Horsepower Calculator 1/8 Mile

Estimate your engine's flywheel horsepower based on your 1/8 mile drag racing performance.

Method 1: Using Elapsed Time (ET)

Include driver and fuel

Method 2: Using Trap Speed

Include driver and fuel

Understanding 1/8 Mile Horsepower Calculations

In drag racing, the 1/8 mile (660 feet) is a common metric used to gauge a vehicle's acceleration and power output. While dyno tests are the most accurate way to measure engine output, mathematical formulas based on real-world track performance can provide a very close estimate of flywheel horsepower.

The Formulas Used

This calculator utilizes two primary mathematical models derived from the physics of mass and acceleration:

• The ET Formula: HP = Weight / (ET / 5.854)³
• The Speed Formula: HP = Weight * (Speed / 234)³

ET vs. Trap Speed: Which is more accurate?

Trap speed is generally considered a better indicator of actual horsepower because it is less affected by the vehicle's "launch" or traction. A car with poor traction might have a slow (high) ET, but its trap speed will still reflect the power the engine is producing. ET-based calculations are highly dependent on the 60-foot time and the efficiency of the suspension and tires.

Key Factors to Consider

To get the most accurate result from this 1/8 mile horsepower calculator, ensure you are accounting for the following:

1. Total Weight: This must include the car, the driver, all fluids, and the fuel load at the time of the run.
2. Wheel HP vs. Flywheel HP: These formulas typically estimate Flywheel (Crank) Horsepower. If you are comparing this to a chassis dyno, remember that drivetrain loss (transmission, differential) usually accounts for a 15-20% difference.
3. Weather Density: Air temperature and humidity affect engine performance. A run on a cold night will yield more power than a run on a hot, humid afternoon.

Example Calculation:
If your car weighs 3,200 lbs and you ran a 6.80 second 1/8 mile at 102 MPH:
– Based on ET: Approx 373 HP
– Based on Speed: Approx 265 HP (Wheel) vs 380 HP (Estimated Crank)

function calculateByET() { var weight = parseFloat(document.getElementById('weight_et').value); var et = parseFloat(document.getElementById('et_value').value); var resultDiv = document.getElementById('res_et'); if (isNaN(weight) || isNaN(et) || weight <= 0 || et <= 0) { resultDiv.innerHTML = "Please enter valid numbers."; return; } // Formula: HP = Weight / (ET / 5.854)^3 var hp = weight / Math.pow((et / 5.854), 3); resultDiv.innerHTML = "Estimated HP: " + hp.toFixed(2) + " hp"; } function calculateBySpeed() { var weight = parseFloat(document.getElementById('weight_speed').value); var speed = parseFloat(document.getElementById('speed_value').value); var resultDiv = document.getElementById('res_speed'); if (isNaN(weight) || isNaN(speed) || weight <= 0 || speed <= 0) { resultDiv.innerHTML = "Please enter valid numbers."; return; } // Formula: HP = Weight * (Speed / 234)^3 // This is a standard variation of the Keasler/Hale formula var hp = weight * Math.pow((speed / 234), 3); resultDiv.innerHTML = "Estimated HP: " + hp.toFixed(2) + " hp"; }