Overall rating:

Good points: terrific engine, quality chassis, great noise.
Not so good: it certainly does not come cheap.

Key Facts

Model tested: Audi A6 3.0 TDI Bi-Turbo S-Line
Pricing: £47,020 (A6 Bi-Turbo starts at £44,670)
Engine: 3.0-litre twin-turbocharged diesel V6
Transmission: eight-speed Tiptronic, four-wheel drive
Body style: four-door saloon
Rivals: Audi S6, BMW 535d, Porsche Panamera Diesel
CO₂ emissions: 169g/km
Combined economy: 42mpg
Top speed: 155mph (limited)
0-62mph: 5.1 seconds
Power: 313hp at 3,900rpm
Torque: 650Nm at 1,450rpm

Our view:

Last year we drove a prototype version of the Audi A6 Bi-Turbo fitted with an electric rather than mechanical first turbo (read that story here) and while impressed by the performance of the early development car we were less than enamoured with the looks. Here is a car that packs near supercar performance yet the looks of a £30,000 poverty spec A6 - not really the look one would expect of a car that commands a £45,000 (at least) price tag.

However, having spent a week with the traditionally turbocharged car I find it is those looks that are half of this car's appeal. Allow me to explain: I like sleeper cars - cars that go like stink yet look like butter wouldn't melt in the mouth. For years my lottery car has been an original shape Audi RS6 Avant Plus, a 476hp battleship that can easily accommodate a trip to Ikea. Over the years there have been many pretenders to this crown like the BMW 6 Series Gran Coupé and current shape Range Rover Vogue, but were my numbers to come up tomorrow it would be the high power A6 diesel that would be sitting on the driveway.

And that is because, under the usual Audi identikit body, which has become ubiquitous on Irish roads over recent years, is a 3.0-litre V6 engine that packs 313hp and 650Nm of torque. It is the same engine that is fitted to the Audi SQ5 - the first diesel powered 'S' car, and if you believe the marketing bluff, one that can trace its lineage to Audi's Le Mans winning cars.

Debatable parentage aside the Bi-Turbo is quick - and not just diesel quick. Find a straight stretch of road on which to open the taps and it will hit 62mph from standstill in 5.1 seconds before sailing along to an electronically limited top speed of 155mph. To put that turn of pace into perspective the twin-turbocharged, petrol-fuelled, V8-powered Audi S6 is only three tenths quicker to 62mph than its diesel sibling. But while the 420hp S6 is marginally quicker than the Bi-Turbo in those terms, it comes nowhere near to the 42mpg fuel economy.

But unlike the RS 6 (or even the S6) there are no visual identifiers to the power this car possesses. There are no swollen arches, no aluminium wing mirrors, there aren't even any distinctive badges to prise off to complete the Q-car look. The only difference between this and a 177hp 2.0-litre TDI version of the Audi A6 is the fact that its oversized brake discs necessitate 18-inch wheels at minimum and even middle-management sales reps might have specified larger rims from the options list.

With so much power to transfer to the tarmac, Audi's quattro all-wheel drive is fitted as standard to the Bi-Turbo - and it needs it. At low revs the engine ticks over as you would expect an Audi diesel to: it's smooth and refined, but with a little bit of tell-tale diesel clatter evident. Take the revs past 2,500rpm, at which point the larger, second turbocharger kicks in, and things change remarkably. It is at this point that the speedo begins to spin quicker than your eyes can comprehend and you are forcibly pinned to the soft Valcona leather bound seat back.

Quick cars will do this, but there are few that will hold you there until you lift off the throttle or the eight-speed Tiptronic transmission grabs another of its eight speeds. While this is happening (and assuming you have selected Dynamic mode from the Audi Drive Select system, which turns the suspension, steering and gears up to eleven) the exhaust emits a noise more akin to that of a petrol V8. This is accomplished via an actuator built into the exhaust system that superimposes an oscillating pattern over the 'natural' exhaust sound to produce a distinctive rumble. Yes, it is a fake noise, but one that befits the performance of the car. We can forgive it more than the 'piped in' audio of the BMW M5 or even the 'piped-out' of the Audi S6.

Were it my money I'd opt for the less showy SE trim that does without the S-Line body kit and includes 18-inch wheels, but that is a case of personal preference rather than a slight at the test car. I would also make sure I ticked the box for the Sports differential that transforms the handling from that of an accomplished saloon into a back road blaster once the road ahead clears.

Alternatives:

Audi S6: more powerful, more agile and more special to look at. The twin-turbo V8 has a prodigious petrol thirst compared to the diesel however, even with stop-start and cylinder-on-demand technology.

BMW 535d: the one time king of the performance diesel saloon. Offers similar punch to the Bi-Turbo but does not feel quite as special.

Porsche Panamera Diesel: stuck with the single turbo version of the Bi-Turbo's engine, so lacks the grunt but makes up for it in prestige and the way it drives.