What does bhp mean

Overview

When you look at the specifications of a car, motorcycle, or other internal combustion engine, you'll often see a figure for its power output, typically measured in horsepower (HP). However, you might encounter different types of horsepower ratings, such as Brake Horsepower (BHP) and Gross Horsepower (GHP). Understanding the distinction is crucial for accurately assessing an engine's capabilities.

What is Brake Horsepower (BHP)?

BHP, or Brake Horsepower, is a measurement of an engine's power that has been adjusted to account for the power lost due to internal friction within the engine itself. To measure BHP, a dynamometer (often called a "dyno") is used. This device is attached directly to the engine's crankshaft, essentially "braking" its rotation at a controlled rate. The force required to maintain this controlled braking is measured, and from this, the engine's torque and horsepower output at the crankshaft can be calculated.

The "brake" in Brake Horsepower refers to the "brake" mechanism used on the dynamometer to absorb the engine's power and measure its output. It's important to note that BHP measures the power directly at the crankshaft, before it is transmitted through the drivetrain (transmission, driveshaft, differential, axles) to the wheels. Therefore, the power actually reaching the wheels (wheel horsepower) will be less than the BHP due to frictional and mechanical losses in these components.

BHP vs. Gross Horsepower (GHP)

The other common rating is Gross Horsepower (GHP). GHP is a theoretical measurement of an engine's power under ideal conditions, without any accessories attached and without accounting for frictional losses. This means GHP figures are typically higher than BHP figures for the same engine. In the past, GHP was sometimes used for marketing purposes, as it presented a more impressive number. However, BHP is considered a more practical and realistic measure of an engine's output because it reflects the power available after internal engine losses, which are unavoidable.

The Society of Automotive Engineers (SAE) established standards for measuring horsepower. Initially, measurements were often quoted as Gross Horsepower (GHP). However, as engines became more complex with added accessories like alternators, power steering pumps, and air conditioning compressors, and as manufacturers sought more standardized and realistic testing, the SAE developed the SAE Net Horsepower standard. SAE Net Horsepower is very similar to BHP, as it measures power with standard accessories installed and accounts for frictional losses. In modern automotive contexts, when manufacturers state horsepower without specifying "gross," they are usually referring to a net rating that is comparable to BHP.

Why is BHP Important?

BHP provides a more accurate representation of the engine's raw power potential before it is diminished by the vehicle's drivetrain. While wheel horsepower is the ultimate measure of what reaches the tires, BHP is a useful benchmark for comparing engines directly. It allows consumers and engineers to understand the fundamental power-generating capability of the engine itself. A higher BHP generally indicates a more powerful engine, capable of producing more torque and accelerating the vehicle more quickly, assuming similar drivetrain efficiencies.

When comparing different vehicles, it's essential to ensure you are comparing like-for-like ratings. If one manufacturer quotes BHP and another quotes GHP, the comparison can be misleading. Most reputable sources and manufacturers today will quote either BHP or SAE Net Horsepower, which are closely related and provide a more honest assessment of engine performance than older GHP figures.

How is BHP Measured?

The measurement of BHP is performed using an engine dynamometer. The engine is removed from the vehicle and mounted on the dynamometer. A "brake" is applied to the engine's output shaft (crankshaft). This brake resists the engine's rotation, allowing the dynamometer to measure the torque the engine is producing at various speeds. Horsepower is then calculated from the torque and engine speed (RPM) using the following formula:

Horsepower = (Torque × RPM) / 5252

This calculation gives the Brake Horsepower (BHP) because it measures the power at the crankshaft, after accounting for the engine's internal friction and any load applied by the dynamometer's braking system.

Conclusion

In summary, BHP is a critical metric for understanding an engine's performance. It represents the power generated by the engine's combustion process minus the power lost to internal friction. While not the final power delivered to the wheels, it serves as a standardized and realistic measure for comparing the inherent power output capabilities of different engines.