IMSA classes

GTP : the class was introduced in 1981. It was aimed at trying to end up the Porsche 934/935 domination of the late 70s. It was a non-production based formula, rules by a set of scale to weight/displacement.

Lights : the equivalent of GTP cars, but for small displacement engines. Jim Downing suggested the name. He also won most of the titles.

GTO : introduced in 1971, production based formula for international sports cars, this class was an equivalent of the FIA Group 4 rules. Engines were over 2,5L. For a long Time the main category, it was later shadowed by the GTX and GTP class. It was also the most popular class. The cars would later run their own races, except for the enduros.

GTU : also introduced in 1971, it was based on the FIA Group 2 rules. Engines were under 2,5L and some GTU races were run in the late 70s and early 80s. Displacement was raised to 2,8L then 3,0L in the late 80s.

AAGT : American born cars were given more latitude to compete with the foreign makers. The class was created in 1974 and produced the most spectacular racers. Al Holbert won two titles with such cars.

GTX : introduced in 1978, it was an attempt at breaking the Porsche stranglehold. It benefited to the germa n make, who fielded the biggest GTX entries. BMW was the main competition in this class.

TO : pony cars with up to 5,0L displacement. The category was dropped in 1976 and TOs were mixed with GTO cars.

TU : same cars, with small engines up to 2,0L displacement. Cars were mixed with GTU cars.

The TU class was dropped in 1974.