WHP to Crank HP Calculator
Dyno measured power
FWD (Front Wheel Drive)
RWD (Rear Wheel Drive)
AWD (All Wheel Drive)
Manual Transmission
Automatic / Torque Converter
Dual Clutch (DCT/PDK)
Adjustable manually
Estimated Crank Horsepower
0 HP
Power lost to drivetrain: 0 HP
Based on a 15% loss factor.
Based on a 15% loss factor.
Understanding the WHP to HP Calculation
When modifying cars or analyzing dyno results, understanding the difference between WHP (Wheel Horsepower) and HP (Crank/Brake Horsepower) is critical. Manufacturers advertise the horsepower generated at the engine's crankshaft (BHP), but a chassis dynamometer measures the power that actually reaches the ground (WHP).
The difference between these two numbers is caused by parasitic drivetrain loss. As power travels from the engine through the flywheel, transmission, driveshaft, differential, axles, and finally to the wheels, friction and inertia consume a percentage of that energy.
How to Convert Wheel HP to Crank HP
Since drivetrain loss is a reduction of the original engine power, the correct mathematical formula to reverse this process divides the wheel horsepower by the remaining percentage of efficiency.
The Formula:
Crank HP = Wheel HP / (1 – (Drivetrain Loss % / 100))
Example: If a car makes 300 WHP on a dyno and has a 15% drivetrain loss:
300 / (1 – 0.15) = 300 / 0.85 = 352.9 Crank HP.
Typical Drivetrain Loss Estimates
While every car is different, the automotive community generally accepts the following ranges for estimating drivetrain loss based on layout and transmission type:
| Drivetrain Layout | Manual Trans | Automatic Trans |
|---|---|---|
| FWD (Front Wheel Drive) | 10% – 12% | 12% – 15% |
| RWD (Rear Wheel Drive) | 12% – 15% | 15% – 18% |
| AWD (All Wheel Drive) | 18% – 22% | 20% – 25% |
Factors Influencing Drivetrain Loss
- Transmission Type: Traditional torque converter automatics typically "eat" more power due to fluid coupling slippage compared to manual transmissions or modern Dual Clutch Transmissions (DCT).
- Differentials: AWD systems require extra differentials and transfer cases, significantly increasing friction and weight, leading to higher losses.
- Rotating Mass: Heavy wheels, tires, and brake rotors increase the inertia required to spin the wheels, which can read as lower WHP on an inertia dyno.
- Dyno Type: The type of dynamometer (Mustang vs. Dynojet) affects the reading. Mustang dynos represent "load-bearing" scenarios and often read lower than Dynojets.