Ideal exhaust diameter?

[lennon]

In my opinion, 2.5" is perfect for a 2.5/2.7 m20. If you build a 2.8 or 3.0 or something, I'd probably use a 3". Definitely a 3" for an s50 or s52. Of course this is all single pipe, dual would be different... and a lot nicer in some cases.

It's going to sound a lot louder if it comes out underneath the car like Lateracer is describing. Otherwise you should be fine. I have SS rep headers and a 2.5" exhaust with only a Flowmaster muffler in the rear, and it's not half as loud as most Civics, WRXs, Hemi trucks, etc. I ran it for a long time with no muffler at all and it was barely any louder, just a lot less pleasant to listen to. My buddy's 325i vert has the same headers into dual side pipes coming right out below the passenger door. His sounds about 100x louder than mine. Of course, I may be wrong and mine may be very loud and I just don't hear it because it's blasting out behind the car. I wouldn't exactly say my m20 sounds "good"... his definitely doesn't. I think the actual quality of the tone is better with the nice stock dual exhaust, cats, and maybe a good aftermarket muffler. I have a tendency to put loud, straight pipes on cars though. Fo' racin' yo...

[nando]

there's no real advantage to dual exhaust, and I certainly don't think it sounds better (or much different). the gasses are merged in the stock exhaust by the x-pipe just before the cat. BMW probably built a dual exhaust for two reasons - clearance and marketing. just like how the E46 M3 has 4 tips..

[lennon]

i'm no expert, but you're probably right about no performance from dual exhaust on an inline 6 like ours. i do think dual 2.25" sounds different than one 3" or whatever numbers you want to plug in. better is a matter of opinion. nice sound/performance really depends on other things besides just whether the system is dual or single pipe. you could have shitty sounding duals or single. i wouldn't bother with the extra space loss, weight, work/money, etc if i was having an exhaust built for an inline 6 bmw.