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Al Holbert won a lot of races driving this Porsche 962.

The GTP class produced some of the fastest racers in the world. The Lola T600 Chevrolet dominated the 1981 season, then you could find March 82Gs, March 83Gs, March 84Gs. Porsche 962s, Chevrolet Corvettes GTP, BMW GTPs, Nissan ZX Tc GTPs, Eagle Toyota HF89s and MKIIIs.

IMSA has developed rules for cars known as GTP(Grand Touring Prototype) cars.

GTP were two-seaters conceived primarily for competition in closed circuit races. They carried equipment for normal road use and all safety devices. They did need not meet production requirements nor be offered to the public. Car identification was by engine manufacturer first, then the manufacturer of the chassis.

The eligible engines were :
  • Type 1 : 2-valve conventional engine
  • Type 2 : 4-valve conventional production engine up to 6,0L
  • Type 3 : Type 1 engine with a single turbocharger with a 54mm restrictor or twin turbochargers equipped with 38mm restrictors. Max displacement 3,0L.
  • Type 4 : Type 2 engines with a single turbocharger equipped with a 54mm or twin turbochargers equipped with 38mm restrictors. Max displacement 3,0L.
  • Type 5 : rotary engines
Engine modifications were free, except as follows :
  • Type 2 production engines and all Type 1 engines must use the production block or an IMSA approved alternate block.
  • Type 1 engines may use modified or alternate cylinder heads that retain the original number and location of valves, camshafts and spark plugs.
  • Type 2 production engines must use the production cylinder heads that retain the original number and location of valves, camshafts and spark plugs. The heads may be modified but material may not be added.
  • Oil tank was limited to 20 liters capacity.
Chassis :

Wheelbase was free

Overall length : 480cm

Overall width : 200cm

Overall height : min : 100cm max : 110cm

Two seats of similar construction and of equal dimension must be mounted in the cockpit

Air intakes may protrude from the plane of the bodywork provided they did not impair the driver's field of vision.

Coachwork:

All coachwork had to be mounted to the fully sprung part of the chassis.

The coachwork sections had to be contiguous and cover all mechanical components when viewed from above.

Wheels and Tires:

Wheel and tire section width may not exceed 16"(1/2" tolerance").

Track dimension is limited by inner tire clearance and the permitted maximum car width.

Aerodynamic devices:

They were governed by the rules of overall length, width and overhangs of an individual anutomobile.

The rear wing or aerodynamic device may be no higher than 110cm above the ground measured with fuel tanks and driver aboard.