If it has Valvetronic then it's an N55. My guess is that it does given the quoted efficiency numbers, but I would love to know for sure.
Originally Posted by kevinlevrone
All BMW had to do to fix the N55 engine was to fit a Valvetronic head on an N54 and call it N55. Maybe that's what Alpina did...it would explain the fuel efficiency.
That doesn't make sense though. It would make more sense to replace the N55 single turbo charger with a twin turbo setup (possibly adapted somehow from the N54, but possibly not). Changing an induction system is typically a lot less involved than creating a Frankenstein head/block combo, and no significant advantages lie in the N54 short block anyway.