This flywheel horsepower calculator converts wheel horsepower (WHP) measured on a chassis dyno back to crank (flywheel) horsepower — the figure manufacturers advertise. Enter your dyno-measured wheel power and an estimated drivetrain loss to recover the engine's output at the flywheel.
Flywheel HP Formula
Flywheel horsepower — also called crank or brake horsepower — is the power the engine makes at its output shaft, before any of it is lost driving the transmission, driveshaft, differential, and axles. A chassis dyno can only measure power at the wheels (WHP), so to compare against a manufacturer's quoted figure you have to add the drivetrain losses back in. The key math point: a 17% loss means the wheels see 83% of crank power, so you divide WHP by 0.83 rather than multiplying by 1.17.
Why You Divide Instead of Add
This trips up a lot of people. If a drivetrain loses 17%, the wheels receive 83% of the engine's output. To reverse that you divide by 0.83 (≈ +20.5%), not multiply by 1.17. Adding 17% back to 400 WHP gives 468 — but the correct flywheel figure is 482. The gap grows as losses rise, so using the right operation matters for any honest power claim.
Typical Drivetrain Loss by Layout
Losses depend on how many components sit between engine and pavement. Manual gearboxes are generally more efficient than automatics, and all-wheel drive loses the most because it drives two axles.
| Drivetrain | Typical loss | WHP → Crank factor |
|---|---|---|
| FWD manual | 10–12% | ÷ 0.88–0.90 |
| RWD manual | 15–17% | ÷ 0.83–0.85 |
| RWD automatic | 17–20% | ÷ 0.80–0.83 |
| AWD | 20–25% | ÷ 0.75–0.80 |
These are estimates — actual loss varies with gear oil, tire size, dyno type, and even temperature. The "15% rule" is a convention, not a measured constant, which is why two dynos can report different crank numbers for the same car.
Worked Example
This calculator provides estimates based on standard mathematical formulas. Real-world results will vary based on mechanical condition, environmental factors, and other variables.
Frequently Asked Questions
Yes, essentially. Both are measured at the engine crankshaft before the drivetrain.
Flywheel (or crank) horsepower is power measured at the engine's output before any drivetrain losses. It's the figure manufacturers usually advertise.
Divide wheel horsepower by (1 minus the drivetrain loss fraction). For a RWD car with 15% loss, divide wheel HP by 0.85.
Because the drivetrain — transmission, driveshaft, and differential — absorbs power through friction before it reaches the wheels, typically 10–25% depending on layout.
Wheel horsepower is directly measured on a chassis dyno, so it's the real number. Flywheel horsepower is an estimate derived from it using an assumed loss percentage.