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The new N55 is a pretty revolutionary engine. It's the first engine to combine a twin scroll turbo with direct injection and valvetronic w/ double VANOS (this setup lets them modify exhaust valve lift and both intake and exhaust timing).
The reason for switching was to develop a more linear and efficient power curve. The old N54 used two small independent turbo's (they aren't sequential) that were optimized for low end power. The issue here was that they ran out of huff at higher RPM's. The solution was to use either sequential turbo's (one smaller for low end and one larger for high end) or use a single twin scroll design. Since sequentials are complex and trouble prone the choice was easy.
The new design produces the same power, but has a much broader power band with peak torque coming in 1k RPM lower at 1,200. Also, since the turbo itself is larger it is able to produce better boost across the RPM range. The interesting thing was that BMW could have extracted a lot more power out of this setup, but instead chose to maintain the same peak power levels as the N54 and reduce emissions and fuel consumption.
So, the cliff notes reason is that it is simply a more efficient design with greater potential.
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