Because it's not in line with BMW's current efforts. The engine is a complete revamp of the engine in the 550i. BMW seems to be committed to turbocharging, not SC

If they were to supercharge a V8, it would just add extra cost because it would only be used on one car and be bespoke, like the E60's V10.

BMW doesn't seem to be doing that anymore.

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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