As a direct competitor to the Mercedes E63 AMG and BMW M5 which have both been presented a while ago, Audi is planning to offer the RS6, seen here undergoing final tests at the German Nürburgring. The bigger air intakes in the front bumper and the two big oval exhaust pipes are the key clues. While the S models have four round exhaust pipes, the RS models have two oval pipes as their distinguishing feature. Shown in these spy photos is the Avant (station wagon), but it will also be available as a saloon. Powered by the same V10 engine as the S6, but with some help from a turbo delivering about 520-550 bhp, this Audi will give the M5 and E63 AMG a strong challenge. The RS6 will debut in Geneva in March 2007.