original regression analysis ↗These explanations and examples describe the documented model. No manufacturer testing, named expert authorship or independent professional review is claimed. Read the method policy or report a correction.