Just checked realoem:
The 328i uses a different exhaust manifold from the 128i and 325i/330i
The 128i uses the same exhaust manifold as the 328xi, but again not the 325i/330i
The 330i uses the same exhaust manifold as the 323i/325i
The 330xi uses the same exhaust manifold as the 325xi
N51 cars have a different exhaust manifold and different down stream catalyst, shared between the 128i and 328i.
The bottleneck has to be the software, and for all N52 powered cars - headers.
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