[pmeloche]

In the old days BMW had a simple nomenclature for their car: the first digit was the series and the next two digits were the engine displacement in liters
and then a string of letters describing chassis characteristics and additional engine characteristics.

We could write an essay on this but to keep it simple let's look at the difference between 318 and 320. Originally, the 318 had a 1.8L engine that got upgraded to a 2.0L engine while keeping the moniker 318i. BMW then upgraded that engine to develop more power and changed the moniker to 320. A classic example is the 330i becoming 335i. The E9X engine used to be a 3.0L NA developing 255 hp IIRC named 330i. Then BMW added twin turbos to it and it now develops 300hp. They then renamed the model 335i.

Hope it helps clarify thngs a little bit.

Cheers.