The model will feature a power upgrade over the standard A6 Avant and apparently have quad exhaust outlets.
The spy shots indicate that Audi doesn't intend to overhaul the S6 Avant's styling. Up front (compare above), the S6 (left) features boxy intakes at the corners of the fascia, in comparison to the rectangular openings on the standard model (right). The rest of the styling looks generally the same. They share hexagonal grilles and sharp-edged headlights. The big sensor that dominates the center of the vehicle continues to be distracting from the otherwise angular styling.
In profile, this pair is essentially identical. The S6 has a more aggressive appearance by utilizing the lower ride height, lower wheels, and bigger brakes.
The changes are similarly subtle at the back. The S6 packs a quartet of circular exhaust outlets that give it a sporty appearance at the rear. Conversely, the standard A6 opts for style by incorporating a pair of wide, hexagonal pipes sticking out of the tail.
Powertrain details are a mystery for the standard A6, and there's an even bigger question mark over the S6. One possibility would be to use a version of the biturbo 2.9-liter V6 from the RS4 Avant because this powerplant would open up the possibility of an even more powerful mill going into the inevitable RS6.
Inside of the S6 Avant, expect a mix of sportiness and luxury amenities. Front occupants would likely sit in aggressively bolstered seats and enjoy high-tech features like Audi's Virtual Cockpit digital instrument panel. The exposed sensors on the exterior suggest the availability of cutting-edge driver assistance features, too.
The standard A6 sedan will debut in 2018 and will probably arrive at the Geneva Motor Show in March. The Avant will possibly arrive later, and the S6 variants will likely arrive even further down the line. This timeline puts the S6 Avant's probable premiere in very late 2018 or even in 2019.