[atmosphericM]

^ Honestly I am not sure, but from what I've read on the interwebs, a different head is the cause for the slightly lower compression ratio, and supposedly the pistons are the same. Considering the SAP port being found on both heads though, I am starting to wonder, myself.

Some people also say that the N51's secondary cats are a higher cell count, thus being cleaner but also more restrictive - however I've never been able to find definitive info there, either. Of course other small changes exist, as well, such as slightly different radiator, the SAP/SAI system, variable length intake manifold, etc.