Question : E46 engine change

Is it easy to fit an E39 M5 engine into my E46 323Ci or does it involve a lot of problems due to the size of the engine bay ?

The M5 engine may fit but you would most likely want to contact Alpina for more info on that swap. Because of the heavier engine you would throw the balance of the vehicle off, just fyi. How about the E46 M3 motor. M3's are faster than the M5, probably a little more logical and cheaper.

i thought that both the E39 M5 and the E46 M3 had a 0-60 of 4.8 secs. i think thats what BMW NA says. maybe you also include some other things when you speak of the broad term "fast"

Yeah, the M3 seems faster, seat of the pants feel, not sure about the times these days. The M5 engine would cost you alot more to install than the M3 engine. Plus traction would be a major issue also.