Originally Posted by stressdoc
The differences between the N55 and N54 in the 535i are determined by software specs, not inherent differences in the performance potential of the bi-turbo (N54) versus twin-scroll+valvetronic (N55) designs.
Believe me, there are variants of the N55 in R&D that smoke the current N54 & N54B30.
If you ask me, I would say a single bigger turbo can never spool fast enough for low-end torque. A single turbo can be equal, but can never be better than 2 turbos for low-end. Valvetronic does not help much either, because at low RPMs the air volume is low so a throttle plate in the air flow instead of valvetronic doesn't really slow things down. This is where the lack of low-end punch comes from.
Of course, software can bring more additional "punch" but so is the software from GIAC, Evotech, etc. The question is, can it bring the additional punch while keeping the targeted BMW parameters ? If it did (with the current hardware), they would have released it at least in the heavier models like the 535i which would really benefit from more torque.
When I will see an M3 with single turbo and Valveronic I will give the props to N55, but I really believe the next M3 will have a twin-turbo without valvetronic in order to provide low-end torque and a high-RPM redline (where the Valvetronic has problems).