Not that it matters much since they let the 'marketing' guys usurp the
logic in the late '70s.  About the only dependable part left now is
'3', '4', '5', '6', and '7' (with '1' and '2' still to come).

The last two digits once (and nearly do again) meant the engine size
in liters.  Still, there's that '45' turbo b.s. to contend with ...
--
C.R. Krieger
(Been there; done that)

http://www.unixnerd.demon.co.uk/bmw.html.You should get your answers there.