page_title: IMSA GT Championship
section_title: Initial divisions
hierarchical_section_title: IMSA GT Championship / Initial divisions
caption_attribution_description: English: Robin McCall in the Hoerr Racing IMSA GTO Oldsmobile Toronado. Photo taken at the 1987 IMSA Columbus 500 (Columbus, Ohio)
context_page_description: IMSA GT was a sports car racing series organized by International Motor Sports Association. Races took place primarily in the United States, and occasionally in Canada.
context_section_description: The 1971 season was the first racing season, and lasted six races. The early years of the series featured GT cars, similar to the European Group 2 and Group 4 classes, divided into four groups:
  GTO - Grand touring-type cars with engines of 2.5L displacement or more, 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.
caption: Robin McCall in the Hoerr Racing GTO Oldsmobile (1987)
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