out in 1976, TO the following year. Turbochargers were not permitted until the middle of the 1977 season. They were allowed following protests by Porsche's motorsport department, after inspecting Al Holbert's AAGT winning Chevrolet Monza, which had won two titles. Prior to 1977, Porsche privateers struggled with obsolete 911 Carrera RSRs against the AAGT cars. Engine sizes were determined by IMSA officials, who had devised a set of rules to determine fair competition, using a displacement versus minimum weight formula. Turbochargers were taken into account as well as rotary power, fuel injection, and many other engine features. As a result, the new premier class known as GTX (Grand Touring Experimental, which was based on FIA's Group 5), brought on the absolute dominance of the Porsche 935. The 935 became the most successful car in the series. The most successful driver of the 1970s was Peter Gregg, who won championships in 1971, 1973, 1974, 1975, 1978, and 1979. Twin turbos were outlawed at the end of the 1982 season after John Paul Sr. and John Paul Jr. dominated in a modified 935. GT spaceframe era In 1984, all GT cars were required to display a large square decal to identify which category the car competed in. A GTU car, for instance, would have a black U on white, and a GTO car, a white O on black. All others had standard IMSA GT decals. One significant change to the rules during the 1980s was the 2.5 liter limit being increased to 3.0 liters, with the maximum 6.0 liter limit still in place. 3.0L cars were required to weigh , whereas 6.0L cars had to weigh no less than . In an effort to equalize the competition, two-valve turbocharged cars were required to weigh 15% more, and four-valve turbocharged cars 20% more. Electronic fuel injection became common, while ground effects were still prohibited. Steering, braking, transmission, and suspension were left up to the constructor. Bigger, more powerful engines were permitted under homologation rules. The number of valves, ports, and spark plugs were not allowed to be modified from the original configuration. The AAR Toyota team suddenly encountered a daunting problem with the rules when, with Toyota's introduction of the new, fourth generation, Celica for the US market, and the team's first entry into the top-flight GTO category (despite not having won a single GTU title), the team faced the distinct possibility that