The 1991 season brought a fundamental regulatory change: the FIA replaced the fuel-consumption-based Group C category with a new 3.5 Litre Formula using naturally aspirated engines, though heavily penalised Group C cars were allowed to participate in a separate C2 class. Jaguar chose to compete in the new formula.
Because the XJR-14's predecessor, the XJR-11, used a twin-turbocharged 3.5-litre engine derived from the Metro 6R4-based JV6 unit, continuing with it was not a viable option under the new naturally aspirated rules. Jaguar's connection with Ford provided a solution: the 3.5-litre Ford HB V8 Formula One engine, most notably used by the Benetton F1 team, was adopted and detuned to around 11,500 rpm and 650 bhp (compared to approximately 700 bhp in full F1 specification) to enhance reliability.
The abandonment of Group C fuel restrictions and the shift to a smaller, lighter engine changed the aerodynamic philosophy entirely. Downforce became the priority over drag. The new design targeted a kerb weight of 748 kg — far lighter than the previous Group C front-runners — and very high downforce levels, making the XJR-14 substantially faster through corners. The design represented a shift in approach at TWR: previous Jaguar sportscars had been directed by Tony Southgate, whereas the XJR-14 reflected Brawn's Formula One background, working with a large design staff of around 12 people.
Three chassis were built and raced in 1991: numbers 591, 691, and 791. In the early part of the season the XJR-14 was in a class of its own, comprehensively outpacing rivals including the Peugeot 905 and Mercedes-Benz C291. Only the opening race at Suzuka went against Jaguar, widely attributed to misfortune rather than the 905's competitiveness.
From midway through the season Peugeot's revised 905B closed the gap significantly and the two teams fought for victories. Jaguar held on to secure the manufacturers' title with three wins.
For the 1991 24 Hours of Le Mans, Jaguar initially entered two XJR-14s but withdrew them, concerned that the Cosworth HB engine was unproven over a 24-hour distance. Three older XJR-12s were entered in the C2 class instead. Though the XJR-12s did not win, the decision was vindicated by Peugeot's 905 also failing to finish, and by Mercedes withdrawing its C291 in favour of the older C11.
After 1991 Jaguar chose not to continue in Group C, and TWR sent the XJR-14 to North America for the 1992 IMSA Camel GTP Championship. Chassis 691, renumbered 192, debuted at Miami and broke the track record by four seconds. However, without major suspension upgrades the car struggled with the high lateral loads of US circuits and finished third in the championship with two victories.
TWR reached an agreement to supply additional XJR-14 chassis to Mazda for the World Sportscar Championship. Fitted with Mazda-badged Judd V10 engines and rebadged as the MXR-01, these cars were essentially production XJR-14s. Without ongoing development they proved uncompetitive, scoring no wins.
Several years later, TWR resurrected XJR-14 chassis 691 for a new purpose: development of a prototype for Porsche. The car's roof was removed to create an open-cockpit design compliant with the new LMP regulations and the car was renamed the TWR-Porsche WSC-95, carrying a Porsche 3.0-litre turbocharged flat-six. TWR also built a second WSC-95 from scratch. The WSC-95 won the 24 Hours of Le Mans outright in both 1996 and 1997.
The XJR-14 demonstrated how Formula One thinking — lightweight construction, extreme downforce, and a purpose-built normally aspirated engine — could be directly applied to sports prototype racing. Ross Brawn's design influence, later seen across multiple Formula One championships, was already evident in the XJR-14's conceptual clarity and performance margins. That one of its chassis went on to win Le Mans twice as the WSC-95 underlines the fundamental strength of the original package.