The reason we define cars by engine size is habit

The very first motorcars ever made were not defined by their size or shape or weight or price,  but by the power of their engines. In precise and meaningful Horsepower. 

More than 100 years later, cars are still described primarily by their engines, but using a measurement that is not meaningful. Ask anyone what car they have got, and the answer will always include the engine size, in cc,  a 1.3 or a 1.8 or a 3 litre…  (make and model, blah, blah).

Even the law (especially for customs duty) is obsessed with engine size.  Presumably, then, the cubic capacity of an engine is a crucial factor.  But crucial to what? Speed? Power? Fuel consumption?

In fact, engine size in cc doesn’t tell you anything except the volume of space in the combustion chambers.  A higher cc simply means that the space is larger, and that will usually mean the engine is physically bigger.

But bigger does not automatically translate to faster or stronger… or thirstier. Engine size is only one factor — among very many and often more important elements — that determines a vehicle’s “performance”.

TECHNICAL IDEA

This is clearer to understand if you think about how different engines of exactly the same size can be. For example, a 2-litre engine might be found in a Formula One racing car, a family saloon, or a farm tractor. Same size. But speed, power and fuel consumption? Incomparable!

Having grasped the technical idea through that extreme example, even sticking to the humdrum world of only saloon cars, the difference between engines of the same size can still be quite dramatic. For a start, power itself is not one thing; it is at least two: one is the power to accelerate — how rapidly the engine revs can be increased, usually described in “horsepower” (HP). The other is the power to work — the engine’s ability to maintain speed even when the vehicle is heavily laden or climbing a hill, usually described as “torque” (measured in footpounds or Newtonmetres).

There are several dozen aspects of design which determine engine performance in these respects. One is not necessarily “better” than another; every variation has pros and cons for reliability, durability, driveability, comfort and cost — and the balance is chosen according to the engine’s probable market and use.

Engine “size” is just one of the variables. Others include the materials, design, operational system and quality of manufacture of every part of the engine itself, the  compression ratio, stroke proportions, combustion chamber shape, camshaft lift, number of cylinders, number of valves,  valve spring hardness, computer management settings, the ignition system, carburetor/injector, flywheel weight and centrifugal balance…

Then come the variations of the vehicle it is fitted to — its weight, shape… and things like gearing, which determines the relationship between the speed of the engine measured in revolutions per minute of the crankshaft and described as revs (rpm); and the resultant speed of the vehicle (in km per hour). And gearing depends on the gearbox, the differential ratios, and the diameter of the tyres.

The only reason to define cars primarily by their engine size is habit... and want of a better idea.