This is where the RS6 makes all the headlines. It uses a 5.0-litre V10 engine that's based on the same unit found in the Lamborghini Gallardo but fitted with twin turbochargers to produce an incredible 580bhp along with immense pulling power. Thanks to twin exhaust pipes, it emits a deep burble on start-up and there's a meaty roar if you give the throttle a quick prod.
However on the move the engine is strangely muted. There is a brief howl when changing gear at high revs, but overall the engine lacks the rewarding soundtrack of the BMW M5. This doesn't detract from the immense acceleration though and a 0-62mph time of 4.5 seconds (4.6 seconds for the estate) is quicker than an Aston Martin V8 Vantage. But it's the effortless way in which the RS6 gains speed that's most impressive and the engine never feels strained - even at high revs.
Of course, this does come at a price and an average fuel economy figure of 20mpg is woeful by any standards, while hard driving will swiftly see this drop. The only gearbox available is a six-speed semi-automatic that features steering wheel-mounted paddle shifts. It offers amazingly quick and smooth changes, but slightly robs the driver of a sense of involvement from behind the wheel.
For a car that weighs more than two tonnes the RS6 handles incredibly well. The standard quattro four-wheel drive system ensures excellent traction, especially out of slow corners, while there's immense levels of grip - even in the wet. It certainly feels agile for such a large car and the responsive steering reacts to the smallest of inputs. However the steering itself feels a little numb at times, but this is a minor gripe.
The RS6 features an upgraded version of the Electronic Stability Programme that's standard on all A6 models, but it's been tweaked to intervene as late as possible to maximise driving pleasure.