Americans will finally see a rational application of BMW’s Euro-centric EfficientDynamics mantra when BMW unveils six-cylinder versions of its 7-series at the Detroit auto show in January. The 740i will occupy the lowest rung on the company’s executive-sedan ladder, below the plenty-powerful 750i, the extreme 760i, and the slightly ridiculous ActiveHybrid 7. Power comes from BMW’s twin-turbo