Power? Torque? What’s the difference?

The performance of a car’s engine is measured by how much power it produces which is shown in kilowatts (kW) or horsepower (hp) and how much torque it produces which is usually shown in Newton-metres (Nm) or foot-pounds (ft-lbs). To explain what all this means in the most simple terms for you to understand, the power figure is dependent on torque and rpm (engine speed) to determine how fast an engine can turn the crank in a certain amount of time. The torque figure refers to how strong this turning force is. Both of these measurements relate to the rotating force of the crankshaft inside an engine which is mated to a transmission that delivers the power to the wheels.

OK, so how does my Subaru compare to a normal everyday car?

So you’ve just taken delivery of a brand new Subaru Impreza WRX STI and you have a brochure that says your car produces 221kW @ 6000rpm and 350Nm @ 3000-6000rpm. You’re probably scratching your head wondering what all this translates to. To put the 221kW figure into perspective, the average car produces around 80-120kW so 221kW is quite a fair bit more. This figure tells you how fast a car is going to accelerate in terms of speed, not the feeling you get from inside the car. None the less, so far we know that your Subaru is quite fast – awesome!

Now let’s take into account how heavy it is or say you encounter a hill. Common sense dictates that if something is heavy, you have to be strong to push it along a flat surface, let alone push it up a hill. Just like a car – it could be the fastest accelerating engine but what’s the point if it has no torque to back it up? It’s not going to move very far if it has no strength to do the job. That’s what the second figure (350Nm) refers to. The higher this figure is, the stronger the crank’s turning force is. This is the figure that determines how strong the car’s acceleration feels from inside the car!

That means that your STI has 221kW of power, which means it picks up speed quickly and it has 350Nm of torque which means that not only does it quickly accelerate, it has a strong acceleration. Fast acceleration means that you will leave many for dead from the traffic lights and strong acceleration means that it is a much more capable car on flat surfaces or hills, with or without extra weight in terms of speed. There’s no point in having a car with a fast accelerating engine if it’s not strong enough to pull its own weight around. Have you ever wondered why your Subaru is so effortless to drive on the highway even with a fully loaded car compared to an average car? You can thank the bucket loads of torque that your Subaru engine delivers for that.

Why are power and torque figures shown as being at a certain rpm?

Every engine is different so these figures usually give you a pretty good idea an engine’s “sweet spot” in terms of power delivery. In the case of your STI’s power figures mentioned before, the power figure (221kW @ 6000rpm) is a calculation of torque x peak rpm divided by 9550 if you are working with metric figures or 5252 if you are working with imperial figures. So, that works out to be (350Nm x 6000rpm) / 9550 = 219.8kW. Looks like you’re missing 1.2kW… but you get the idea.

The torque figure (350Nm @ 3000-6000rpm) shows that from 3000rpm onwards right upto 6000rpm, your engine is delivering the strongest rotational force it possibly can. This translates to a very nice, powerful and drivable car as you would be spending most of your time driving within this RPM range.

If you now understand what power and torque is, you will then understand why trucks are slow but powerful enough to haul extremely heavy loads. Trucks are powered by diesel engines. Diesel engines produce very little power but they are capable of producing epic amounts of torque. Torque = strength. Lots of strength = able to haul heavy loads.

So there you have it. Torque is everything and power is nothing without it. Many people believed that the more power an engine can produce, the faster it can go. While this may be true to an extent, many failed to truly understand the fundamentals of what power really is. Can you get more power and torque from an engine? Of course you can!

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