I'm open to correction on this, but I think the 328 has the highest emmission levels and is the heaviest engine for fuel consumption and because of this the NCT have their limits for the 3 series based on this engine. This is just what I heard to a question I asked when I got a conversion from a 318 to a 3.0 M3 engine a few years back in my e36, and I was concerned about cunsumption and emmissions!