Designed when the championship was already begun to replace the 642, which proved to be not very competitive and not very reliable, the new car called 643 nevertheless fails to bring the Italian team closer to the top of the category of the 1991 Formula 1 World Championship.
The 643 F1 is the latest and most extreme of the single-seaters developed since the 640 of 1989. Although it is a sufficiently distinct model from the 642, its ancestor, the 643 shares the alternative name of F1-91 with it, it is also used, as mentioned, in the 1991 season.
The most visible differences compared to the 642 are in the aerodynamics, in fact, unlike the duckbill solution of the old single-seater and typical of all the projects starting from the 640, the nose of the 643 adopts a tapered fairing, pointed forward and wider towards the car body, and with a rounded profile, almost flat near the cockpit rib and tending to curve downwards in the forward section.
Furthermore, the whole front part of the car is raised from the ground compared to the old model: this with the dual purpose of preventing the front wing from touching the bottom under the weight of the aerodynamic load, both to increase the air flow that s' slips under the car and is conveyed to the rear extractor.
The sides of the muzzle adopts an unusual black colour typical of the unpainted carbon fiber, replacing the typical Corsican red connoting the rest of the livery. The side bellies are also deeply redesigned, resulting shorter, wider and lower than those on board the 642, without completely abandoning the profile to Coca-Cola.
The twelve-cylinder engine of the 643 differs from that used on the progenitor for the increased cylinder bore, the reduced piston stroke, and the lower displacement. Despite this, it still boasts greater power, such as to improve the relationship with the weight of the car body.
Despite the efforts of the Maranello technicians, unfortunately the 643 is unable to overcome the chassis, obsolescence and reliability problems that plagued the 642.
The competitive debut, on the Magny-Cours track, a circuit characterized by a particularly regular and smooth road surface, is however promising, as Prost is second and Alesi fourth.
As happened with the 642, however, in the following races the gap with respect to the McLaren-Honda and Williams-Renault again widens: the complex chassis-suspension of the 643 is in fact in crisis on the circuits full of ups and downs and generally under braking.
Already starting from the British Grand Prix, raced on the Silverstone circuit, the 643 will begin to show its limits, allowing Alain Prost to take only third place, but more than a minute away.
Distance is also important at Hockenheim, given that in qualifying the two Ferraris are separated by a little less than two seconds from the poleman, and in the race only Alesi manages to reach the finish line in third place, after Alain Prost exits following a duel triggered with Ayrton Senna.
After the defeat at the Hungarian Grand Prix, in Belgium Alesi is leading the race, except that on lap 30 gives up the engine, leaving the victory to Ayrton Senna.
In Monza Alain Prost returns to the podium, thanks to third place; result repeated also on the Estoril circuit, but by the hand of Jean Alesi. Despite the huge gap in qualifying, Ferrari managed to take an encouraging second place in Jerez, Spain, thanks to the careful driving of Alain Prost.
In the meantime, however, the internal situation of the team, already critical by itself, is also souring due to the intolerance of Alain Prost, who finds the car very slow and difficult to drive, complaining in particular about the fragility of the shock absorbers and the hardness of the steering.
After a previous dismissal, retracted immediately before Ferrari's departure for Japan against Alain Prost, on 20 October 1991, in an interview granted at the end of the Japanese Grand Prix, during which Alain talks about a problem at the steering that, having bent as a result of a contact, worked badly on his car, comparing the effort to be applied on the steering wheel to that of a truck, the new management of Ferrari pushes away the French driver, who in the final race of the season is replaced by test driver Gianni Morbidelli.
In the face of six podiums won in the ten races in which 643 took part, four of which with Prost and two with Alesi, the car suffered eight retirements, and only in rare circumstances did it give its drivers the opportunity to fight openly for victory (Magny-Cours and Spa), failing however to establish any fast laps.
The single-seater accuses eight retirements out of ten races during the season, however allowing Ferrari to win five more podiums during the season, including three third places in the British and Italian Grand Prix conquered by Prost, who also obtained a second place in the Spanish Grand Prix, and two third places in Germany and Portugal won by Alesi, for a total of 39.5 world championship points which, added to the 16 obtained by the 642, allow them to take third place in the standings reserved for manufacturers at the end of the season.