the letter O meaning "over 2.5L". The GTO class was dominated by Corvettes, then by Shelby Mustangs, and then various factory teams consisting of Cougars, 280zxs, Celicas and finally, 300ZXs. GTU - Grand touring-type cars with engines of 2.5L displacement or less, the letter U meaning "under 2.5L". The GTU group was dominated by Porsche 914-6 GTs, and SA22 Mazda RX-7s (1978–1985) through the end of the 1980s. TO - Touring-type cars, such as the Chevrolet Camaro with engines of 2.5L or more displacement TU - Touring-type cars with engines of 2.5L or less displacement In essence, these groups had been absorbed from the Trans Am Series. Trans Am would quickly become a support series for IMSA GT. History of the top series in the GT Championship Camel GT era The first champions were Peter H. Gregg and Hurley Haywood, in a Porsche 914-6 GTU. Common winners in these early years of IMSA were the Porsche 911 Carrera RSR, and the Chevrolet Corvette. Camel became the title sponsor during the second season, with the series becoming known as the Camel GT Challenge Series. The sponsor's corporate decal had to be displayed and clearly visible on the left and right sides of all racecars, and Camel's corporate logo patch was also required to be on the Nomex driver suit's breast area, featuring Joe Camel smiling and smoking a cigarette while driving a race car. Initially, all cars were identified with a category tag, stating which category they competed in, but from the middle of the 1975 season on, all cars within the series had to have a rectangular IMSA GT decal, which incorporated its logo on the left, followed by a large GT tag, as well as a Joe Camel decal. Starting fields of 30 or more competitors were not unusual during this era. One of the premiere race events was the Paul Revere 250, which started at midnight of the Fourth of July. The race was conducted entirely at night. In 1975 a new category, All American Grand Touring (AAGT), was introduced to counteract the Porsche dominance in GTO. In 1981, the Bob Sharp Racing team used a loophole in the rules to build a Datsun 280ZX inside the U.S. with a V8 engine from a Nissan President. The car was not a success, however, and it became obsolete when the new GTP category was created. TU would be phased