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I think this kind of explains it...........
The "TwinPower Turbo" moniker initially denoted engines that utilize a twin-scroll turbocharger which was unveiled in 2009 (the calendar year, not the model year) with the N55 and N74 being the first engines to use this technology.[2] Nonetheless, BMW has started to apply the TwinPower label retroactively to earlier turbocharged engines that are still in production like the N54 and N63, although they do not use twin-scroll technology.[3] The N55 is the only single turbo engine in BMW's lineup as of the 2010 model year.
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