The 6R4's name denoted its key specifications: 6-cylinder engine, Rally application, and 4-wheel drive. Austin Rover engaged Williams Grand Prix Engineering to develop the car, and the resulting machine bore almost no mechanical relationship to the production Metro road car beyond a superficial visual resemblance. The structure was a semi-monocoque seam-welded tubular chassis, with the outer bodywork constructed largely from GRP. Only the roof panels (aluminium), the steel doors, and certain remaining panels from the original Metro shell were carried over, and even the doors were concealed behind plastic airboxes. The engine was a bespoke 3-litre V6 designed by David Wood, using some architectural elements derived from the Cosworth DFV Formula 1 engine. It featured twin overhead camshafts and four valves per cylinder and was notably naturally aspirated at a time when most Group B competitors used turbocharged powerplants. The engine was mounted back to front, with its forward end facing the hatchback and the gearbox attached conventionally behind it, placing it in the middle of the vehicle. Permanent four-wheel drive was transmitted via separate prop shafts to front and rear differentials.
The 6R4 was produced in two forms. The Clubman road-going version developed approximately 250 bhp and was the homologation model, with around 200 units built and sold to the public at £40,000 each. A further 20 cars were built to International specifications with a recorded output of over 410 bhp. Rover announced completion of the full homologation requirement by November 1985.
The 6R4 made its WRC debut at the Lombard RAC Rally in November 1985, where works driver Tony Pond finished third overall, a strong result behind two Lancia Delta S4s. That encouraging beginning was not sustained into the 1986 season. Although 6R4s were entered at Monte Carlo, Sweden, Portugal, and Corsica during 1986, none of the cars managed to finish a round. The primary source of problems was the V6 engine, which suffered persistent reliability issues. Halfway through 1986, the FIA banned Group B following a series of fatal accidents involving both competitors and spectators. The ban effectively ended the 6R4's front-line career, though private owners continued to run the cars in various events for the remainder of the year and into subsequent seasons. The car proved formidable in rallycross competition after its WRC career ended.
Austin Rover withdrew from rallying at the end of 1986. The following year, all remaining parts and engines were acquired by Tom Walkinshaw Racing. The 6R4's V6 engine later reappeared in a significantly modified form, fitted with turbochargers, in the Jaguar XJ220 road car. The 6R4 remains one of the most distinctive Group B cars, notable for its naturally aspirated engine philosophy at a time when turbocharged power had become the norm, and for the involvement of Williams Grand Prix Engineering in bringing Formula 1 expertise to rally car construction.