Crank HP (BHP) to Wheel HP (WHP)
Wheel HP (WHP) to Crank HP (BHP)
Custom Percentage (Enter Manually)
Front Wheel Drive (FWD) – ~15% Loss
Rear Wheel Drive (RWD) – ~18% Loss
All Wheel Drive (AWD) – ~25% Loss
Estimated Wheel Horsepower
0 HP
Input Power:0 HP
Drivetrain Loss:0% (0 HP)
This represents the power actually reaching the tires after mechanical losses in the transmission and drivetrain.
Understanding Wheel Horsepower (WHP) vs. Brake Horsepower (BHP)
When measuring vehicle performance, understanding the distinction between Crank Horsepower (often referred to as Brake Horsepower or BHP) and Wheel Horsepower (WHP) is crucial for enthusiasts and tuners alike. The numbers quoted by manufacturers in brochures are almost exclusively Crank Horsepower—the power generated by the engine at the flywheel, with no accessories or drivetrain attached.
Wheel Horsepower (WHP) is the actual power delivered to the ground. This number is measured using a chassis dynamometer (dyno) and is always lower than the crank horsepower due to parasitic losses caused by friction and inertia in the drivetrain components.
What is Drivetrain Loss?
Drivetrain loss refers to the power dissipated as energy travels from the engine, through the transmission, driveshaft, differentials, and axles, finally reaching the wheels. This energy is primarily lost as heat due to mechanical friction and the energy required to spin heavy rotational masses.
Typical Drivetrain Loss Estimations
While every car is unique, the automotive community generally accepts the following ranges for drivetrain loss based on the layout of the vehicle:
Front Wheel Drive (FWD): 10% to 15% loss. FWD systems are generally the most efficient because the engine sits directly over the drive wheels, requiring fewer components (no long driveshaft or rear differential).
Rear Wheel Drive (RWD): 15% to 20% loss. Energy must travel through a transmission, a driveshaft running the length of the car, and a rear differential, creating more friction points.
All Wheel Drive (AWD): 20% to 25% loss. AWD systems are the heaviest and most complex, requiring transfer cases, multiple differentials, and extra axles, resulting in the highest parasitic loss.
How to Calculate WHP from BHP
To estimate your Wheel Horsepower from the manufacturer's rated Crank Horsepower, you apply the estimated drivetrain loss percentage. The formula calculates the remaining power after the loss is subtracted.
Formula: WHP = BHP × (1 – (Loss % / 100))
Example: A car with 300 BHP and a 15% drivetrain loss (0.15):
300 × (1 – 0.15) = 300 × 0.85 = 255 WHP.
How to Calculate Crank HP from WHP
If you have dynoed your car and know the WHP, you may want to estimate what the engine is making at the crank. This requires dividing by the efficiency factor.
Formula: BHP = WHP / (1 – (Loss % / 100))
Example: A car puts down 400 WHP on a dyno with an estimated 20% loss:
400 / (1 – 0.20) = 400 / 0.80 = 500 BHP.
Factors Affecting Accuracy
It is important to note that these calculators provide estimates. Real-world results vary based on several factors:
Dyno Type: Mustang dynos (load-bearing) typically read lower than Dynojet dynos (inertia-based).
Transmission Type: Automatic transmissions usually have higher parasitic losses than manual transmissions due to the torque converter (though modern dual-clutch transmissions are very efficient).
Conditions: Temperature, humidity, and altitude affect engine performance and dyno readings.
function updateLabels() {
var mode = document.getElementById('conversionMode').value;
var inputLabel = document.getElementById('hpInputLabel');
var resultTitle = document.getElementById('resultTitle');
var explanationText = document.getElementById('explanationText');
if (mode === 'bhpToWhp') {
inputLabel.innerText = "Crank Horsepower (BHP)";
resultTitle.innerText = "Estimated Wheel Horsepower (WHP)";
explanationText.innerText = "This represents the power actually reaching the tires after mechanical losses.";
} else {
inputLabel.innerText = "Wheel Horsepower (WHP)";
resultTitle.innerText = "Estimated Crank Horsepower (BHP)";
explanationText.innerText = "This estimates the engine's power at the flywheel before drivetrain losses.";
}
}
function setDrivetrainLoss() {
var type = document.getElementById('drivetrainType').value;
var lossInput = document.getElementById('lossPercentage');
if (type === 'fwd') {
lossInput.value = 15;
} else if (type === 'rwd') {
lossInput.value = 18;
} else if (type === 'awd') {
lossInput.value = 25;
} else {
lossInput.value = ""; // Clear for custom
lossInput.focus();
}
}
function calculateWHP() {
// Get input values
var hpInput = document.getElementById('hpValue').value;
var lossInput = document.getElementById('lossPercentage').value;
var mode = document.getElementById('conversionMode').value;
// Elements to update
var resultBox = document.getElementById('resultBox');
var finalHpDisplay = document.getElementById('finalHp');
var inputPowerDisplay = document.getElementById('inputPowerDisplay');
var lossDisplay = document.getElementById('lossDisplay');
var hpLostDisplay = document.getElementById('hpLostDisplay');
// Validation
if (hpInput === "" || isNaN(hpInput) || hpInput <= 0) {
alert("Please enter a valid horsepower value.");
return;
}
if (lossInput === "" || isNaN(lossInput) || lossInput = 100) {
alert("Please enter a valid drivetrain loss percentage (0-99).");
return;
}
var hp = parseFloat(hpInput);
var lossPct = parseFloat(lossInput);
var lossDecimal = lossPct / 100;
var result = 0;
var hpLost = 0;
// Calculation Logic
if (mode === 'bhpToWhp') {
// Calculating WHP from BHP
// WHP = BHP * (1 – loss)
result = hp * (1 – lossDecimal);
hpLost = hp – result;
} else {
// Calculating BHP from WHP
// BHP = WHP / (1 – loss)
// Example: 255 WHP / 0.85 = 300 BHP
result = hp / (1 – lossDecimal);
hpLost = result – hp;
}
// Display Logic
resultBox.style.display = "block";
finalHpDisplay.innerText = Math.round(result) + " HP";
inputPowerDisplay.innerText = hp;
lossDisplay.innerText = lossPct;
hpLostDisplay.innerText = Math.round(hpLost);
}