The DFV had established itself as the most successful Formula One engine in history by the late 1970s, but its suitability for endurance racing was limited by crankshaft vibrations that damaged components under sustained running. When new Group C regulations came into force in 1982, Cosworth produced the DFL as a dedicated sportscar derivative, enlarging the DFV's basic 90-degree V8 architecture to displacements of 3,298 cc and 3,955 cc through wider bore and longer stroke dimensions. The 3.9-litre version was always referred to in the sport as the 3.9, despite technically displacing just under 4.0 litres.
Both versions shared the fundamental engineering architecture of the DFV but were substantially re-engineered for the demands of endurance racing. Neither fully solved the vibration issues that had dogged the parent engine in long-distance events.
The DFL made its competition debut in the 1982 World Endurance Championship season, where the 3.3-litre variant powered the new Rondeau M382 to three podium finishes including an outright victory at the 1,000 km Monza event. Despite this promising start, the season ended with Rondeau in second place in the championship points behind Porsche — a creditable result that somewhat overstated the engine's reliability.
At the 24 Hours of Le Mans the DFL proved consistently problematic. From 34 Group C1 class starts between 1982 and 1984, the DFL produced only three finishes, none of them reaching 300 completed laps. The 3.9-litre version fared worst, failing to establish itself as competitive at any level and disappearing from C1 competition entirely by 1985.
The DFL found its most consistent success in the fuel-restricted C2 class, introduced in 1983 as a category for smaller-engined prototypes. Low-power tuning mitigated the engine's vibration-induced reliability problems by reducing mechanical stress, and the DFL became the most widely used engine in the class. Cars powered by the 3.3-litre DFL — predominantly Spice and Tiga prototypes run by privateer entrants — won four C2 class championships and five C2 class victories at Le Mans between 1985 and 1990. The best distance ever covered by a DFL-powered car at Le Mans came in the 1988 race, when a C2 machine completed 351 laps.
Cosworth subsequently produced 3.5-litre variants of the DFV family for Group C use when the FIA signalled a transition toward F1-derived engines in the early 1990s. The DFZ was adapted for sportscar use by Spice Engineering in 1990 but caused significant problems due to vibration, with Gordon Spice estimating it cost approximately four times more to run than the DFL. The DFZ did find consistent results in the lower-powered FIA Cup class in 1992. The DFR variant, used by the Spice team in 1990 and Euro Racing in 1991, achieved two C1 third-place finishes but was never able to challenge the dominant Peugeots, Jaguars, and Sauber Mercedes. The last start for a DFL-family engine in the C1 class at Le Mans came in 1988.
The DFL represented Cosworth's attempt to leverage the DFV's proven architecture into endurance racing, with limited success at the top level but genuine competitive value in privateer hands. Its five Le Mans C2 class victories and four C2 class championships are a tangible legacy, even if the engine never threatened to win the overall race as the standard DFV had done in 1975 and 1980. The DFL programme demonstrated both the strengths and the inherent limitations of adapting a short-distance racing engine for endurance events, and influenced Cosworth's subsequent decisions around the DFZ and DFR programmes.