Rwhp to Hp Calculator

RWHP to HP Calculator

Manual / Rear-Wheel Drive (15%) Manual / Front-Wheel Drive (12%) Automatic / Rear-Wheel Drive (20%) All-Wheel Drive (25%) Custom Percentage

Estimated Crank Horsepower (HP)

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function calculateCrankHP() { var rwhp = parseFloat(document.getElementById('rwhpInput').value); var lossPercent = parseFloat(document.getElementById('lossInput').value); var resultDiv = document.getElementById('hpResult'); var crankValueDisplay = document.getElementById('crankValue'); var lossDetail = document.getElementById('lossDetail'); if (isNaN(rwhp) || isNaN(lossPercent) || rwhp <= 0 || lossPercent = 100) { alert("Please enter valid positive numbers. Drivetrain loss must be less than 100%."); return; } // Formula: HP = RWHP / (1 – (Loss / 100)) var lossDecimal = lossPercent / 100; var crankHP = rwhp / (1 – lossDecimal); var lostHP = crankHP – rwhp; crankValueDisplay.innerText = Math.round(crankHP) + " HP"; lossDetail.innerText = "Approximately " + Math.round(lostHP) + " HP lost through the drivetrain (" + lossPercent + "%)."; resultDiv.style.display = "block"; }

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Understanding RWHP vs. Crank Horsepower

In the automotive world, not all horsepower is created equal. When a manufacturer advertises a car's power, they are typically referring to Crank Horsepower (Brake Horsepower or BHP). This is the power measured directly at the engine's flywheel before it goes through the transmission and axles.

Rear Wheel Horsepower (RWHP) is the actual power that reaches the ground. As power travels from the engine through the gearbox, driveshaft, differential, and axles, a portion is lost due to friction and rotational mass. This is known as parasitic loss or drivetrain loss.

Typical Drivetrain Loss Figures

While every car is different, tuners generally use these standard percentages to estimate the gap between the dyno and the flywheel:

• Manual / Front-Wheel Drive: 10-13% loss.
• Manual / Rear-Wheel Drive: 15% loss.
• Automatic / Rear-Wheel Drive: 18-22% loss (Torque converters create more heat and friction).
• All-Wheel Drive: 20-25% loss (Powering two differentials and four wheels increases resistance).

How to Calculate Engine HP from RWHP

To convert wheel horsepower back to crank horsepower, we use the following formula:

Crank HP = RWHP / (1 – Loss Percentage)

Example Calculation

If your car produces 400 RWHP on a chassis dyno and you have a manual RWD transmission (15% loss):

1. Convert 15% to a decimal: 0.15
2. Subtract from 1: 1 – 0.15 = 0.85
3. Divide RWHP by that number: 400 / 0.85 = 470.6 Crank HP

Note: This calculator provides an estimate. Real-world losses depend on fluid temperatures, tire pressure, gear ratios, and the specific type of dynamometer (e.g., Mustang Dyno vs. Dynojet).