This year in Detroit BMW showed a slew of new, small cars like the 1- and 2-Series, and a lighter-weight M3 and M4, in its latest effort to tighten and refine its burgeoning lineup. Mercedes, with its new 4-cylinder AMG engine, and Audi, with the S8 and sporty A3, did the same. The idea with each seems to be that smaller vehicles with turbocharged engines offer efficiency gains with uncompromising performance, plus a healthy profit margin to boot. . . .