Understanding Power Transmission in Vehicles

Horsepower, Wheel Power & Actual Power Explained
By Boosted Budget


Power transmission is a crucial part of how vehicles operate. Whether you're cruising in a sedan or racing in a sports car, it's important to understand how power flows from the engine to the wheels. This knowledge not only enhances your understanding of vehicle performance but also helps when comparing specs or planning modifications.

In this post, we’ll explain the three most important types of power in automotive systems:

Horsepower (HP)

Brake Horsepower (BHP)

Wheel Horsepower (WHP)


1. Horsepower (HP)

Horsepower refers to the theoretical power output of the engine without accounting for any losses. It is measured at the crankshaft.

👉🏻 What is Horsepower?

The term was introduced by James Watt and equates to the power required to move 550 pounds one foot in one second. In modern metrics:

👉🏻 1 HP = 745.7 Watts

Horsepower is often used as a marketing figure but doesn’t reflect the power that actually reaches the wheels.

👉🏻 How It’s Measured

Horsepower is calculated using mathematical formulas based on torque and engine RPM, usually measured using an engine dynamometer before any drivetrain components are connected.


2. Brake Horsepower (BHP)

Brake Horsepower represents the engine’s actual output after internal engine losses such as friction, water pump resistance, alternator load, and exhaust restriction.

👉🏻 Why “Brake” Horsepower?

It gets its name from the use of a brake dynamometer, which applies resistance to the engine’s output shaft to measure power.

👉🏻 BHP = HP - Engine Losses

This is a more accurate reflection of the engine’s output before the power travels through the drivetrain.

👉🏻 Common Losses Include:

  • Engine friction
  • Cooling and charging system loads
  • Heat and exhaust resistance


3. Wheel Horsepower (WHP)

Wheel Horsepower is the true usable power—it’s the power that actually makes it to the ground through the tires.

👉🏻 Why It Matters

WHP is measured using a chassis dynamometer, with the vehicle fully assembled and driven under load. It accounts for all power losses in the drivetrain including:

  • Transmission
  • Driveshaft
  • Differential
  • Bearings
  • Wheel and tire friction

👉🏻 WHP = BHP - Drivetrain Losses

Typical Drivetrain Losses

  • Front-Wheel Drive (FWD): 10–15%
  • Rear-Wheel Drive (RWD): 15–20%
  • All-Wheel Drive (AWD): 20–25%

👉🏻 Comparison Table

Power Type Measurement Point Description Typical Loss

HP Crankshaft Theoretical Engine Output 0%

BHP Engine Output (after accessories) Real Engine Output 10–20%

WHP Drive Wheels Actual Road Power 15–25%

👉🏻 Simple Analogy

Think of power transmission like water flowing through pipes:

HP: Water available at the tank

BHP: Water after passing filters and valves

WHP: Water that reaches the faucet


Final Thoughts

Knowing the differences between horsepower, brake horsepower, and wheel horsepower gives a clearer picture of a vehicle’s true performance. So, when comparing vehicles or reading spec sheets, always ask: Where is that power measured?

Stay tuned for our next article, where we'll break down torque vs. horsepower and how they affect acceleration and towing.

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