Brake Horse Power: Understanding the Force Behind Your Classic

Brake horse power explained
Brake horsepower explained User:Sgbeer; adapted by User:Martinvl, CC BY-SA 3.0 , via Wikimedia Commons

Brake horsepower (bhp) is a vital aspect of an engine’s performance in vehicles, referring to the usable power that an engine can generate.

When assessing the capabilities of an automobile, understanding the difference between total engine power and brake horsepower is crucial. The term “brake horsepower” stems from the need to determine the net power output of an engine by discounting friction losses in various engine components, such as the gearbox, water pump, and generator.

Brake Horse Power

In this article we will explain the different terminologies to describe power.

The concept of brake horsepower is relevant not only in conventional petrol or diesel-powered vehicles but also in electric vehicles. With the rise of electric vehicles, a new measurement unit called kilowatts (kW) has emerged, allowing for better comparisons of power outputs across different types of vehicles.

Whether it’s a traditional internal combustion engine or an electric motor, understanding the meaning and implications of brake horsepower is essential to grasp the overall dynamics and efficiency of the powertrain.

Key Takeaways

  • Brake horsepower refers to the usable power generated by an engine, accounting for frictional losses
  • The concept applies to both conventional petrol or diesel engines and electric vehicles
  • Familiarising ourselves about brake horsepower helps us understand the power capabilities of various types of vehicles

History of Brake Horsepower

James Watt and the Concept of Horsepower

The history of brake horsepower can be traced back to the industrial revolution when the Scottish engineer James Watt sought to compare the power output of steam engines to that of horses.

Watt observed that a horse could turn a mill wheel at a certain rate, doing work at a consistent pace. He then equated this work rate to a unit of measurement, which he named “horsepower” (HP).

Horsepower became a standard unit for measuring the capacity of engines and machinery. It allowed people to understand the efficiency and capability of new technology by comparing it to the familiar power of horses.

As engines evolved, so did the need for more precise measurements and terms to describe the specific types of power generated by engines, leading to the development of brake horsepower.

Development of Brake Horsepower Measurement

Brake horsepower (BHP) is the amount of work generated by a motor under ideal conditions, without the consideration of the effects of any auxiliary components that may slow down the actual speed of the motor.

This power output is measured at the engine’s output shaft, providing a more accurate representation of the engine’s performance.

The concept of brake horsepower originated to better evaluate the efficiency and power that engines were actually able to deliver, compared to the power developed in the combustion chambers. The power generated within the combustion process is typically higher than what is actually delivered, due to friction and mechanical losses.

By measuring brake horsepower, it became possible to precisely compare engine performance and capacity.

Defining Brake Horsepower

Brake horsepower (bhp) is a crucial unit of measurement used to indicate the actual usable power that an engine generates. This measurement helps us understand the efficiency of an engine, as it considers the power losses associated with various mechanical components.

Relation to Horsepower and Metric Horsepower

Brake horsepower is closely related to horsepower (hp) and metric horsepower (PS or pferdestärke). All three measure an engine’s output, but they differ slightly in their focus.

  • Horsepower: Originated by James Watt, this measurement quantifies the work produced by an engine at a given time. It is predominantly used in the United States and the United Kingdom.
  • Brake Horsepower: BHP measures the engine’s output whilst accounting for power losses that occur due to friction in auxiliary components – such as gearbox, alternator, and water pump. This measurement provides a more accurate representation of an engine’s true performance.
  • Metric Horsepower: Commonly used in European countries, metric horsepower, or PS, is similar to regular horsepower. It equals about 98.6% of the imperial horsepower.

Calculating Brake Horsepower

To calculate brake horsepower, one must account for the power losses in an engine’s mechanical parts. This can include the gearbox, alternator, cooling pump, and power steering system. One common method for calculating BHP is the use of a dynamometer, which measures the engine’s output at the crankshaft, whilst simulating real-world operating conditions.

Considering the engine’s gross horsepower and power losses in various components, brake horsepower can be mathematically determined using the following formula:

BHP = Gross Horsepower – Power Losses

In summary, brake horsepower is an essential measurement that accurately reflects an engine’s actual usable power. By comparing BHP to the more traditional imperial horsepower and metric horsepower units, we gain a better understanding of an engine’s true performance in various driving scenarios.


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Brake Horsepower in Vehicles

Car Engine BHP Ratings

Brake horsepower (BHP) is a vital metric when evaluating a vehicle’s engine performance and efficiency. In cars, BHP ratings often vary depending on the engine size, type, and configuration.

For example, a modern small hatchback with a 1.2-litre petrol engine may have a BHP rating of around 80, while a high-performance sports car with a 5.0-litre V8 engine could boast over 400 BHP.

Manufacturers often provide BHP ratings to help consumers understand a car’s capabilities, such as acceleration and power delivery.

SUV and Truck Power Ratings

SUVs and trucks typically have higher BHP ratings than smaller cars. These vehicles are designed to handle demanding tasks like towing or off-road driving. A midsize SUV might have a BHP rating between 150 and 300, while a full-size truck may feature over 400 BHP.

It should be noted that diesel engines often produce more torque, contributing to the vehicle’s overall power and towing capacity. Therefore, brake horsepower figures for diesel-engined SUVs and trucks can be higher than their petrol counterparts.

Impact on Performance

In general, higher BHP ratings lead to better performance, with quicker acceleration and higher top speeds. The BHP of a vehicle’s engine influences its ability to:

  • Accelerate quickly
  • Maintain high speeds
  • Tow heavy loads
  • Drive smoothly up steep inclines

However, it’s essential to consider other factors like weight, aerodynamics, and drivetrain when assessing a vehicle’s overall performance. For example, a lightweight sports car with 300 BHP might have quicker acceleration than a heavy SUV with the same BHP rating. Additionally, factors such as gearbox ratios and brake systems can also influence performance.

Brake horsepower plays a significant role in a vehicle’s engine performance and capabilities. While higher BHP ratings generally lead to better performance, it’s crucial to consider other vehicle components and characteristics that could affect the overall driving experience.

Engine Dynamics and Brake Horsepower

Engine Efficiency and Frictional Losses

In any internal combustion engine, a certain amount of power is lost due to friction between engine components and other mechanical losses related to the transmission, (particularly automatics), alternator, and power steering system.

These frictional losses decrease the overall efficiency of the engine, which is the ratio of usable power output to the total power produced by the engine. Brake horsepower (bhp) measures the actual usable power produced by the engine, taking into account these losses. A higher bhp indicates better engine efficiency and improved performance.

Torque and Power

Torque is a measure of the rotational force generated by the engine, while power refers to the rate at which work is done. In an engine, torque is produced by the combustion of fuel, which causes the engine’s pistons to move and the crankshaft to rotate.

The rotation of the crankshaft transfers the torque to the wheels of the vehicle, enabling it to move. Brake horsepower is a measure of the actual power delivered to the vehicle’s wheels, accounting for the various losses that occur within the engine and ancillary systems.

To put it in simpler terms:

  • Torque: Rotational force produced by the engine
  • Power: Rate of work done by the engine
  • Brake horsepower: Actual usable power output of the engine, considering frictional losses

BHP and Engine Capacity (cC)

Engine capacity, commonly measured in cubic centimetres (cc), or cubic inches (CI in the USA), refers to the total volume of the engine’s cylinders. A larger engine capacity generally means that more fuel and air can be combusted, resulting in higher power output and torque.

However, a larger engine is not always synonymous with higher brake horsepower. Other factors such as engine efficiency, design, and materials used can also influence brake horsepower.

For example, two engines with the same capacity may have different brake horsepower values due to differences in their internal components, manufacturing processes, or engine management systems. Consequently, it is essential to consider both engine capacity and brake horsepower when evaluating an engine’s performance and efficiency.

Measurement Techniques

Dynamometer Use

A key tool in measuring brake horsepower is the dynamometer. This device connects to an engine and applies a load, simulating real-world conditions. By measuring the torque exerted by the engine and its rotational speed, the dynamometer calculates brake horsepower.

This method ensures accurate and consistent readings, making it essential for determining the overall efficiency and power output of an engine.

Calculating Power Losses in Drivetrains

Power losses in drivetrains are crucial to understanding the true performance of a vehicle. To calculate the brake horsepower, one must consider the total engine power and account for the power lost through friction in the engine’s mechanical parts, gearbox, water pump, and generator. This helps measure the ‘useful’ power an engine can produce, giving a more accurate representation of its performance.

From Engine Power to Wheel Horsepower

Engine power and wheel horsepower are related, with some power being lost through the drivetrain. Measuring wheel horsepower (WHP) is essential to know the actual power reaching the vehicle’s wheels.

This is done by taking the brake horsepower and accounting for power losses in the drivetrain. A rolling road is often used to assess this.

By considering factors such as torque and wheel horsepower, the dynamometer provides a reliable measurement of the true performance of a vehicle.

Comparing Power Units

Kilowatt versus Brake Horsepower

Kilowatts (kW) and brake horsepower (bhp) are both units used to measure the power output of an engine. While 1 kW is equal to 1,000 watts, 1 bhp is equivalent to 745.7 watts. Kilowatts offer a more uniform method of measuring power and are widely used by engineers worldwide. To convert between kW and bhp, you can use the following conversion ratio: 1 kW equals approximately 1.341 bhp.

In the context of cars, the power output of the engine is measured in bhp, focusing on the amount of energy left after accounting for various losses such as those due to the gearbox, alternator, and water pump. When comparing the two units, kilowatts can offer a more efficient and standardised approach for measurements.

BHP and Other Power Units in Different Regions

Various power units are used in different regions around the world to measure engine power. In the UK, the brake horsepower (bhp) is commonly used to express the power output of automotive engines.

In Europe, however, Pferdestärke (PS) is the German equivalent of brake horsepower; it is comparable to bhp and frequently used in countries like Germany and Austria.

Another power unit used primarily in continental Europe, particularly France and Italy, is the cheval vapeur (CV). This is another unit for expressing engine power, which is very similar to bhp and PS but has a slight difference in calculation, making it slightly smaller than bhp.

In contrast, the United States tends to use horsepower (hp) as the primary unit, which is slightly different from bhp but is similar enough for most purposes.

In summary, power units can vary between regions, with the UK utilising brake horsepower, while Europe prefers PS or CV, and the US primarily using horsepower.

Regardless of the region, understanding the various measurements and their conversions can be useful when comparing engine performance with different power units.


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Factors Affecting Brake Horsepower

Auxiliary Components and BHP

Brake horsepower (BHP) is influenced by a variety of auxiliary components, which can improve or hinder an engine’s overall performance.

For example, the efficiency of the air intake and exhaust systems can affect power output. High-quality air filters and exhaust systems allow for better airflow, resulting in increased BHP. On the other hand, subpar components or clogged systems may decrease the efficiency of the engine, leading to a lower BHP.

Air Intake and Exhaust Systems

  • High-quality air filters: improve engine efficiency
  • Efficient exhaust systems: enhance power output
  • Clogged or subpar components: decrease engine efficiency

Impact of Vehicle Weight on BHP

The weight of the vehicle also has an impact on BHP and its ability to propel the car forward. A lighter vehicle requires less power to accelerate, resulting in a more effective BHP.

Conversely, a heavier vehicle necessitates greater power from the engine, which can negatively affect the BHP in relation to the vehicle’s acceleration capabilities.

Vehicle Weight and BHP

  • Lighter vehicles: require less power to accelerate
  • Heavier vehicles: require more power, may negatively affect BHP

Engine Design Variations

Different engine designs can result in variations of BHP as well. Several factors, such as engine efficiency and design, fuel quality, and octane rating, can contribute to the overall BHP.

Engine manufacturers constantly strive to optimise these factors to improve performance and power output. Also, advancements in technology and innovative designs play a crucial role in enhancing engine capabilities and ultimately boost BHP.

Engine Design Factors

  • Engine efficiency:
    • Optimal engine design: improves performance
    • Suboptimal design: hinders power output
  • Fuel quality: affects BHP
  • Octane rating: influences engine performance


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Brake Horsepower in Electric Vehicles

Battery Performance and BHP

Battery performance plays a significant role in determining the brake horsepower (BHP) of electric vehicles (EVs).

In general, an electric car consumes approximately 1,500 kilowatt-hours (kWh) per year, with variations depending on individual usage patterns. As the battery supplies power to the electric motors, it is crucial for it to have a high energy density and efficient discharge rate to maintain and optimise the BHP.

Some key factors affecting battery performance and, subsequently, BHP in electric vehicles include:

  • Capacity: Measured in kilowatt-hours (kWh), it determines the range and power available to the electric motors.
  • Discharge rate: Indicates the rate at which the battery can supply power to the motors, directly influencing BHP.
  • Thermal management: Efficient cooling and heating of batteries ensure optimal performance and reduced heat-related energy losses that can affect BHP.

Electric Motors and Power Ratings

In an electric vehicle, brake horsepower refers to the mechanical output power derived from the motor’s shaft. The input power is different from the BHP because of various losses such as mechanical friction and heat generation in the motor and powertrain. It is important to note that BHP is also measured in horsepower (HP) or watts.

Understanding various components and power ratings of electric motors can further help in comprehending BHP calculations in EVs:

  1. Motor efficiency: It represents the ratio between the output power (BHP) and the input power from the battery. Higher efficiency results in a higher BHP and improved vehicle performance.
  2. Power factor: This term refers to the ratio of real power to apparent power in an electric motor. Having a higher power factor indicates efficient utilisation of input power, resulting in a better BHP rating.
  3. Voltage and amperage: As stated by the power law, voltage (E) and amperage (I) determine the wattage or power consumption of a motor. There are 746 watts in one horsepower, so every time a motor develops or uses 746 watts, it has done one horsepower of work.

In summary, the BHP of electric vehicles is influenced by both battery performance and electric motors’ characteristics. Factors such as battery capacity, discharge rate, thermal management and motor efficiency play a crucial role in determining the overall BHP rating in EVs.

Brake Horsepower in Context

Real-world Applications

One key application of bhp is in vehicle acceleration. A higher bhp indicates greater usable power, allowing a vehicle to achieve faster acceleration and perform better overall.

Moreover, bhp is an essential factor for automotive enthusiasts seeking a deeper understanding of their vehicle’s capabilities and limitations. By comparing the bhp values of different engines, one can reliably assess the potential performance between them.

Legal and Regulatory Aspects

Brake horsepower is not only relevant from a performance standpoint but also plays a role in legal and regulatory aspects. Vehicles’ bhp ratings can ultimately impact taxation rates, insurance premiums, and emission regulations.

For instance, a higher bhp value may result in increased road tax or higher insurance rates due to the assumption of greater power and performance.

Furthermore, some countries or regions have strict regulations concerning the maximum bhp allowed for specific vehicle categories or road usage conditions. Compliance with these rules is essential to ensure both legal operation and conformance with environmental standards [3].

In summary, brake horsepower is an indispensable measure of an engine’s usable power, both for assessing acceleration capabilities and meeting legal and environmental regulations. By understanding the implications of bhp, one can fully appreciate the benefits and constraints associated with various engine designs and make informed choices about their vehicle’s performance and usage.

Frequently Asked Questions

What determines the brake horsepower rating of a vehicle?

The brake horsepower (BHP) rating of a vehicle is determined by the power output of its engine.  BHP serves as a valuable metric for comparing engine power by incorporating the expected basic loads and providing a more realistic assessment of its actual performance.

How does brake horsepower relate to the overall performance of a vehicle?

Brake horsepower is an important aspect of a vehicle’s performance, as it measures the power of an engine without losses that occur due to factors such as friction and mechanical inefficiencies.

Higher BHP generally translates to better acceleration and higher top speeds, although other factors like weight, aerodynamics and transmission play important roles as well.

What are the primary benefits of a higher brake horsepower in cars?

A higher brake horsepower in cars offers improved acceleration, quicker 0-60 times, and potentially higher top speeds. This can result in a more responsive and enjoyable driving experience. However, it is important to note that better performance may come at the cost of increased fuel consumption and emissions.

How can one convert brake horsepower to watts?

To convert brake horsepower to watts, you can use the following formula:

Watts = BHP × 745.7

Where 1 BHP equals 745.7 watts. This conversion allows for a more standardized comparison of power outputs across different vehicles and industries.

Can brake horsepower figures be a reliable indicator of a vehicle’s engine strength?

While brake horsepower figures can provide a useful comparison of a vehicle’s engine strength, it is important to consider other factors such as torque, weight, and aerodynamics when assessing overall performance.

BHP can be a reliable starting point for comparing vehicles, but a more comprehensive evaluation should consider a range of metrics and dynamics.




Brake horsepower (BHP) has a significant impact on a vehicle’s performance, as it represents the actual usable power produced by an engine. By considering power losses due to friction and mechanical inefficiencies, BHP provides a more accurate representation of the engine’s output.

Essential factors, such as the engine’s power and efficiency, contribute to the performance capabilities of the vehicle, making BHP a critical metric for understanding and improving performance. In general, a higher BHP rating would result in better acceleration and top speed, enhancing driving experience.

Primary Components Involved:

  • Engine
  • Gearbox
  • Water pump
  • Mechanical components

In aviation, BHP plays a vital role in determining the power output of an aircraft engine at the power output shaft, providing valuable insights into the aircraft’s performance. This information allows pilots, engineers, and designers to optimise their operations and achieve desired outcomes.

When choosing a vehicle or evaluating engine performance, it is crucial to understand the implications of BHP ratings. This knowledge enables drivers to make informed decisions and appreciate the engineering prowess and potential of their chosen vehicles.

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