The mid-size sedans and estate wagons from Audi stand in size between compact A4 and full-size A8. The series consists of three sedans and two wagons that vary through powertrains yet a unibody structure shared by all contains lightweight aluminum structural components. Base A6 3.0 sedan and A6 3.0 avant wagon use Audi's impressive 3.0-liter V6 worth 220 hp. A five-speed Tiptronic automatic is standard with sport settings, but the multitronic CVT is offered on FrontTrak models. Audi's quattro traction system is optional for sedan or wagon. Next step up in the series leads to the A6 2.7T sedan tipping decidedly to the sporty side of performance with a unique bi-turbo 2.7-liter V6 aboard for 250 hp. The two turbo-chargers -- one for each of two cylinder banks -- take up less space than a single large unit and the small size means faster response from accelerator pressure to firing time and higher torque at lower engine speeds. Then A6 4.2 sedan stocks the powerful 4.2-liter dual cam V8 out of Audi's ultimate S8 for massive muscle. And for ultimate strength in a wagon, the sport-tuned S6 avant also gets the V8. S6 avant rolls on 17-inch Avus alloy wheels with ZR performance tires, has stiffer suspension settings on a lowered platform and puts a Bose sound system in the cushy cabin.