The chassis was built from aluminium with carbon fibre used at stress points and was based on the reasonably successful 1983 Williams FW08C. The engine cover had to be redesigned as the car was powered by a smaller, but more powerful () V6 engine rather than the Cosworth V8 that powered the FW08C. The front of the car was also redesigned giving the car cleaner aerodynamics. The FW09 was introduced at the South African Grand Prix, the last race of the 1983 season, which was nothing more than a shakedown exercise. Rosberg showed the potential of both car and engine by qualifying sixth and finishing in fifth place while team mate Jacques Laffite started 10th (after failing to qualify the old car at the previous European Grand Prix at Brands Hatch), but spun off under braking for Crowthorne Corner at the end of the long main straight on lap two, his car ending up in the tyre barrier and out of the race.