Whether you’re a keen reader of car reviews, you’ve been thinking of buying your first car, or you’re in the market for something else, then chances are, you’ll be used to seeing the term horsepower. But what does it mean?
With all the other figures used to explain vehicle performance, horsepower is just one of the measurements that can confuse the uninitiated. And when you factor things like brake horsepower into the mix, then you’re bound to be even more befuddled too.
So, if horsepower has left you scratching your head in the past, this article is for you. Below, we’ll define horsepower in more detail, look at how it’s measured and how it compares to other measurements.
In its simplest form, horsepower (hp) is the figure used to measure the power of an engine. Basically, it’s how much ‘work’ the engine is capable of.
If you want to get technical about things, then power is a measurement of work divided by time, while work can be defined as force multiplied by distance. It’s these equations where horsepower has its origins…
Back in 18th-century Scotland, inventor James Watt sought to compare the power of horses to then-burgeoning steam engines. To do this, he measured the pulling power of horses up a mine shaft.
Many calculations later, he found that one horsepower equated to roughly 33,000 foot-pounds per minute of work. Horsepower as we (kind of) know it today was born.
But back to the present day. Generally, the higher the horsepower, the more power the engine will generate and put out.
That said, although horsepower figures will give you the broad strokes of a vehicle’s performance, you can’t really directly link power to vehicle speed. The speed of a car depends on a few different things.
Take its power-to-weight ratio, for instance. This is the amount of power a car produces in relation to its weight. So, a small city car with 75hp will be much faster than a larger, heavier SUV – even if the SUV has the same amount of power.
Now that we’ve defined horsepower, what then, does brake horsepower refer to? Aren’t they the same thing? Well, not quite.
Brake horsepower refers to the horsepower of the car after accounting for the frictional losses in power from the engine. For this reason, brake horsepower is always less than a car’s horsepower, though there’s not much in it.
1hp is the equivalent of just under 0.99bhp, although the difference is more noticeable when things are scaled up. So, a car with 150hp will have a brake horsepower of 147.
You may have seen the acronym PS used in car ads or reviews at some point too. Another measurement of power, it’s the most popular measurement used by car manufacturers based in Germany; the name literally translates to “horse strength”.
Essentially, it’s the metric equivalent of a single horsepower: 1PS equals 0.9863hp. In France and Italy, the terms cheval-vapeur and cavallo vapore are used, but they mean the same thing as pferdestarke.
To add to the confusion, kilowatts (kW) are yet another metric used to measure engine power. Introduced in 1992 by the European Union (and popularly used in Australia), kW tends to be a smaller number, so a lot of manufacturers tend to stick with bhp, because of course, the bigger bhp figure sounds more impressive. It’s all a matter of sales, of course!
The use of kW as the EU’s preferred unit of power has been growing in popularity too, thanks in large part to electric vehicles. Since electrical power is most commonly quoted in kW, and we’re seeing electric vehicles become more commonplace with each year, it makes more sense that car manufacturers would sell electric cars with units that people are familiar with.
You’re bound to have heard of this impressive-sounding word before. A favourite of Jeremy Clarkson et al, torque certainly conjures up images of mighty motors speeding across picturesque vistas, but is it another unit of power? For the sake of this article, thankfully not.
Measured in either Nm (newton metre) or lb/ft (pound-foot), torque is a unit of how much turning force a car can produce. More specifically, it’s a rotational force that results in acceleration.
So, essentially, it’s responsible for how quickly a car gets from that all important 0-60mph range. The higher the torque, the quicker it can accelerate and achieve its 0-60mph.
If speedy accelerating from stationery, more power when overtaking and the ability to tow heavy objects is important to you, then the higher the torque number, the better.
There’s a chance you’ll also see a torque figure followed by an engine speed (such as 1,500-2,500rpm). This is an indication of a car’s peak torque around this number of revs, which is why it’s better to change up a gear if the revs are too high. Keep in mind that with electric cars, there’s no need to build up your revs since torque is delivered instantly.
So, when it comes to acceleration, which is better: more torque or more power? Generally, the more horsepower your car has, the faster it’ll be able to reach its top speed. But where torque is concerned, things aren’t so clear cut. What you need to look out for is where peak torque arrives and how long it can be sustained for.
Say a car with 200hp and 200Nm of torque, with a peak torque of 6000rpm, is pitted against another car with 200hp, but only 150Nm of torque, though its peak torque is at 1,500rpm. Although it has less torque, the second car would accelerate quicker than the first.