Horsepower is an important way to rank vehicle performance, but there are three variations of horsepower, which are HP, BHP, and WHP. Each uses different standards in their performance rating, so it is important to know how each measure is calculated.
Keep reading to learn more about the differences between HP, BHP, and WHP, and the purpose of each measuring designation when evaluating vehicle output.
What is HP?
In its more basic definition, “HP” is just the measurement of an engine’s horsepower – without considering some addition factors, specifically friction. HP is almost never an accurate measurement of anything a vehicle can do.
It is reported that the average car will lose 10%-20% of HP before that engine power gets to the drive wheels and ultimately, HP isn’t really very useful when considering how a vehicle will actually perform.
How is Horsepower Measured?
Horsepower (HP) is calculated from an engine’s displacement, stroke and RPM. The horsepower is derived from an engine-specific calculation. This is based on the engineer’s calculation alone; in other words, this is the calculation that is called indicated horsepower.
So, there isn’t any reason to believe a vehicle could generate this kind of output in reality.
What is BHP?
Brake horsepower (BHP) is a measurement of an engine’s output before it passes through a vehicle’s transmission and driveline. BHP is primarily what the engine could output in a perfect world scenario.
As much as 15%-20% of an engine’s BPH is used up by friction and wear, and damage, before it can even get delivered to the pavement. However, it’s still far more realistic than just “HP” when representing the output of the vehicle.
How is Brake Horsepower Measured?
BHP is measured using a “brake style” dynamometer. A dynamometer measures, at the flex plate (automatic) or flywheel (manual), where its power can be measured at the output stage in a clearly measurable fact.
However, these readings are not adjusted to indicate the loss of power that takes place during the vehicle’s powertrain.
What Is WHP?
“WHP” simply refers to Wheel Horsepower, the measure of the output a vehicle can produce and transfer to the pavement at its drive wheels.
This may be the most accurate representation of a vehicle’s output, as WHP factors in external forces that can cause losses in power, like friction and wear of the mechanical parts of a vehicle.
Due to this fact, a given vehicle should be able to replicate its recorded WHP in a real-world scenario.
How Is Wheel Horsepower Measured?
Wheel horsepower is tested by a chassis dynamometer that uses a roller underneath the vehicle’s drive wheels to measure torque and rotational speed.
Testing equipment like that mentioned above can be found in many performance/speed shops and is even a staple of the drag racing community.
What is The Measurement Standard of Horsepower?
Currently, the vast majority of vehicle manufacturers are measuring the horsepower of vehicles using brake horsepower (BHP) as their standard, whether the vehicle is a sports car, a minivan, or an economy car.
While this measurement might be relatively accurate when it comes to measuring an actual engine’s output, it is far less accurate when addressing the power delivery to the wheels.
However, we have come a long way from where we used to be when it comes to testing.
Until the early 70’s, it was standard practice to conduct these tests with non-production headers and without any belt-driven accessories (alternator, power steering pump, etc.).
However, this information was not ideal, and that practice was stopped somewhere between the late 70’s the early 80’s, to give consumers a real chance at performance numbers.
What are the Factors of Drivetrain Power Loss?
As previously stated, there is a significant amount of power lost between a crankshaft and a vehicle’s wheels.
This power loss is known as drivetrain power loss and is responsible for disputes about the relationships between BHP and WHP!
Sometimes, limiting factors in an environmental sense don’t aid an engine’s output, and the difference can be astronomical.
Here are some common reasons for drivetrain power loss.
1. Temperature.
Generally speaking, cooler air is more oxygen-rich than warm air, so cooler air is paramount to achieve optimal combustion.
Generally speaking, a vehicle with a higher temperature will not achieve the same performance as a vehicle operating at a temperature more conducive to optimally efficient combustion.
2. Altitude.
As you move up in altitude, the output of a vehicle’s powertrain will change significantly due to the change in oxygen density. Since oxygen is a needed catalyst for combustion, thinner air at altitude will champers power output.
3. Transmission.
All vehicles lose some horsepower from the output of an engine to the vehicle’s wheels through conventional transmission operation. All this drag from the torque converter, clutch packs, bearings, and seal, and from standard windage and simple friction takes horsepower.
4. Wear.
As components wear, the drag associated with moving components in a vehicle’s drivetrain will change, too. Worn internal parts of a transmission and an aging driveline assembly are common sources of power loss. Worn u-joints could also add to the potential power loss inside a driveline.
Do Electric Cars Have Horsepower?
As electric cars are gaining more notoriety in the mainstream, many manufacturers have decided how to represent vehicle performance.
Initially, kilowatt outputs were used to measure performance, but many, and in particular American-manufactured auto brands, felt it best to convert wattage to a nominal horsepower number, as consumers were more familiar with looking at a horsepower number when defining engine performance.
One Kilowatt is 1.34 horsepower, so generating a fair and likely accurate measure of performance is simple, and most customers should understand the measure of performance.