Defining Horsepower (HP)
Horsepower (HP) is a common measurement used to rate the power of an engine. It was invented by James Watt in the 18th century to compare the output of steam engines to the power of draft horses.
One horsepower is defined as the power needed to lift 550 pounds by one foot in one second. It’s a theoretical number used for comparison purposes rather than an actual physical measurement.
How horsepower is calculated
Horsepower is calculated by this formula:
HP = (Torque x RPM) / 5,252
- Torque is measured in pound-feet (lb-ft)
- RPM is revolutions per minute
The 5,252 figure comes from 33,000 lb-ft/minute being equivalent to 1 horsepower.
Uses of horsepower
Horsepower is commonly used to rate the power output of automobile and marine engines. Manufacturers advertise the max horsepower of their engines to give buyers a sense of their performance capability.
Horsepower indicates an engine’s potential but not how much is ultimately used to drive the wheels and propel the vehicle. That’s where brake horsepower comes in.
Understanding Brake Horsepower (BHP)
Brake horsepower (BHP), also known as wheel horsepower, measures the power delivered by an engine at the driveshaft or crankshaft to actually move the vehicle.
How BHP is measured
BHP is measured by an engine dyno with torque and RPM sensors while the engine is running with a load applied to simulate real driving conditions. This accounts for power losses within the engine and drivetrain.
BHP will always be less than an engine’s gross horsepower since some power is lost to friction, pumping losses, and driving accessories.
Uses of brake horsepower
BHP gives a more real-world measurement of usable power output. It better represents the power delivered to the wheels to accelerate a vehicle.
BHP is useful for quantifying an engine’s true performance rather than just its theoretical capabilities. It accounts for inefficiencies so is more meaningful for vehicle operation.
Explaining Power Output (PS)
Power output or PS is a measurement used in Europe for engine power, similar to horsepower. One PS is equivalent to 0.73549875 kW or roughly 0.98632 horsepower.
How power output is measured
Like horsepower, PS is calculated from an engine’s torque and RPM. The formula is:
PS = Torque (in Nm) x RPM / 9.5488
So it’s based on metric units instead of imperial ones.
Uses of power output
PS is the standard for stating engine power in many European countries since it uses the metric system. Car brochures and ads in Europe often list power output in PS rather than horsepower.
While less common in the US, some automakers like BMW still use PS alongside HP for engine specs. It gives an equivalent power rating for European customers.
Key differences between HP, BHP, and PS
While HP, BHP, and PS all quantify engine power, there are some key differences between these specifications:
- HP is calculated from torque and RPM so only reflects the theoretical maximum capability
- BHP is physically measured by a dyno and accounts for real-world losses
- PS uses a formula like HP but with metric units
- HP rates gross engine power before drivetrain losses
- BHP measures net power delivered to the wheels
- PS provides engine power in metric units commonly used in Europe
So in summary:
- HP is engine power potential
- BHP is real usable power
- PS is engine power in metric units
Converting between HP, BHP, and PS
The three units use different formulas but can be converted with these relationships:
- 1 HP = 0.7457 kW
- 1 kW = 1.341 HP
- 1 PS = 0.7354 kW
- 1 kW = 1.359 PS
- 100 HP = 74.57 kW = 93.41 PS
- 150 PS = 110.31 kW = 147.1 HP
- 200 BHP = 149.2 kW = 199.6 HP
So while they represent engine power differently, you can convert between HP, BHP, and PS.
Which specification is most important?
For determining real-world vehicle performance, brake horsepower (BHP) is the most useful specification. It directly measures the engine power that gets transmitted to the wheels through the drivetrain.
BHP accounts for internal engine and drivetrain losses, giving a better sense of the usable power. Horsepower ratings can overestimate potential since they don’t consider these losses that reduce usable output.
While PS is useful for European customers, BHP makes the most sense for quantifying true usable engine power in any vehicle. Pay attention to BHP numbers when comparing vehicle performance.
Horsepower (HP), brake horsepower (BHP), and power output (PS) all indicate engine power but have key differences:
- HP is a theoretical maximum rating
- BHP measures real net engine power output
- PS provides power in metric units used in Europe
For determining real-world vehicle performance, BHP is the most insightful specification to look for. It directly accounts for losses so represents the true engine power delivered to accelerate the vehicle.
Q: Is horsepower the same as brake horsepower?
A: No, horsepower (HP) rates an engine’s theoretical maximum capability while brake horsepower (BHP) measures the actual usable power output at the wheels. BHP is always lower than HP.
Q: When would you use power output (PS) instead of horsepower (HP)?
A: PS is used as the standard for stating engine power in many European countries since it uses the metric system. Car specs in Europe often list PS rather than HP.
Q: Which engines can have higher horsepower or brake horsepower?
A: Gross horsepower tends to be higher since it doesn’t account for losses. Brake horsepower is lower since it factors in internal losses that reduce the net power delivered to the wheels.
Q: How do you convert between PS and HP?
A: 1 PS = 0.7354 kW, and 1 kW = 1.341 HP. So 100 PS x 0.7354 kW/PS x 1.341 HP/kW = 99 HP.
Q: Why is brake horsepower more useful than horsepower for real-world driving?
A: Brake horsepower better represents usable power since it directly measures net engine output at the wheels after drivetrain losses. This gives a more realistic gauge of acceleration capability.