Audi is presenting three new S models at the 2011 Frankfurt Auto Show next month, two of which, the new S8 and S7 Sportback, we’ve already seen.

The third S model is the new 2013 Audi S6, which will be making its debut in both sedan and Avant wagon bodystyles but is only destined for sale in the U.S. as a sedan.

The new S6 is scheduled to go on sale in Europe in the spring of 2012 and should arrive in U.S. not long after.

It will come packing a twin-turbocharged 4.0-liter V-8 rated at 420 horsepower and 405 pound-feet of torque, the latter coming on from just 1,400 rpm. You may recall that the previous model came with a 5.2-liter V-10, however, the downsized V-8 not only offers similar performance, it’s also a whole lot more fuel efficient.

Average gas mileage is 24.25 mpg in the S6 and 24.0 mpg in the S6 Avant for the European combined cycle.

The force-fed V-8 will accelerate the S6 from 0 to 60 mph in only 4.8 seconds, while the S6 Avant is just 0.1 of a second off that pace. Top speed is an electronically limited 155 mph for both models.

2012 Audi S6 Avant

2012 Audi S6 Avant

A number of technologies contribute to this top result, including brake energy regeneration, engine start-stop and Audi’s new “cylinder on demand” engine management system. This latter feature can deactivate four of the engine’s eight cylinders during low load situations, such as cruising on the highway.

Drive is sent to all four-wheels via a seven-speed S tronic dual clutch transmission. The quattro permanent all-wheel drive system gets a self-locking center differential and torque vectoring feature. Speed adjustable steering is available as an option, as is Audi’s sport differential, which actively distributes power between the rear wheels.

The S6 and S6 Avant come standard with 19-inch aluminum wheels, with 20-inch wheels remaining an option. Standard brakes are internally ventilated steel units, though buyers can opt for carbon ceramic units.

Both cars also ride on an adaptive air suspension, which in its most hardcore setting can lower them by up to 10 millimeters.

2012 Audi S6

2012 Audi S6

Inside, height-adjustable sport seats with power lumbar supports and embossed S logos are standard. Audi offers two other versions as options: the power-adjustable comfort seats with memory function and the power-adjustable S sport seats with integrated head restraints.

Also standard are xenon headlights, a head-up display, a 360 degree camera based parking assistant, and Audi’s MMI interface with Bluetooth connectivity.

Opting for the range-topping MMI Navigation Plus package gives you an eight-inch LCD display, hard-drive based entertainment system, WLAN capability and a 15-speaker Bang & Olufsen sound system.

The S6 Avant, with its 59.33 cubic-feet of storage space, tips the scales at 4,300 pounds, while the S6 sedan is not much better at 4,177 pounds.

Look out for the debut of the 2013 Audi S6 and S6 Avant at the 2011 Frankfurt Auto Show on September 13, where they will be joined by the 2012 Audi S8 and 2013 Audi S7.

In the meantime, click here to follow our complete coverage of the event.