He also added an IMSA GTP title during 1983 in a Chevrolet and Porsche powered March 83G when Porsche were unable to make their 956 eligible for competition that year. February 27, 1983, he won the Grand Prix of Miami. Holbert finished fourth in the 1984 Indianapolis 500, and led the Porsche IndyCar effort in 1987–1988. He won the 24 Hours of Le Mans in 1983, 1986, and 1987, the 24 Hours of Daytona in 1986 and 1987 and the 12 Hours of Sebring in 1976 and 1981. Holbert was the head of the Porsche North America's Motorsports Division and ran his own racing team, Holbert Racing. He clinched two more IMSA GTP championships back to back in both 1985 and 1986 driving a Lowenbrau sponsored Porsche 962. In 1988, Holbert realised that the Porsche 962 that had brought him success in his earlier years was becoming outmoded by the newer generation of racers from the likes of the Jaguar XJR-9 and the Electramotive's Nissan GTP ZX-Turbo. His plan was to build an open top Porsche-engined racer for customer teams. Porsche eventually built such a car nearly a decade later, although the WSC-95 would never be built for customer teams as Holbert and Porsche intended.