IMSA GT

The 1971 season lasted six races. The series was originally aimed at two FIA stock car categories, running GT (Groups 3 and 4) and touring (Groups 1 and 2) classes divided into four groups:

GTO — Grand touring cars with engines of 2.5L or more. Initially dominated by Corvettes, then Shelby Mustangs, later by Cougars, 280ZXs, Celicas, and finally 300ZXs.

GTU — Grand touring cars with engines of 2.5L or less. Dominated by Porsche 914-6 GTs and Mazda RX-7s (1978–1985).

TO — Touring cars with engines of 2.5L or more.

TU — Touring cars with engines of 2.5L or less.

These groups were absorbed from the Trans-Am Series, which quickly became a support series for IMSA GT. Camel became the title sponsor during the second season; the corporate Joe Camel decal was required on all racecars and driver suits.

The first champions were Peter Gregg and Hurley Haywood, sharing a Porsche 914-6 GTU. Common winners in the early years included the Porsche 911 Carrera RSR and the Chevrolet Corvette. Gregg dominated the 1970s, winning championships in 1971, 1973, 1974, 1975, 1978, and 1979.

In 1975, John Bishop introduced the All American Grand Touring (AAGT) category to counteract Porsche dominance in GTO. TU was phased out in 1976 and TO the following year. Turbochargers were not permitted until mid-1977, allowed following protests by Porsche's motorsport department after inspecting Al Holbert's AAGT-winning Chevrolet Monza, which had won two titles. In 1977, a new premier class known as GTX (based on FIA Group 5) was introduced, bringing the absolute dominance of the Porsche 935, the most successful car in the series' history. Twin turbos were outlawed at the end of 1982 after John Paul Sr. and John Paul Jr. dominated in a modified 935.

In 1981, the Bob Sharp Racing team used a loophole to build a Datsun 280ZX with a V8 engine from a Nissan President; the car was unsuccessful and became obsolete when the GTP category was created. From 1984, all GT cars were required to display a large square decal identifying their category. The 2.5-litre displacement limit was raised to 3.0 litres; two-valve turbocharged cars were required to weigh 15% more and four-valve turbocharged cars 20% more.

The AAR Toyota team entered the top-flight GTO category with the new fourth-generation Celica, powered by the 4T-GTE engine producing around 475 hp (354 kW) — drawn from the car's Safari Rally-winning Group B predecessor. To race with a rear-wheel-drive conversion, the team persuaded IMSA to change the rules permitting cars to race with other than their original drivetrain. Drivers including Chris Cord, Willy T. Ribbs, and Dennis Aase piloted the car, which was dominant in its class before the team moved to GTP. The Mazda RX-7 won its class at the IMSA 24 Hours of Daytona ten years in a row starting in 1982, and won eight IMSA GTU championships in a row from 1980 through 1987, recording its one-hundredth class victory on September 2, 1990.

During the 1989 season, the Audi 80 with its Quattro four-wheel-drive system competed, but lost the manufacturers' and drivers' titles — the latter with Hans-Joachim Stuck driving — after two race retirements. The Roush Racing Mercury Cougar XR7 and Clayton Cunningham Racing's Nissan 300ZX were its main rivals, the Nissan taking seven wins from fifteen that season.

In 1981, purpose-built GTP (Grand Touring Prototype) cars appeared, similar to the FIA Group C cars introduced to the World Endurance Championship from 1982. The key difference was that GTP had no fuel-consumption emphasis. Brian Redman became the first GTP champion, driving a Lola T600 with a Chevrolet engine. March also fielded prototypes; Al Holbert won the 1983 championship in a Chevrolet-powered car before switching to Porsche power. Randy Lanier won in 1984 with Chevrolet power.

The Porsche 962 dominated from 1985 to 1987. Nissan took control in 1988 but faced challenges from Jaguar, Porsche, and Toyota throughout the following three years. Toyota and Dan Gurney's All American Racers were quickest in 1992 and 1993 at the end of the GTP era, campaigning the Eagle MkIII — a car so dominant it was blamed for the demise of the class. The Camel Lights category, a lower-powered prototype class introduced in 1985, was won first by Argo Racing Cars and then by Spice Engineering.

The GTP category was credited with innovations including antilock brakes, traction control, and active suspension. The GTP cars ran their last race on October 2, 1993, at Phoenix.

Following successful heart surgery in 1987, John Bishop began to rethink his priorities. In January 1989, Bishop and France sold the series to Mike Cone and Jeff Parker, owners of Tampa Race Circuit. The new owners relocated IMSA headquarters from Connecticut to Tampa Bay and installed Mark Raffauf as president. Cone and Parker subsequently sold to businessman Charles Slater; both lost millions attempting to revive sagging TV ratings.

By 1992, multiple factors contributed to the GTP category's decline: Porsche shifted focus to its IndyCar program (Porsche 2708) instead of building a 962 successor — a follow-up car had been proposed by Al Holbert before his death in an aircraft accident in 1988. Japanese factory teams withdrew as Japan's economy deteriorated, and diminishing variety in the field drove away race fans.

In 1993, IMSA introduced the World Sport Car (WSC) category to replace GTP and closed-top Camel Lights cars. WSC cars were open-top, flat-bottomed sports prototypes with production engines. They debuted at the Miami Grand Prix with a single entry and struggled with reliability. At the 1994 24 Hours of Daytona, eight WSC cars competed; two finished, with the lead WSC car finishing ninth behind GT cars. The Ferrari 333 SP debuted at Road Atlanta and won on its first appearance; Oldsmobile won the manufacturers' title over Ferrari by four points that year.

In 1995, the Riley & Scott Mk III emerged as a new rival to the Ferrari 333 SP. Ferrari helped the category score an overall win at the 12 Hours of Sebring and took the manufacturers' title. The Ferrari 333 SP and Riley & Scott (running Oldsmobile and Ford engines) dominated from 1995 through the end of IMSA in 1998.

In 1996, Slater sold the organization to Roberto Müller and Wall Street financier Andy Evans, also an IndyCar owner and owner-driver of the Scandia WSC team. Evans and VP of marketing Kurtis Eide renamed the organization Professional Sports Car Racing (PSCR). In 2001, Don Panoz purchased PSCR to consolidate the sanction for his American Le Mans Series, renaming the sanctioning body IMSA.

A breakaway series forming in 1998, the United States Road Racing Championship, failed by 1999. Grand-Am Road Racing, supported by NASCAR's France family, then launched with the Daytona Prototype class and eventually merged with ALMS in 2014 under IMSA sanction to create the IMSA SportsCar Championship. The WeatherTech Championship is considered the official continuation of the original IMSA GT Championship.

This article is based solely on the supplied corpus. No external sources were consulted; claims that could not be substantiated against the corpus were omitted under the drop-the-claim rule.