Originally Posted by Sniz
I would expect the n55 to have a little less low end grunt, but be surperior up top where it really helps for performance reasons.
a n54 powered vehicle can certainly pull my M3 on low end rpm pulls, but once i'm spooled, good bye.
same idea but the n55 wont be as laggy as a larger turbo for many reasons, twin scroll and valvetronic being 2 of the main ones.
Avoidance of lag has always been a top priority for BMW for these bread&butter cars. Twin scroll is another way of getting boost at low rpm/pressure -- a small impeller with little inertia, and then a big one for serious boost (like the turbo in the diesel). The twin turbo design did much the same -- little turbos spin quickly, two turbos push more air.
Edit: misuse of term "impeller", see comments below.