What does horsepower mean? And what is torque?

caravan towing

Bruce Newton

Posted December 21, 2021


We’ve all heard the term, but does anyone know what horsepower really means? And what does it have to do with torque?

First of all, a bit of a history lesson. Power in its most simple definition means using strength to overcome resistance and create movement. 

The emergence of the steam engines in 18th century Britain prompted Scottish engineer James Watt to compare this new device with a draft horse to quantify its capabilities. Hence, the term horsepower was born.

Watt even defined one horsepower as the power required to lift 33,000 lbs exactly one foot in one minute. 

You often see horsepower shortened to ‘hp’ as a measure of a car’s engine output, but you also might see it as ‘PS’ – a term German car companies use that expands to Pferdestrke, or horse strength.

The official rating Australia and much of the world uses is kilowatts (kW). 


Measuring power

What about the internal combustion engines that sit under the bonnet of the cars, SUVs, and utes most of us drive? Their power isn’t measured by hitching up to a 33,000 lb concrete block and hitting the throttle. No, it’s all more scientific than that.

So there are some basics to remember. Torque, or pulling power as it is also known, is the rotational force produced by an engine’s crankshaft. The ‘work’ is that force acting over a distance, where ‘power’ is how much work is being generated. The power number is arrived at by multiplying torque by engine speed.

The basic rule is the more power generated the more work is done and the faster your vehicle can accelerate from 0-100kmh, the higher the top speed, and so on (there are lots of provisos here: gearing, vehicle weight, aerodynamics, etc; but the fundamental rule is correct).

Traditionally, the bigger the engine, the better off you are because there is inevitably more torque to work with. 

Turbocharged and supercharged engines also have more torque because they pump more air into the cylinder. More air leads to more torque, which leads to, yep, more power. 

Why some people love power

In an internal combustion engine, power tends to increase with revolutions per minute (rpm) but not in a purely linear way. This is because torque is not constant through the rpm range. 

Torque tends to be very low at low rpm, peaks at mid-rpm, where the ability of the engine to maximise the amount of air going into each cylinder is at its optimum and then drops away at higher rpm.

Because power equals torque multiplied by rpm, power tends to increase the more you rev the engine, even as torque is dropping off at high rpm.

So that’s an explanation of why horsepower is the term focussed on by performance car lovers and in motorsport - because maximum throttle high-rpm tuning works in that environment.

Of course, boys love to compare, so there are pub bragging rights about power numbers. Through the generations of locally-built Ford and Holden performance cars, peak power kept creeping up with each new iteration of Falcon and Commodore V8 topping the other by a few kilowatts.

It is not unusual for the peak power figure to also become part of a car name. Jaguar, Land Rover, Lotus, Renault, Skoda, and Volkswagen all currently do this, for a wide variety of vehicles including quite bland entry-level models.

 

Car showing horsepower and torque caravan

Torque, or pulling power as it is also known, is the rotational force produced by an engine’s crankshaft. Image: Getty. 


 

Is torque more important?

For all the fixation on top-end power figures amongst car makers, there’s actually much to be said for paying attention to the torque figure.

As we’ve already noted, torque production tends to be at its peak in the mid-range. The downside of that is less power than at peak engine speed, but for 99 per cent of drivers and 99 per cent of the time, that’s going to be irrelevant. 

In the mid-range, most engines are running in their most comfortable state, and therefore not consuming as much fuel or being worn out as quickly. Because you’re driving in the fat of what’s called the ‘torque curve’, you’re getting the most efficient combination of torque and power.

It really comes in handy when accelerating to pass traffic, climbing a hill, or towing. Automatic transmissions also tend to be tuned to take advantage of that torque delivery - often keeping the engine in the sweet spot of the torque delivery.

Of course, on a steady state on a freeway, a car will happily run at very low engine speeds, meaning its generating little torque or power and uses only a little fuel. Some modern car engines even have a function that allows cylinders to be deactivated in this situation, allowing V8s to turn into V4s and so on.   

Going electric

Not many of us are buying electric vehicles yet, but that will change. In contrast to a petrol engine, one of the key traits of an electric motor is it generates maximum torque at zero rpm. That’s because its delivery of fuel – electricity in this case – is fundamentally instant.

Electric current arrives and immediately creates torque through the magnetic attraction between the armature and motor housing. The rich torque available at such low motor speeds is very effective at launching the vehicle, providing a seemingly instantaneous response and very satisfying acceleration.

Urban traffic is where electric vehicles really shine with great torque for launch combined with the ability to re-generate electricity from frequent stops and negligible power consumption when halted in traffic.

The downside is that at present, e-motors can’t sustain that high output because resistance builds in the system and reduces torque and power. But electric cars get away with less peak power than internal combustion because the torquey initial response more than makes up for the difference.

 

Looking at what's important when getting cover for your motor vehicle? 
Protect what’s important with RACV Motor Insurance →


Related reading