Here is why they are changing from N54 to N55. Apparently it was meant to happen the whole time.
"This has the same advantages as two small turbines (as on the N54); it's just combined in one housing (in the N55)," he added. BMW could have used a single, twin-scroll turbocharger on the N54 engine, Holder said, but company officials were concerned the quality would not have been up to par at the time the 335i coupe launched in 2006.
Quote comes from:
http://blogs.insideline.com/straight...55-engine.html