Engine Horsepower Calculator · Practical guide
ET Method vs Trap Speed Method: Which Estimates Horsepower Better?
Trap speed isolates engine power; ET measures the whole run. Using both brackets your true horsepower within about 5-10%.
At a glance
Two Formulas, Two Perspectives
Both methods estimate horsepower from quarter-mile performance, but they read different things. The ET method uses elapsed time: HP = Weight / (ET / 5.825)^3. The trap speed method uses finishing velocity: HP = Weight x (Trap Speed / 234)^3. Because they weight the launch differently, the same car often produces two slightly different answers.
Interactive tool
Engine Horsepower Calculator
Enter your own values, then compare the result with the examples below.
Results
Same Car, Both Methods
Take a 3,500-pound car that runs 13.5 seconds at 102 mph:
| Method | Input | Estimated HP |
|---|---|---|
| ET Method | 13.5 seconds | 281 HP |
| Trap Speed Method | 102 mph | 290 HP |
The midpoint — about 285 HP — is a solid working estimate, and the small gap suggests a reasonably clean launch.
When to Trust Each One
- Trust trap speed when the launch was ugly — wheelspin, bogging, or a slow reaction time barely affects the number at the finish line.
- Trust ET when you want to judge the whole combination, including traction, gearing, and driver skill.
- ET reads low, trap reads normal means power is fine but traction or launch technique is costing time.
- Trap reads low, ET reads normal often signals a car that launches hard but runs out of breath up top, or a high-drag body.
Accuracy Expectations
For a healthy street car on a good pass, expect both methods to land within 5-10% of a chassis dyno number once you add typical drivetrain losses back in. Neither formula knows your altitude, transmission slip, or aerodynamics, so treat the results as well-informed estimates rather than measurements — and always use full race weight including the driver.
Frequently Asked Questions
Why do the two methods give different answers?
ET is heavily influenced by the first 60 feet of the run, while trap speed reflects sustained power over the full distance. Traction, gearing, and aerodynamics push the two estimates apart.
Which method should I use for a front-wheel-drive car?
Trap speed is usually the better choice for front-wheel drive because weight transfer off the drive wheels makes launches inconsistent, which distorts ET-based estimates.
Can I average the two results?
Yes. Averaging the ET and trap speed estimates is a common way to get a single working number, and the spread between them tells you how much the launch influenced the run.
Do these methods work for motorcycles?
Approximately, but motorcycle aerodynamics and rider weight dominate the result, so estimates run less accurate than for cars. Use total weight of bike plus rider.