The difference between horsepower and engine cc

What you need to know:

The main difference between horsepower and cc lies in its definition; horsepower is the measurement of power of the engine while cc is the measurement of the volume or size of the engine. No matter how different the two words seem, they bear a significant relation with each other and can also be converted to the other.

Horsepower and engine cc are two very diverse concepts that incidentally are not mutually exclusive. Both are measurable, but one needs slightly more proprietary tools to quantify than the other. Let us break it down.

Engine capacity

We start with the easier one; engine cc. CC means cubic centimetres, and is a unit of volume. The number preceding that unit is the total volume swept by all the engine cylinders. I will assume you know what a cylinder is. This is how to calculate the engine cc:

Get the volume of one cylinder. This is simple primary school calculation; base area times height. Base area is simply the area of a circle because cylinders are circular. So, pi times the square of the radius of the cylinder gives you the base area, then multiply this by the height of the cylinder, which we call the stroke. Once you have this, then multiply that figure by the number of cylinders.

The end result is the engine capacity in whatever units you were working with; Americans sometimes refer to cubic inches rather than cubic centimetres. To give the capacity in litres, simply divide the figure you got by 1,000.

Calculating

Practical example: I have a BMW E34 525i. I know the nomenclature is already a giveaway of its engine capacity but let us pretend we have not seen it. I want to know the engine capacity of this car. The technical specs indicate a bore of 84mm (8.4cm) and a stroke of 75mm (7.5cm). What is its engine capacity?

We start with the area of one cylinder; pi (3.14) times the square of the radius. The radius of the cylinder is half the bore, so it is 4.2 cm. The area (pi times the square of the radius) is 3.14 times 4.2 times 4.2 again. This gives 55.39. We multiply this base area, times the height which is the stroke of the engine: 55.39 times 7.5, to get 415.42. This is the volume of one cylinder.

But my car has six cylinders, so I have to multiply the volume per cylinder times number of cylinders to get the engine capacity. 415.42 times six, which is 2492.5. My car’s engine capacity is 2493cc, which is typically rounded off to 2500cc. That is what the “25” in “525i” stands for. To express this figure in litres, divide the figure by 1,000 to get 2.5. Mine is a 2.5 liter engine.

I said this is the easier one because you do not need any special tools to measure engine capacity, a ruler is enough. A ruler, a pen and paper are all you need to measure engine capacity (also called displacement). Horsepower is a whole other kettle of fish.

Horsepower

Engine power is typically expressed in horsepower but sometimes in other units such as kilowatts. This is how much work an engine can do. Please note; it is not how heavy a load it can pull, that is torque. It is how much work, or how fast it can apply that torque.

The best way to express it is this way. There is a man. This man can lift a 90kg sack, tops. At 91kg, he cannot move that sack at all. So, the maximum torque he develops is 90kgm. There are two subsequent scenarios that follow, both of which explain what power is.

Once he has lifted that sack, power is how fast he can run while carrying that sack. Some people can run fast with that 90kg sack despite being unable to lift 91kg. Some people will run slower with that 90kg sack but this will not allow them to lift 91kg either. The reasons behind this are too technical to get into here, but they are why automotive engineers are well paid and also why engines cost so much money.

The second scenario is slightly similar in that our man here can lift a 90kg sack a metre off the ground (torque). Let us, for the sake of example, assume he is lifting 90kg sacks off the ground and loading them on a conveyor belt that is exactly a metre high. Power is how many 90kg sacks per second or per minute he can load onto the conveyor belt. Again, some people can load those sacks really fast despite the fact that they cannot lift 91kg. Others will load the sacks slower but this will not allow them to lift 91kg either. So, in a nutshell, power is the rate of doing work. But what is torque?

Torque

Torque is the ability to do work. In the example above, the torque applied is 90kg, which is the ability to move 90kg a distance of one metre. Sounds simple, no? It is, until you have to wrap your head around how torque is applied. This is where the issue of transmissions (a.k.a gearboxes) comes in, but again, we will not go down that rabbit hole just yet.

Pundits of boxing know the fact that Mike Tyson’s right hook was measured at 60kg. If he throws that right hook over a distance of one metre, can we then say that he has 60kgm of torque? This implies that there is enough torque in his right hand to go to tug-of-war against a Landcruiser VX packing a twin-turbo diesel V8 (63.7kgm). Why buy a V8 when you can get Mike Tyson to push you around with one hand, then?

Many of us, myself included, can actually lift a 90kg sack one metre high before breaking into a sweat, triceps start vibrating at low frequency, veins start showing on our necks and our girlfriends ask us to stop the nonsense before we throw out our backs and have to visit a physiotherapist. Does that mean I have more torque than Mike Tyson’s coma-inducing Fist of Righteousness? Does that mean I have the same torque as a 67-seater turbocharged Isuzu bus (98kgm)?

While I have oversimplified the concept of torque, in the real world, torque is applied in a rotational manner, so torque is in reality a twisting force, not a linear push.

Relation between torque and rpm

Ideally, there is a certain range within the rev range, what we call the ‘‘torque band’’ where the maximum torque can be applied, before the torque drops off as the rpms go higher. The analogy here is simple; you can lift a sack one metre off the ground. There is a limit to how fast you can do the lifting before you either cave in, or have to reduce the load in order to move any faster.

That is why vehicles have transmissions. These help the engine to stay within the torque band, the optimum load-lugging rev range, while increasing speed.

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