What is Car Horsepower?

When it comes to purchasing a new or used car, a key specification that captures the attention of drivers is knowing how much horsepower it has. While factors like fuel economy and space in the cabin also matter, horsepower remains the benchmark of performance potential for most motorists. 

Whether you're looking at the vast selection of SUVs, the formidable power of 4x4s, the hastiness of a hot hatch, or the speed of a sports car, knowing what determines a car’s horsepower and how much you require under the bonnet can certainly help with your purchasing decision. 

In this straightforward guide to car horsepower, we’ll cover topics including what exactly horsepower is and how it’s measured, make the comparison between horsepower and brake horsepower, and reveal how horsepower differs from torque. Read on to find out more… 

What does horsepower mean in a car?

Horsepower (hp) in a car refers to the maximum power the car's engine can produce. Horsepower measures an engine's power output, so the more horsepower a car has usually means it has the capabilities of faster acceleration and increased towing capacity, purely because there is more power available.

The average horsepower for a car is anywhere from 100hp to 200hp, depending on the make and model of the vehicle. For high-performance sports cars, the range of horsepower increases from 300hp anywhere up to 500hp.  There are a number of factors that determine engine horsepower, such as:

• Engine size (larger engines can produce higher levels of horsepower)
• Engine configuration (the number of cylinders plays a vital role)
• Forced induction (additional turbochargers and superchargers boost an engine's horsepower output)
• Air intake and fuel delivery components
• Engine design characteristics

Why is it called horsepower?

Horsepower is so called because the unit of power was originally defined based on how much power a horse can produce. It was in the late 18th century when James Watt was working on early steam engines and needed a way to market the idea that his creations could replace horses for doing laborious work.

He ran experiments to determine how much force a horse could exert over time while pulling heavy loads. This established a value for a single horse's working capacity which Watt then used as a benchmark, hence "horse power" was used as a basis to rate his engines' power output and selling potential compared to the four-legged kind. 

Even though the calculations have evolved over time (for example, 1 horse doesn’t equate to one unit of horsepower), the name has stuck to be a measure of power for the cars we know and love today.

How is horsepower calculated?

Horsepower is calculated using a fairly complex formula that takes into account an engine's rotational speed - revolutions per minute (RPM) - and torque output. Specialised equipment measures rotational force and RPMs to calculate power output through a relationship linking torque, rotational speed, and defined constants ultimately expressed as horsepower. 

Dynamometer testing allows monitoring of horsepower and torque across an engine’s full operating range to identify its peak horsepower output. 

Why is horsepower also referred to as PS?

Horsepower is also commonly referred to as ‘PS’ because they both measure the same thing - an engine's power output. The terms differ because ‘PS’ is of German heritage and stands for Pferdestärke, which is the German word for horse strength or horsepower, and tends to be a more European term. 

1 PS was originally defined as the power needed to lift a mass of 75 kilograms by 1 meter in 1 second. This worked out to an approximate value of 1 PS = 0.985 HP (horsepower), so PS and HP measure are essentially the same thing. Over time the values have become commonly interchanged. 

Manufacturers may choose to use PS or HP depending on the origin country, but both tell you how much peak power the car's engine can produce.

Is horsepower the same as brake horsepower?

No, horsepower and brake horsepower are slightly different ways of measuring an engine's power output. Horsepower (hp) refers to an engine's maximum gross horsepower; this is the peak horsepower an engine can generate before any losses in performance are detected. 

Brake horsepower (bhp) is equal to one horsepower, but instead measures the force needed to brake the vehicle in question. Therefore, this imperial unit of power is used only the measure the power available at the shaft of an engine, as it has nothing to do with acceleration performance. 

An engine's brake horsepower will always be slightly lower than its gross horsepower rating, this is because power loss due to friction comes into play. For example, an engine rated at 300hp may deliver around 280bhp, which is why horsepower numbers tend to get featured more prominently by manufacturers.

What is the difference between horsepower and torque?

Torque is a measurement of an engine's rotating force or ‘twisting’ power typically measured in pound-feet (lb-ft) or Newton metres (Nm). Engine torque determines an engine's ability to perform work and accelerate at low RPMs (revolutions per minute) or speeds. Higher torque at low RPMs allows more power for tasks like pulling heavy loads.

This differs from horsepower which measures an engine's total power output typically over its entire operating speed range. Most car engines are tuned to provide a balance of both relatively low-end torque and high horsepower at higher RPMs. Checking both of these specs gives a more accurate picture of what overall performance can be expected.

New and used cars with power in the pedal at EMG Motor Group

When buying a car, horsepower is often one of the key specifications used to market both new and used vehicles for good reason - it directly relates to the real-world performance that drivers can feel when behind the steering wheel. 

For many buyers, pairing their horsepower needs can narrow down their choices and make the whole car purchase process simpler. Whether the priority is responsive performance or maximum efficiency, here at EMG Motor Group, we can talk you through our selection of cars designed for a more exciting everyday driving experience.