The project was defined by Stuart Turner in the spring of 1983, shortly after he was appointed head of Ford Motorsport in Europe. Turner enlisted the support of Walter Hayes, Ford's vice-president of public relations, who had previously championed both the Ford GT40 Le Mans programme and the Cosworth DFV Formula One engine. Together they approached Cosworth, where an in-house project based on Ford's T88 engine block — a twin-cam, 16-valve unit known internally as the YAA — provided the technical foundation.
Cosworth agreed to develop a turbocharged version, designated YBB, provided the engine would produce at least 150 kW in road form and that Ford accepted a minimum order of 15,000 units. Ford required only around 5,000 engines for homologation but accepted the full order; the additional 10,000 engines later motivated the development of a second-generation four-door Sierra RS Cosworth.
Ford chose the Sierra because it offered rear-wheel drive and acceptable aerodynamics, while a racing variant could simultaneously improve the model's reputation — the Sierra had attracted negative public reaction since its 1982 introduction. Lothar Pinske oversaw the distinctive bodywork, including the prominent rear whale-tail wing, front intake opening, and widened wheel arches. After initial reluctance from Ford management regarding the wing's visual impact, Pinske prevailed by demonstrating it was essential for stability at 300 km/h.
The car was publicly unveiled at the Geneva Motor Show in March 1985. In practice, production began in summer 1986 at Ford's Genk factory in Belgium. Of 5,545 cars produced, 500 were sent to Aston Martin Tickford for conversion to the RS500 Cosworth.
The RS500 was developed to create an evolution-specification racing car. Announced in July 1987 and homologated in August 1987, it featured an uprated Cosworth engine with a thicker-walled cylinder block, a larger Garrett AiResearch T04 turbocharger, a larger air-to-air intercooler, a second set of four fuel injectors and a second fuel rail (inactive in road trim), an uprated fuel pump, and output increased to 227 PS at 6,000 rpm in road form. The rear tailgate gained a lower secondary spoiler beneath the standard whale tail, with a gurney flap added to the upper wing. A redesigned front bumper removed the fog lamps in favour of additional brake-cooling intakes. Exactly 500 RS500s were built, all right-hand drive, for the UK market only.
The RS500 dominated touring car racing globally from 1987 onward. At the 1987 World Touring Car Championship, Ford took pole position at all six remaining rounds and won four of them. The works Eggenberger Motorsport team claimed the entrants' World Championship. The RS500 won the 1988 European Touring Car Championship entrants' title and the 1988 Deutsche Tourenwagen Meisterschaft, with Klaus Ludwig taking the drivers' championship in the Ford Team Grab car and Wolf Racing winning the team title. In Australia, Dick Johnson Racing dominated the 1988 and 1989 Australian Touring Car Championships, with Dick Johnson and John Bowe finishing one-two in both seasons. The RS500 won the Bathurst 1000 twice: in 1988 with Tony Longhurst and Tomas Mezera, and in 1989 with Johnson and Bowe. The car also won the British Touring Car Championship in 1990, with Robb Gravett taking the drivers' title. Motorsport Magazine described the RS500 as statistically the most successful road-derived racing car of all time, winning 84.6 percent of all races it entered.
In rally competition the rear-drive Sierra Cosworth was used on tarmac events in the World Rally Championship from 1987. Didier Auriol won the 1988 Corsica Rally outright, the only occasion that season when Lancia was beaten in direct competition. Jimmy McRae took the British Rally Championship in a Sierra in both 1987 and 1988.
The second generation was a four-door saloon assembled at Genk, retaining the YBB engine in slightly revised form. Its more conservative rear wing gave a drag coefficient of 0.33, allowing a marginally higher top speed of 150 mph and a 0–60 mph time of 6.1 seconds. Approximately 13,140 examples were produced in 1988 and 1989.
The third generation, launched in January 1990, added four-wheel drive to make the car competitive in the World Rally Championship. The Ferguson MT75 gearbox, which had been under consideration since 1987, was not available until late 1989. The revised engine was designated YBG for catalytic converter-equipped cars and YBJ without, producing marginally more power to compensate for the additional 100 kg weight. The 4x4 contested full WRC programmes in 1991 and 1992 in the hands of drivers including Francois Delecour and Massimo Biasion, taking several top-three finishes but never winning a WRC round. Production of the 4x4 continued until the end of 1992, when the Sierra was replaced in the rally programme by the Escort RS Cosworth.
The Ford Sierra RS Cosworth is widely regarded as one of the definitive performance cars of the 1980s. Its dominance contributed to the decline of the Group A format, as the RS500 remained competitive for more than three years after homologation without requiring a replacement. The car remains popular in historic racing and national-level rally competition, valued for its combination of power, reliability, and relative accessibility to private competitors.
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