Horsepower Estimation Methods
Both methods estimate engine horsepower from quarter-mile drag strip data. Vehicle weight should include the driver, passengers, fuel, and anything in the car during the run.
Elapsed Time (ET) Method
The ET method uses the total time to cover a quarter mile (402.3 meters):
HP = Weight ÷ (ET ÷ 5.825)³
This formula works best when the driver applies maximum power from start to finish with consistent traction.
Trap-Speed Method
The trap-speed method uses the vehicle's speed at the quarter-mile finish line:
HP = Weight × (Speed ÷ 234)³
Use the speed at the finish line, not the average speed. This method can be more accurate because trap speed is less affected by launch technique and traction issues at the start.
Example
A 3,800 lb vehicle with a 14.5-second quarter mile:
- ET Method: 3800 ÷ (14.5 ÷ 5.825)³ = 3800 ÷ 15.43 ≅ 246.3 HP
Same vehicle with a 95 mph trap speed:
- Trap Method: 3800 × (95 ÷ 234)³ = 3800 × 0.0669 ≅ 254.2 HP
Factors That Affect Accuracy
- Drivetrain loss: These formulas estimate crank HP. Wheel HP is typically 10-20% less due to transmission and differential friction.
- Altitude & weather: Higher altitude and hotter temperatures reduce air density and engine output. Correction factors exist but aren't included here.
- Traction: Wheel spin wastes energy and inflates ET / reduces trap speed, leading to underestimated HP.
- Aerodynamics: At higher speeds, drag becomes significant. These formulas don't account for vehicle aerodynamics.
- Launch technique: The ET method is sensitive to reaction time and launch consistency.
Typical Quarter-Mile Times
| Vehicle Type | ET (seconds) | Trap Speed (mph) | Est. HP |
|---|---|---|---|
| Economy car | 17-19 | 75-85 | 100-150 |
| Mid-size sedan | 14-16 | 90-105 | 200-300 |
| Sports car | 12-14 | 105-120 | 300-500 |
| Muscle car / tuned | 10-12 | 120-140 | 500-800 |
| Top Fuel dragster | 3.6-3.8 | 330+ | 11,000+ |