1/8 Mile Horsepower Calculator

1/8 Mile Horsepower Calculator

Enter your vehicle details below:

Estimated Performance

Note: Speed-based calculations are generally more accurate for actual engine potential.

function calculateHP() { var weight = parseFloat(document.getElementById("vehicleWeight").value); var speed = parseFloat(document.getElementById("trapSpeed").value); var et = parseFloat(document.getElementById("elapsedTime").value); var resultBox = document.getElementById("hpResultDisplay"); var speedOut = document.getElementById("speedHpResult"); var etOut = document.getElementById("etHpResult"); if (isNaN(weight) || weight 0) { // Formula: HP = Weight * (Speed / 234)^3 var hpSpeed = weight * Math.pow((speed / 234), 3); speedOut.innerHTML = "HP (from Speed): " + Math.round(hpSpeed) + " whp"; } if (!isNaN(et) && et > 0) { // Formula: HP = Weight / (ET / 5.85)^3 var hpEt = weight / Math.pow((et / 5.85), 3); etOut.innerHTML = "HP (from ET): " + Math.round(hpEt) + " whp"; } if ((isNaN(speed) || speed <= 0) && (isNaN(et) || et <= 0)) { speedOut.innerHTML = "Please enter either Speed or ET."; } }

Understanding 1/8 Mile Horsepower Calculations

In drag racing, the 1/8 mile sprint is a standard measure of a vehicle's acceleration and power-to-weight ratio. Because short-track racing focuses heavily on the "launch" and mid-range torque, calculating horsepower from these numbers provides a unique look at how much power your engine is actually putting to the ground.

The Mathematical Formulas

This calculator utilizes two primary industry-standard formulas derived from physics to estimate "Wheel Horsepower" (WHP):

• Trap Speed Method: HP = Weight * (Speed / 234)³. This method is usually preferred because trap speed is less affected by traction issues at the starting line.
• Elapsed Time (ET) Method: HP = Weight / (ET / 5.85)³. This method reflects your total run efficiency, including how well the car "hooked" at the start.

Why is Trap Speed HP Different from ET HP?

It is common to see two different numbers if you provide both inputs. If your Trap Speed HP is significantly higher than your ET HP, it usually indicates a traction problem. Your car has the power to go fast (Trap Speed), but it struggled to get off the line efficiently (ET).

Example Calculation

Imagine a muscle car with a total weight (including driver) of 3,600 lbs. If that car crosses the 1/8 mile mark at 92 MPH with an ET of 7.6 seconds:

1. Speed Calculation: 3,600 * (92 / 234)³ ≈ 218 WHP.
2. ET Calculation: 3,600 / (7.6 / 5.85)³ ≈ 164 WHP.

In this scenario, the discrepancy suggests the car may have experienced significant wheel spin, as the trap speed indicates the engine is capable of much more than the final time suggests.

Factors That Affect Accuracy

While these formulas are mathematically sound, real-world variables can influence the results:

• Density Altitude (DA): High humidity or high elevation reduces engine power.
• Transmission Type: Modern dual-clutch transmissions shift faster than manual gearboxes, often resulting in higher ET-based HP figures.
• Aerodynamics: Though less critical in the 1/8 mile than the 1/4 mile, drag still plays a role in final trap speeds.
• Drivetrain Loss: This calculator estimates power at the wheels. To estimate crank horsepower, you typically add 15-20% to the result.