ET Method Horsepower Calculator

How to Estimate Horsepower Change from Quarter-Mile ET

The ET method horsepower calculator estimates the change in rear-wheel horsepower between two quarter-mile runs at the same race weight. By comparing elapsed times before and after a modification, you can quantify the power gain or loss without strapping the vehicle to a dynamometer.

The ET Method Formula

The horsepower change is calculated using Roger Huntington's empirical drag-racing formula:

$$ \Delta HP = \frac{W}{\left(\frac{ET_2}{5.825}\right)^3} - \frac{W}{\left(\frac{ET_1}{5.825}\right)^3} $$

Where W is the race weight in pounds (vehicle + driver + fuel), ET₁ is the quarter-mile time before the modification, and ET₂ is the time after. The constant 5.825 comes from Patrick Hale's regression analysis linking quarter-mile ET to rear-wheel horsepower.

How to Use This Calculator

Enter the vehicle's race weight (including driver and fuel), the quarter-mile ET before the modification, and the ET after the modification. The calculator instantly shows the horsepower gain or loss, HP before and after, kilowatts gained, and metric PS gained. The percent change gives context for how significant the gain is relative to the baseline power.

ET Method vs. Trap Speed Method

The ET method is more sensitive to launch quality and traction than the trap speed method. If the vehicle hooked up better on one run than the other, the ET method may overstate or understate the true power change. When launches are inconsistent, use the 0-60 Quarter Mile Calculator instead, which relies on the vehicle's speed at the quarter-mile mark and is less affected by the first 60 feet. For engine modification planning, also check the Compression Ratio Calculator.

Practical Applications

Drag racers use this calculator to evaluate the effectiveness of bolt-on modifications such as cold air intakes, exhaust systems, ECU tunes, and cam swaps. For a broader power analysis, pair this with the Engine Horsepower Calculator. Restorers can verify whether engine rebuilds delivered the expected power improvement. Even motorcycle tuners can use the formula — provided the weight includes the rider and the ETs are measured at the same track on similar days.

Common Mistakes

The most common error is comparing runs at different weights. If the vehicle was lighter on the second run (less fuel, no passenger), the calculated gain will be overstated. Similarly, comparing runs from different tracks or vastly different weather conditions introduces variables that mask true engine output. Always compare runs at the same weight, the same track, and similar air density.