What is Brake Horsepower (BHP)? BHP vs WHP Explained

In the world of cars, not all "horsepower" is created equal. You might see a car advertised as 300 HP, but a dyno test shows only 250 HP. Why? The difference lies in BHP versus WHP.

Quick Answer (TL;DR)

• BHP (Brake Horsepower): Power measured at the engine crankshaft (no gearbox, alternator, etc.). This is the advertised number.
• WHP (Wheel Horsepower): Power measured at the tires (after gearbox, driveshaft, axles). This is the real-world number.
• The Gap: WHP is typically 15-25% lower than BHP due to friction losses.

What is Brake Horsepower (BHP)?

Brake Horsepower is the measure of an engine's horsepower before the loss in power caused by the gearbox, alternator, differential, water pump, and other auxiliary components.

Why "Brake"?

The name comes from the measurement device called a Prony Brake (or later, a water brake dynamometer). This device applies a braking load to the engine's spinning output shaft to measure torque and RPM, which are then calculated into horsepower.

• Standard: Most factory car specs (e.g., "450 HP") are actually BHP (measured at the crank/flywheel) under ideal laboratory conditions (SAE J1349 standards).

What is Wheel Horsepower (WHP)?

Wheel Horsepower is measured at the driven wheels, usually on a chassis dynamometer (rolling road).

• This is the power that actually pushes the car forward.
• It accounts for all the parasitic losses in the drivetrain:
  • Transmission friction
  • Differential gears
  • Driveshaft weight/inertia
  • Axle friction
  • Tire rolling resistance

Drivetrain Loss: The 15% Rule

The difference between BHP and WHP is called Drivetrain Loss.

General Rule of Thumb Estimates:

1. Front-Wheel Drive (FWD): ~10-15% Loss

   • Most efficient (fewer shafts).
   • Example: 200 BHP Engine ≈ 170-180 WHP.

2. Rear-Wheel Drive (RWD): ~15-20% Loss

   • Long driveshaft and rear differential add weight/friction.
   • Example: 400 BHP Engine ≈ 320-340 WHP.

3. All-Wheel Drive (AWD): ~20-25% Loss

   • Most complex (transfer case, two differentials, multiple shafts).
   • Example: 300 BHP Engine ≈ 225-240 WHP.

Calculation Example

Car: Subaru WRX STI (AWD)

• Factory Rating: 305 BHP
• Expected Loss: ~20%
• Calculation: 305 × (1 - 0.20) = 244
• Real WHP: You usually see ~240-250 WHP on a dyno.

Car: Honda Civic Type R (FWD)

• Factory Rating: 306 BHP
• Expected Loss: ~12%
• Calculation: 306 × (1 - 0.12) = 269
• Real WHP: Often dynos higher (~280 WHP) because Honda creates efficient drivetrains (or underrates the engine!).

Marketing vs. Reality

Car manufacturers sell BHP because it's the bigger number. Tuners and racers care about WHP because it's the honest number.

Underrated Engines: Some manufacturers (like Porsche, BMW, McLaren) are famous for advertising BHP numbers that are actually closer to WHP.

• Example: A BMW rated at 500 HP might put down 480 WHP, implying the engine really makes closer to 550 BHP.

Frequently Asked Questions

Is BHP the same as Crank HP?

Yes, essentially. "Crank Horsepower," "Brake Horsepower," and "Engine Horsepower" generally refer to the same flywheel measurements.

Can I calculate my BHP from a WHP dyno sheet?

Yes, approximately. Divide your WHP by the efficiency factor.

• RWD (15% loss):

  WHP / 0.85 = BHP

• AWD (20% loss):

  WHP / 0.80 = BHP

Does tire pressure affect horsepower?

Yes! On a dyno, low tire pressure increases rolling resistance, lowering your WHP score. Always dyno with correct pressures.

Conclusion

• BHP is what you brag about at the pub (Engine power).
• WHP is what wins races (Power to the ground).

When modifying a car, always use WHP (dyno results) to track progress, as it captures the efficiency of the whole system, not just the engine.

Last Updated: January 30, 2026
Reading Time: 8 minutes