The five-seater's exterior is very smart, especially in 'S-Line' specification. Great S-Line features include huge 20-inch alloys as standard, discrete design lines and an overall graceful stance. Not only does the car look awesome, but it can go too! Audi has fitted its most powerful V6 diesel ever, and that means the Bi-Turbo can go from a standing start to 100km/h in just 5.1 seconds!

Inside the cabin it's all classy A6 with, of course, sportier seating up front. The layout is the familiar Audi experience we know well. Seating is comfortable and, despite 20-inch alloys, the ride is civilised and composed. The boot is huge too, and features remote key fob opening.

With the current A6, Audi showcased a host of new electronic aids and features like the highly-illuminating LED headlights (Audi was first to market with these), but the Bi-turbo, with 313bhp and an incredible 650nm of torque, is all about the driving experience. The two turbochargers used are different sizes, with a smaller turbo providing initial additional engine boost, followed by a larger turbo that takes over to deliver the high-end power.

The Bi-Turbo also makes a glorious sound when you accelerate hard, thanks to a 'Sound Actuator' in the exhaust system. It is important drivers get to connect with their machines and enjoy the power delivery - you certainly can in Bi-Turbo. Audi says the actuator system "essentially consists of a speaker, which imposes a defined oscillation pattern on the exhaust flow and transforms the typical TDI exhaust note into a more sonorous roar that is closer to the output of a high-performance petrol engine".

Power is delivered through an eight-speed 'Tiptronic' transmission (with paddle shifts) to the latest-generation, Quattro all-wheel-drive system that has a self-locking centre differential and torque vectoring. An optional 'Sports' differential (that limits wheel spin and power loss) was fitted to my test car and this made the rear of the car more controllable and far more interesting, especially when driving enthusiastically.

Audi's 'drive select' adaptive dynamics system, which allows the driver to control the throttle response, transmission shift points and steering feel depending on their mood, makes driving the A6 Bi-Turbo a joy. I spent most of the time in 'individual' mode; it allows you to fine-tune the driving experience to your tastes. I set all the options to 'dynamic' bar the steering, which I put in 'comfort' mode. Auto and economy settings are also available.

As saloon cars go, the A6 is one of the best in its class, but add in Quattro all-wheel drive, a sports diff and a powerful twin-turbo 313bhp/650nm diesel engine and the result is one of the best cars I've driven. The immense torque, coupled with four-wheel drive traction, is a beautiful combination. It's economical too: capable of over 44mpg and produceing a relatively small CO2 of 169g/km, which means it falls in to tax band 'D' for annual road tax (€570).

The €72,120 (OTR) A6 Bi-Turbo S-Line could never be called cheap, but it is one of the most complete machines you can drive.

Michael Sheridan