AND is has a 2 litre engine, well - 1998 or whatever.
The 316/318/320 are all the same size engine, just have different power outputs - so you call tell people that you have a 2 litre 
Surprising I know, but check the handbook, the 3XX is just a marketing designation
Same as the 525d, 530d and 535d all have 3.0 diesel engines after 2007 
Really? so the 535d is just a chipped 530d? Thought it had its own new engine :-/
Not quite, this is how the 3 engines are:
525d - 204 BHP Small-ish Turbo
530d - 242 BHP Large Turbo
535d - 286 BHP Small Turbo & Large Turbo.

How is the power output achieved then :question if they are all the same size lump :-/
Different sized Turbo's on the diesels, different engine mapping on the petrols, some I think IIRC have different manifolds, like your 318i will have I think 143 BHP, but the 320i I think has a different manifold and a couple of other little mods and it has 170 bhp

My 5 series has a 218 BHP 2.5 engine, the 523i has the same engine but 192 BHP due to a different manifold. But again the 525i after 2007 was changed to a more fficient 3.0 engine but still retains exactly the same power output, where as the 530i after 2007 has 272 BHP with the same engine. You could probably get you petrol mapped in a slightly different way and it'd give you 320i performance
