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The International Motor Sport Association was founded in 1969. They organized one race that year. In 1971 the first whole IMSA season consisted of six races was held. In the in the early seasons the cars similar to European Groups 4 and 2 were divided into four groups: GTO, GTU, TO and TU. That meant GT or Touring under or over 2500 cc. Usual winners of the early years of IMSA GT races were Porsche 911/Carrera RSR and Chevrolet Corvette. Gregg/Haywood with Porsche 914/6 GTU became the very first champions.
Later in 1978 a new premier group GTX brought absolute dominance of Porsche 935, which became the most successful car in the series. The most successful driver of seventies was Peter Gregg, champion 1971, '73, '74, '75, '78 and '79.
In 1981 for the first times real prototypes appeared in the new category IMSA GTP and Brian Redman became the first IMSA champion driving a prototype, Lola T600 Chevrolet. March also started In GTP class (champions 1983 - Al Holbert, 1984 - Randy Lanier). Year 1984 new Porsche 962 debuted and became best car between 1985-1987. After that IMSA enjoyed a Nissan era with Geoff Brabham at the wheel. Toyota's drivers were quickest in 1992 and 1993 but it was the end of high tech/high speed GTP prototypes in IMSA. During whole this GTP period Jaguar tried to win the series. Originally with American Group 44 car, later with Tom Walkinshaw's World Championship winning TWR based Jaguars. They won many races but never became champions. There were also some other big manufacturers present in GTP class such as BMW, Ford, Chevrolet-Lola and later Mazda. Along with 'big' GTP cars a Camel Lights category was created in 1985 for smaller prototypes. Argo's drivers were first champions but later were beaten by Spice's drivers. Another well known cars in the class were also Tiga, Royale, Alba, Fabcar and later Kudzu.
IMSA got quite new face in 1994 when new open top WSC sportscars replaced previous GTP and Lights closed cars. Ferrari built a 333SP prototype for this new category and immediatelly became the car to beat. First WSC champion was however Wayne Taylor with Kudzu because Ferrari weren't ready to start in early season races at Daytona and Sebring. In 1995 a new Ferrari rival appeared: Riley & Scott Mk III. Only these two cars were able to win any race untill the demise of IMSA in 1998.
During 1997 season was IMSA renamed to Professional SportsCar Racing (PSCR) and was known under this name untill end of 1998. For the 1999 season SPORTS CAR, the sanctioning body, decided to drop their own championship in order to sanction a new American Le Mans Series which arose from initiative of Mr. Don Panoz.
In IMSA many groups with separate classifications always existed. Each era had its own top class, initially it was IMSA GTO, then IMSA GTX, later IMSA GTP and finally IMSA WSC. Other categories had sometimes their own separate races, while all categories together ran usually only in long distance races such as 24 Hours of Daytona or 12 Hours of Sebring.
Many years GT cars in IMSA raced in two categories. GTO were Grand Touring cars over 2.5 litres and GTU cars Grand Touring under 2.5 litres. In 1992 premier GT category was renamed to GTS because of sponsorship reasons and the GTO title was used for former American Challenge cars that supported IMSA GT races for many years. New GT categories were established in 1995 to move closer to european rules: GTS-1 and GTS-2. In 1997 GTS-2 became GTS-3 and new GTS-2 group was announced.
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