Why is 500cc Per Cylinder the Goldilocks Number for Engines?

The car industry is in a period of huge upheaval, especially in the power train department. The V12 is dead, the V10 is hanging on with its fingernails and even the V8 and V6 are under threat as a long list of manufacturers opts for a four-cylinder two-litre turbo engine. The list of manufacturers that has opted for this configuration is long, but why has this become the Goldilocks format for so many?

When you do the maths, you will notice that everyone is now going for 500cc per cylinder, and this isn’t just the in-line fours. V6s have all become 3 litres, V8s have all become 4 litres and V-10s have all become 5 litres, heck, even those like Porsche with a flat-six have opted for a 500cc per cylinder per pot with the venerable flat-six in the 911 now being a 3-litre (500cc x 6 cylinders =3000cc, geddit?).

Engineers have long been looking for the sweet spot and 500cc is what is described as the Goldilocks spot: not too big, not too small. However, it is not about the size; it is about the shape. It turns out that the format of the combustion chamber makes all the difference when it comes to the amount of power you can get out of your fuel, and a slightly undersquare format is where the sweet spot is.

If you go smaller and cylindrical, then the surface area to volume ratio increases so that the cooling of the flame front on the cylinder walls results in the fuel warming up the engine coolant rather than pushing the cylinder where you want it to go. If you take it too far the other way and opt for a flattened disc, the flame front doesn’t reach the cylinder wall and so there will be unburnt fuel and increased emissions.

Looking closer at the migration, it would appear it is not really the size of the cylinder that is important but the shape of the combustion area or stroke. Here it turns out that most of the car companies are trying to achieve a nearly ‘square’ combustion area with a bore-to-stroke ratio of just less than one; in other words, where the stroke is just longer than the cylinder is wide. Thus, most 2-litre engines now have a bore of slightly less than 86mm and a stroke around the 90mm mark, creating an almost ‘square’ combustion area.

It turns out that the 500cc is not really a magic number, as long as the bore-to-stroke ratio is kept almost one and a ‘square’ combustion chamber is created. However, manufacturers have sort of come to the conclusion that 500cc is a number suitable for most applications—not too big and not too small. In fact, just right. The size at which the power and torque and fuel consumption meet in a holy trinity that proves to satisfy regulators, manufacturers and the public alike. Then you need to consider in the age of modulation, there are obvious cost-savings by having a standardised 500cc cylinder across all manufacturers’ powertrains whereby you can ‘share’ the parts bin.