The 1991 season was a troubled transitional period for endurance racing. FISA had introduced new 3.5-litre non-turbocharged regulations — mirroring contemporary Formula 1 — but very few manufacturers had built cars meeting the specification. The Group C category was therefore kept alive as "Category 2" alongside the new "Category 1" formula. Category 2 cars were penalised with 100 kg of ballast, except for Mazda's rotary-engined cars, which were permitted to race at their normal weight — a loophole that would prove decisive.
FISA mandated that the top ten grid positions must be occupied by Category 1 cars regardless of their qualifying times, creating a grid where the slowest front-row qualifiers were separated by over 30 seconds from the fastest Group C cars starting just behind them.
The race also inaugurated Le Mans' new pit complex, built at a cost of over £12 million to replace the facility constructed after the 1955 disaster. It provided 46 pit spaces, each five times the size of the previous ones, a 2,900-seat grandstand, race control centre and media centre.
Jaguar, returning as defending champions with back-to-back wins in 1988 and 1990, entered the new XJR-14 designed by Ross Brawn with a Ford-Cosworth HB Formula 1 engine producing 650 bhp, plus four older XJR-12 Group C cars. Sauber-Mercedes returned after skipping 1990, entering the new C291 with a purpose-built 3.5-litre flat-twelve for the Mercedes Junior Team of Karl Wendlinger, Michael Schumacher and Fritz Kreutzpointner, alongside three veteran C11 turbo cars for Jean-Louis Schlesser, Jochen Mass, Alain Ferte, Jonathan Palmer, Stanley Dickens and Kurt Thiim.
Peugeot made its Le Mans debut as a works team with the new 905, a carbon-composite machine running an 80-degree V10 developing 600 bhp, managed by Jean Todt. Keke Rosberg and Yannick Dalmas drove one car; Mauro Baldi and Philippe Alliot the other.
Mazda entered three cars including two examples of the improved 787B, developed with larger wheels, revised suspension and improved aerodynamics. The quad-rotor engine maintained 710 bhp while the 787B's heaviest example at scrutineering was still 150 kg lighter than the lightest Porsche in their class. The team knew 1991 would be its last chance to win Le Mans before rotary engines were banned under the incoming regulations. Volker Weidler, Johnny Herbert and Bertrand Gachot drove the lead 787B.
Peugeot dominated the opening hour from pole position, but both cars were eliminated early — one by a fuel fire in the pits, the other by engine failure. Mercedes then ran 1-2-3 for several hours. Schlesser and Mass in car number 1 held a comfortable lead through the night, while the Mercedes Junior car with Schumacher set a new lap record of 3:35.5, cutting nearly five seconds from the previous mark set by Nissan.
Jaguar struggled throughout with excessive fuel consumption, forced to run five to six seconds per lap slower than their true pace. Multiple off-road excursions and mechanical incidents cost the team heavily. The leading Joest Porsche of Derek Bell and Hans-Joachim Stuck retired with persistent water leaks.
The Mazda 787B of Weidler, Herbert and Gachot ran steadily throughout, lying third at the halfway point. The rotary engine's lighter weight under the regulations gave the car a structural advantage as attrition mounted around it.
With three hours remaining, after leading for almost 17 hours, the Schlesser/Mass/Ferte Mercedes came in with coolant temperature spiking: the water pump belt had failed. The damage was terminal. The Mazda screamed past into the lead and Herbert brought the car home to win by two laps over the IMSA-specification Jaguar XJR-12 of Davy Jones, Raul Boesel and Michel Ferte.
The Mercedes Junior team finished fifth despite losing time with gearbox and undertray problems, winning the Index of Energy Efficiency. Only 12 cars were classified at the finish from 38 starters — the smallest starting grid in nearly 60 years.
Mazda's victory remains the only Le Mans win by a rotary engine and the only outright victory by a Japanese manufacturer. The 787B's characteristic four-rotor howl at full speed became one of the most recognisable sounds in motorsport. Rotary-engined cars were banned from the top class the following year.
The race also marked one of Michael Schumacher's rare endurance appearances before his Formula 1 career took hold. His lap record and the aggressive pace he demonstrated in sharing the Mercedes Junior car were noted across the paddock.
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