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The categories : what are they?

John Bishop had helped draw FIA rules for the World Championship of Makes. The FIA Appendix J group 2 and 4 cars had only the few races to run in the US. He wanted a series based upon familiar looking cars and he thought that a Championship was needed to allow these cars to run on a regular basis. He wanted to go his own way, so he created the IMSA Camel GT Championship that would involve these types of cars :

  • GTO based on FIA Group 4 cars, over 2,0Liters displacement
  • GTU based on FIA Group 4 cars, under 2,0Liters displacement
  • TO based on FIA Group 2 cars, over 2,0Liters displacement
  • TU based on FIA Group 2 cars, under 2,0Liters displacement

All four categories were based on the FIA Rules set by Paris international homebased organisation. The main differences between the European rules and the IMSA rules were a greater liberty for tire width and cars could have less weight. Cars could also be stripped of every useless device.
These categories did evolve with time and the IMSA customized its rule book to suit to its needs. John Bishop's openminded spirit was always all ears to his racers, he wanted to equalize everyone's chance to win.
These categories were to be run until 1973, then the TO and TU were dropped, and all the cars were regrouped into the GTO and GTU categories.

Holbert_1GTO : Al Holbert Porsche Carrera RSR at Mid Ohio in 1974

Copyright Mark Windecker

Hotschkisporsche77GTU : John Hotschkis Porsche 914/6 at Mid Ohio in 1977

Copyright Mark Windecker

Wallemazda73TU : Ray Walle Mazda RX2 at Mid Ohio in 1973.
Copyright Mark Windecker

Kwechcapri73TO : Horst Kwech Ford Capri at the same race.
Copyright Mark Windecker

In 1975, Porsche was the ever winning car, John Bishop devised a new category, aimed at breaking this domination, the AAGT was created.

Moyercorvette77AAGT : James Moyer at Mid Ohio in 1977.

Copyright Mark Windecker

AAGT standed for All American GT. These cars were to be American built ones, with a set of ultra liberal rules. Bigger engines, wider tires, less weight : all of this set was made to break the European domination in GT racing. The Chevrolet Monza was the best example of what could be done with two titles in 1976 and 1977 for Al Holbert.
In 1976, Porsche introduced its 934 Turbo, a true racing car with tremendous horsepower. John Bishop did not allow this car in his series as he thought that racing costs would rise. In fact, Porsche did have a year of waiting and, in 1977, the 934s were showing the way to the future. Cars would be allowed to run with wider tires and big wings. But they did not dominate the series and the races were always spectacular.

For 1978, things went the turbo way, as the Porsche 935 was the car to own, and the rules did change a lot.
GT designed  cars that conformed to FIA  Group 4 rules. They all run on smaller wheels. in 1979, Porsche 934 Turbos were allowed to run in this class.
GTX : similar to Group 5 cars. Lightweight bodywork, no restriction on fender shapes, chassis reinforcement and big wings were allowed. Engine modifications were permitted as long as the original block was retained. Turbocharging was the way to go. Wheel widths were restricted to 16 inches. It was a cousin from the FIA Group 5 and Porsche 935 were the cars to beat, BMW 320 Turbos sometimes proved very fast, if not reliable. They did remain production based cars, but only had their appearance, as they were true racing cars.

Whittingtonporsche79GTX : Bill Whittington on his Porsche 935 at Mid Ohio 1979.

Copyright Mark Windecker

AAGT : cars were no longer limited to 350ci, they could go to 454ci and use aluminium blocks as well. Even turbocharging was permitted in AAGT. 6liter cars could use 20inch wheels and larger engines could use any width.
GTU rules did not change, it was more a displacement change with the 2,0Liter limit moving up to 2,5L, then 2,8L and finally 3,0L in 1985.
American Challenge cars were allowed to run with the big cars, mainly in 1978, but then they would run their own races and have their own championship. They allowed intermediate American Sedans, with a light range of modifications permitted.

1978buick_1American Challenge : Gene Felton Buick Skylark at Road Atlanta in 1978.

Photo courtesy Gene Felton

In 1981, the IMSA went quite another way, and introduced a brand new category, the GTP.

Fieldlola82Ted Field Lola T600 Chevrolet at Mid Ohio 1982

Copyright Mark Windecker

  • GTP: a non production based category intended to break the Porsche 935 Turbos domination.

A wide variety of engines would be available with different flavors, and chassis were quite liberal. John Bishop did finally create his own rules system with a weight to displacement grid. Starting from a 350ci Chevy, he devised a system that could fair to everybody. The bigger the engine was, the more weight it would carry. He proved quite good as this set of rules did match everyone's will.
The first true GTP was Brian Redman's Lola T600 which crushed the opposition in 1981. In 1982, some teams switched to GTPs and the GTX cars were allowed to be further developed. Tube framed GTX cars appeared and it was John Paul Jr who won the Championship on a set of Porsche 935s and a ... Lola T600. From that moment, the GTPs would be the cars that would rule the Championship.
In 1984, Porsche introduced its famous Porsche 962, a car that would dominate the series until 1988, with Al Holbert winning the title in 1985 thru 1987. Holbert Racing was nearly a factory team as they were very well supported and quite professional. Al Holbert would soon become the President, for a short while, of Porsche Motorsport North America.

A new era was to begin, that era was to last until 1993 and made IMSA one of the most prestigious road racing series in the World. In 1985, in order to allow small displacement GTP cars to enter in a more competitive way, the Camel Lights category was created and found a relative success for small teams willing to run GTP cars with smaller engines. These cars were to be powered by non turbo engines, and their displacement could not be in excess of 3,0Liter with a weight of 700 kilos.

These are the different IMSA rules as of 1985.

  • GTP
  • GTX
  • GTL
  • GTO
  • GTU

Fuel tank capacity : GTU : 110Liters Every other classes : 120Liters

TableauMaximum displacement in Cu Centimeters


Minimum weight in KG

Engine eligibility

  • Type 1 : Volume-produced 2-valve conventional engines and rotary engines
  • Type 2 : Racing engines and volume-produced 4-valve conventional engines
  • Type 3 : Surpercharged Type 1 engines(restricted to single turbo and single ignition)
  • Type 4 : Supercharged Type 2 engines

Loringdenali87Camel Light : David Loring Denali Mazda at Columbus 1987

Copyright Mark Windecker

After the Porsche era, it was the Nissan and Toyota domination. Those were factory efforts and racing costs rose up. Jaguar and Chevrolet tried to get their share but with no success. All Those cars were to be seen until 1993. Fields were getting thin and something had to be done.
After twenty years of involvement in IMSA racing, John Bishop felt it was time to take a little rest. He would finally sell his IMSA series to businessmen and the spirit would be definitively gone. The original moto "Racing with a difference" was gone.
A new concept was to be introduced : the World Sports Cars did appear in IMSA. They were the future of IMSA.


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