Yup, the N55 seems to have hit a wall in terms of tunabilty. Something that BMW seems to know because the 335is and 1M use the older N54 I6TT, instead of the uprated N55

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2011 BMW X5 xDrive 35d Vermillion Red Metallic/Black Nevada Leather
Options: ZCW, ZPP, ZRC, ZTP, 300, 386, 496, 4AB, 6FL, Combox, LED Coronas. 04/11 Production