In 1963, Iso founder Renzo Rivolta commissioned Giotto Bizzarrini to develop a competition variant of the Grifo. Bizzarrini had left Ferrari in 1961 following the famous internal dispute that also saw the departure of several other senior engineers; he subsequently set up Prototipi Bizzarrini in Livorno, where he consulted for several manufacturers including ATS, Lamborghini, and Iso.
Bizzarrini developed two versions simultaneously: the A3/L (Lusso, for road use) and the A3/C (Corsa, for racing). The road car's body was designed by Giorgetto Giugiaro at Bertone; for the racing version, Bizzarrini created a dramatically modified alloy body. A significant engineering change distinguished the two cars — in the A3/C, Bizzarrini moved the engine approximately 40 mm rearward relative to its position in the road car, making the racer a front, mid-engine layout that improved weight distribution. This required a modification so awkward that mechanics had to remove a section of the dashboard to adjust ignition timing.
Bizzarrini later described the A3/C as his "Improved GTO," a direct reference to the Ferrari 250 GTO whose development he had led before his departure from Maranello. Both cars were unveiled at the 1963 Turin Motor Show — Bertone displayed the A3/L prototype while Iso showed the unpainted A3/C competition car.
The A3/C's competition career centred on the 24 Hours of Le Mans, the most prestigious endurance race of its era.
In 1964, a prototype A3/C was entered at Le Mans, driven by Edgar Berney and Pierre Noblet. The car ran well until brake problems forced a lengthy two-hour pit stop. Despite the delay, the car resumed and finished 14th overall — a creditable result for a first appearance with a car built on a much tighter budget than the factory Ferrari and Ford entries it faced.
In 1965, the A3/C returned to Le Mans and improved significantly, finishing 9th overall. This placed it among the leading sports cars of its displacement class and validated Bizzarrini's approach. The result was achieved against competitive machinery from manufacturers with far greater resources.
The simultaneous development of road and racing cars led to complications over intellectual property. The production Grifo GL, which entered production in 1965, was manufactured at Bresso, while the A3/C continued to be produced at Piero Drogo's Sports Cars of Modena workshop in Emilia-Romagna, under Bizzarrini's direct supervision.
A dispute between Rivolta and Bizzarrini over the use of the Grifo name led to the partnership dissolving. From that point the road car continued as the Iso Grifo GL, while Bizzarrini developed his racing car independently under his own name — the car becoming the Bizzarrini A3/C and ultimately the Bizzarrini 5300 GT Corsa. Only 22 examples of the Iso-badged Grifo A3/C were completed before the split ended that line of development.
The A3/C was powered by an American Chevrolet small-block V8 engine, consistent with Iso's practice of using American powertrains for cost and reliability reasons. The modified alloy body was aerodynamically optimised for high-speed circuit racing, a significant departure from the steel construction of the road car. The mid-forward engine placement improved the car's handling balance compared to a conventional front-engine layout.
The Iso Grifo A3/C is significant both as a racing car that punched considerably above its weight at Le Mans, and as the work through which Bizzarrini established his post-Ferrari reputation. The car's 9th-place finish at Le Mans in 1965 against far better-funded factory entries remains its defining achievement. The split with Iso allowed Bizzarrini to continue developing the concept under his own name, producing a small series of road-going and competition cars that maintained the same engineering philosophy. The original A3/C, built in just 22 units, is regarded as a rare and historically important competition car of the 1960s GT era.