In the automotive world, not all horsepower is created equal. When a manufacturer claims a car has 400 horsepower, they are almost always referring to Crank Horsepower (also known as BHP or Brake Horsepower). This is the power measured directly at the engine's flywheel before it goes through the transmission.
Wheel Horsepower (WHP) is the actual power that reaches the ground. This is the number you see when you put a car on a chassis dynamometer (dyno). Because of physics, some power is lost as it travels through the drivetrain.
What causes Drivetrain Loss?
Drivetrain loss is the energy consumed by the components sitting between the engine and the tires. Common factors include:
Transmission Friction: Moving gears and hydraulic fluid in automatics creates heat and resistance.
Driveshafts and U-joints: Spinning heavy metal components requires energy.
Differential: Turning the power 90 degrees (in RWD/AWD) creates friction.
Axles and Bearings: Every rotating part adds a small amount of drag.
Standard Drivetrain Loss Estimates
While every car is different, the automotive industry generally uses these estimates for drivetrain parasitic loss:
Front-Wheel Drive (FWD): 10% to 15% loss. (Most efficient due to fewer components).
Rear-Wheel Drive (RWD): 15% to 18% loss. (Standard for sports cars).
All-Wheel Drive (AWD): 20% to 25% loss. (Heaviest loss due to multiple differentials and transfer cases).
How to Use the Calculator
To calculate your estimated crank horsepower from a dyno sheet, select "Convert WHP to Engine HP," enter your dyno number, and select your drivetrain type. Conversely, if you want to know how much power your new engine build will actually put to the ground, use the "Convert Engine HP to WHP" mode.
Example: If your RWD car makes 340 WHP on a dyno and you assume a 15% drivetrain loss, your engine is likely producing approximately 400 HP at the crank.
function updateLabels() {
var mode = document.getElementById("calcMode").value;
var powerLabel = document.getElementById("powerLabel");
var powerInput = document.getElementById("powerInput");
if (mode === "whpToHp") {
powerLabel.innerText = "Wheel Horsepower (WHP):";
powerInput.placeholder = "e.g. 300";
} else {
powerLabel.innerText = "Engine Horsepower (Crank/BHP):";
powerInput.placeholder = "e.g. 350";
}
}
function updateLossValue() {
var drivetrain = document.getElementById("drivetrain").value;
var customContainer = document.getElementById("customLossContainer");
var lossInput = document.getElementById("lossPercentage");
if (drivetrain === "custom") {
customContainer.style.display = "block";
} else {
customContainer.style.display = "none";
lossInput.value = drivetrain;
}
}
function calculatePower() {
var mode = document.getElementById("calcMode").value;
var inputPower = parseFloat(document.getElementById("powerInput").value);
var lossPercent = parseFloat(document.getElementById("lossPercentage").value);
var resultArea = document.getElementById("resultArea");
var mainResult = document.getElementById("mainResult");
var resultTitle = document.getElementById("resultTitle");
var lossDetail = document.getElementById("lossDetail");
if (isNaN(inputPower) || inputPower <= 0) {
alert("Please enter a valid horsepower number.");
return;
}
if (isNaN(lossPercent) || lossPercent = 100) {
alert("Please enter a valid drivetrain loss percentage.");
return;
}
var lossDecimal = lossPercent / 100;
var finalPower = 0;
var difference = 0;
if (mode === "whpToHp") {
// Formula: Crank HP = WHP / (1 – Loss)
finalPower = inputPower / (1 – lossDecimal);
difference = finalPower – inputPower;
resultTitle.innerText = "Estimated Engine Horsepower (Crank)";
mainResult.innerText = Math.round(finalPower) + " HP";
lossDetail.innerText = "Estimated loss of " + Math.round(difference) + " HP through the drivetrain (" + lossPercent + "%)";
} else {
// Formula: WHP = Crank HP * (1 – Loss)
finalPower = inputPower * (1 – lossDecimal);
difference = inputPower – finalPower;
resultTitle.innerText = "Estimated Wheel Horsepower (WHP)";
mainResult.innerText = Math.round(finalPower) + " WHP";
lossDetail.innerText = "Approx. " + Math.round(difference) + " HP lost before reaching the wheels (" + lossPercent + "%)";
}
resultArea.style.display = "block";
}