According to Autocar this morning, BMW’s M division has built a special lightweight one-off version of the E60 M5 as part of the 25th anniversary celebrations for the iconic four door saloon.

Revealed this morning at BMW’s prototype workshop at the Nurburgring in Germany, the celebratory M5 is described by M division’s head of development, Albert Biermann, as “the M5 CSL we never built”.

Among the changes brought to the new car is a heavily modified version of the standard M5’s 5.0-litre V10 engine. The highly strung, 90-degree unit has been enlarged with a longer stroke taking its capacity up to 5.5-litres, making it the largest engine to ever feature in an official M-car.

Also included as part of the engine overhaul is a carbonfibre manifold boasting a increased capacity air box and a secondary oil cooler – the latter requiring modifications to the front bumper, which gains an additional air duct in the place usually dedicated to the license plate.

Official power and torque figures have not been released but Biermann hints the output peaks at “around 580bhp and close to 400lb ft”

Channelling the added reserves back to the rear wheels is a beefed up version of the M3’s seven-speed double clutch gearbox. The Getrag engineered unit, operated via steering wheel mounted paddles or push/pull gear lever, supplants the standard M5’s sequential seven speed manual unit, offering smoother and more rapid gear shifts.

Further changes centre around the body, which receives a carbonfibre roof as part of a weight saving program that has pared a claimed 50kg from the standard M5’s 1830kg kerb weight.

Yet despite the investment that has gone into creating the new uber-M5, Biermann says it is destined to remain a one-off.

“Right now we’re concentrating on the next-generation M5. This car has been fun to develop but there are no plans for production,” he says.

Read the rest of this article over at Autocar.

[Photos: Autocar ]