Estimate engine horsepower from quarter-mile performance and vehicle weight.
These formulas estimate wheel horsepower from a car's measured quarter-mile performance. They were derived by Wallace Racing from large datasets of drag-race results. The trap speed formula is more physics-based (terminal velocity scales with cube root of power); the ET formula is a pure empirical regression. Both assume a stock or mildly modified, reasonably aerodynamic vehicle. Heavily modified or oddly shaped vehicles will show greater error.
Trap Speed method
HP = weight (lbs) × (trap speed mph / 234)³
Elapsed Time method
HP = weight (lbs) ÷ (ET in seconds / 5.825)³
Use the total vehicle weight at the strip: driver weight + fuel + any passengers. This is heavier than the manufacturer's curb weight. A typical passenger car adds 150–200 lbs over curb weight for the driver and a half-tank of fuel.
They are accurate to within 3–5% for relatively stock, production-bodied vehicles. Significant aerodynamic modifications (wings, diffusers, drag body kits) alter the relationship between trap speed and power. For highly modified cars, a chassis dyno is more reliable.
Both methods have ~5% uncertainty. If they disagree significantly, check that you're using the actual vehicle weight (not manufacturer curb weight) and that the run was a clean pass (no wheel spin, good reaction time). The trap speed method is generally more reliable because it is less sensitive to launch technique.
These formulas estimate at-the-wheels (or at-the-rear-axle) horsepower from performance data. Typically crank HP is 10–20% higher than wheel HP depending on drivetrain losses.