Audi is fitting a 12-cylinder petrol engine into the A8 to make it the world's most powerful production saloon.

The 6.0-litre, 48-valve unit has a W-cylinder layout and a mighty 420 horsepower output, creating a new flagship A8 model - the A8 L 6.0 quattro. It goes on sale in Germany this month and costs £70,000. Audi is the only manufacturer to use a 12-cylinder engine arranged in the W formation in volume production. It says the major benefit of this design is its space efficiency. Despite additional cylinders and a larger displacement, the 6.0-litre W12 engine actually takes up no more space than the 4.2-litre V8 engine in the A8 range.

Another major advantage is that it enables power to be transmitted to all four wheels, via the standard five-speed Tiptronic gearbox, bringing benefits in traction and control. All other conventional front-mounted V12 engines drive only rear wheels because there is normally insufficient space within the engine bay for the additional components.

To create the ultimate luxury Audi, the W12 engine has been installed in the long-wheelbase version of the A8, which is 130mm longer than the standard car and provides rear seat passengers with an impressive 70mm of extra legroom, plus additional head, elbow and shoulder room. The W12 engine, in common with Audi's equally spectacular V8 TDI unit, will not be fitted to right-hand-drive versions of the current A8 saloon, but will form part of the UK line-up of future generation A8 models.