THE biggest engine to go into an Audi has made its debut in the company's flagship long wheelbase limousine.

The 6.3-litre, 12 cylinder monster gives the luxury A8 awesome firepower yet loses none of the car's serene quality.

Uprated from the previous 6.0-litre W12 it now delivers 500ps resulting in performance not far short of that available in the Le Mans derived R8 supercar.

The A8 L may weigh the best part of two tonnes but can accelerate from standstill to 60mph in less than 4.7 seconds.

Not surprisingly such engineering doesn't come cheap and the A8 L W12 carries a price tag of £93,465.

And even in this heady echelon there are extras to be had. The car we sampled was fitted out with a Bang & Olufsen hi fi and rear seat TV screens which took the price up to £104,505.

Rivals such as the Jaguar XJ, Mercedes S-Class and BMW 7 Series can be had for less and Audi knows it has to deliver a premium package.

This is chief executive territory and in typical Audi fashion the A8 is packed with refinement but packaged in such an understated manner.

The car is more than 17 feet long but there is nothing ostentatious about its appearance.

Inside it is a different story with hide upholstery, excesses of leg room and gadgets front and rear.

The emphasis is on back seat luxury with zoned climate control throughout the car, LED ambient lighting and sumptuous trim including bespoke gloss black inlays and even a vanity mirror.

In the rear armrest are controls for the ventilation and entertainment system - and there are more buttons than on the dash of many a family car - while the DVD flat screens are mounted on the rear of the front seats.

On the road the A8 L is supremely quiet. The car is fitted with an eight speed auto box with seamless changes and for something of such proportions it is remarkably rewarding to drive.

Power delivery is smooth and controllable - docile when needed but with mighty acceleration available if required.

Its lightweight aluminium construction allied to the quattro all wheel drive set up allows the car to be incredibly nimble and when it comes to handling the A8 L is absolutely composed - more so than some of Audi's smaller models.

The new engine now has Audi's fuel stratified injection system which has noticeably improved efficiency.

Audi claims that W12 will average 22.8 to the gallon and that's what we saw on the trip computer.

For a car in this league its emissions of 290g/km are to be commended and overall it is almost ten per cent more fuel efficient than the previous model.

Other features exclusive to the W12 include all-LED headlamps, powered door closure - including the boot - and in keeping with its ‘executive on the move' status the car is WiFi enabled, allowing up to six devices to be hooked up to the Inter