Originally Posted by Dackelone
It might be that the cars with the secondary cats are SULE cars (ie CA specs). ?
The three stage intake manifold is also called: DISA. IF you google that you will find lots of info about how it works. BMW has used that DISA intake manifold on lots of other models as well. Some of the German tuners can reprogram the two stage intake (DISA too) to make a big Hp increase. Normally they can get around 275 PS with just a chip tune on a N52.
The N52 with the secondary cats is ULEV II-compliant. The N51 has lower compression, 3-stage intake manifold, secondary cats, secondary air injection, and ozone-reducing radiator and is SULEV II-compliant. The N52 without the secondary cats is Bin 5-compliant (i.e., LEV II) and is only available outside CARB-compliant states, but I'm not sure if maybe some of those states offer the ULEV II N52 by choice/default, or how exactly it works.