For the following year, Bishop brought in sponsor R. J. Reynolds and in 1975, introduced a new category called All American Grand Touring (AAGT). In 1977, the series would go through a series of major changes. IMSA permitted turbocharged cars for the first time as well as introducing a new category called GTX for cars based on the Group 5 rules. In 1981, after Bishop decided to not follow FIA's newly introduced Group C rules, so he introduced the GTP class for sports prototypes. In 1989, Bishop sold off his organization. After a period of decline in the early 1990s, the Worlds Sports car category was introduced in 1993 to replace the GTP category in 1994.
After a period of multiple ownerships, the organization was eventualy renamed Professional Sports Car Racing (PSCR). In 1999, PSCR decided to drop their own championship in order to sanction a new American Le Mans Series. Despite various names, the GT series was known commonly as the IMSA series as it had been the organization's dominant series.
Initially, cars were marked visibly with its category tag, stating which category they belong, but onward from the middle of the 1975 season, all cars within the series had to bear a rectangular IMSA GT decal, which incorporated its logo on the left followed by a large GT tag. as well as Joe Camel decal.
Starting fields of 30 or more competitors were not unusual during this era. One of the premiere race events was the "Paul Revere 250" which started at the stroke of midnight of the 4th of July. The race was conducted entirely during the night from start to finish.
In 1974 a new category called All American Grand Touring (AAGT) was introduced to counteract the Porsche dominance in GTO.
This category did not run without controversy. In 1981, Bob Sharp Racing team used a loophole in the rules to build a Datsun 280ZX inside the U.S. with a V8 engine from a Nissan President. This car was not a success and became obsolete when the new GTP category was created. The TU would be phased out in 1976 along with the TO for the following year.
Turbochargers were not permitted until the mid-1977 season. They became permitted after protests by Porsche's motorsport department after inspecting Al Holbert's AAGT winning Chevrolet Monza, which had won two titles. Prior to 1977, Porsche privateers struggled with obsolete 911 Carrera RSRs against the AAGT cars.
Engine sizes were determined by IMSA officials, who had devised a set of rules to determine fair competition, using a displacement versus minimum weight formula. Turbochargers were taken into account as well as rotary power, fuel injection, and many other engine features.
As a result, the new premier class known as GTX (Grand Touring Experimental, which was based on FIA's Group 5), brought on the absolute dominance of the Porsche 935. The 935 became the most successful car in the series. The most successful driver of the seventies was Peter Gregg, who won championships in 1971, 1973, 1974, 1975, 1978, and 1979. Twin turbos were eliminated at the end of the 1982 season after John Paul and John Paul, Jr. dominated in a modified 935.
One big change to the rules during the 1980s was the 2.5 liter ceiling having been changed to 3.0 liters, with the maximum of 6.0 liter ceiling still in place. As for the complex set of series rules, 3.0L cars was required to weigh 1900 lb. whereas 6.0L weighed no less than 2700 lb. 2 valve turbocharged cars were given an extra 15% weight and 4 valve turbocharged, 20% extra. Electrical fuel injection was to become common and ground effects were prohibited.
Steering, braking, transmission and suspension could be left up to the constructor. Bigger and more powerful engines were permitted under homologation rules. Other items that had to be left as unchanged were the number of valves, ports and spark plugs of the original configuration.
Also within the category, one of the biggest changes as one team discovered a loophole was the AAR Toyota team when, with its introduction of the new fourth generation Celica for the US market and the teams first entry into the top flight GTO category, despite not having to win one single GTU title, the team was to face a possibility that they would end up racing a front wheel drive car until they managed to persuade IMSA to rework the rules, determining that a car does not have to race in its original drivetrain, therefore with a redesigned chassis, the car was converted to rear wheel drive, . Another advanced features is the 4T-GT engine, from its Safari Rally winning Group B predecessor producing of around 475hp. and its features mentioned above. Piloted by the likes of Chris Cord, Willy T. Ribbs and Dennis Aase, the car was proven to be dominant within its class until the teams defection to GTP, utilizing the same engine, becoming dominant once again. Other teams would follow this example, notably Chevrolet Beretta (in the Trans-Am series) and Mazda MX-6 (in IMSA GTU).
The Celica was one of the few example of cars that had broken away from its production GT derivatives of the earlier years as with full spaceframe chassis, they became serious race cars. By 1987, the category became dominated by factory teams, with testing sessions becoming common and rules tailored to welcome them in, rather than turn them away, other than that, cars were required to resemble their showroom counterpart with oversized fenders as they were not permitted to be over the width of 79 inches.
There were no restrictions to body materials as most teams favored removable fiberglass and one of the only that remains of the production counterpart is the sheet steel roof.
Another car that exploited the rule system was Audi's 90, with its highly advanced four wheel drive system, the car had a potential to become a car to beat during the 1989 season, although the car was proven to be dominant, despite heavy competitions from two factory teams, Roush Racing Mercury Cougar XR7 and Clayton Cunningham Racing Nissan 300ZX, taking seven wins out of fifteen, as Audi stayed away from the early season endurance classics (Daytona and Sebring) as well as having two cars out of the race in two different rounds, therefore costing them the title for both makes and driver (Hans-Joachim Stuck).
Another manufacturer to experience a run of wins was Mazda. After some success by the Mazda RX-2 and Mazda RX-3, the Mazda RX-7 won its class in the IMSA 24 Hours of Daytona race ten years in a row, starting in 1982, and won the IMSA GTU championship each year from 1980 through 1987. The car went on to win more IMSA races in its class than any other model of automobile, with its one hundredth victory on September 2 1990.
In 1981, purpose-built GTP prototypes, similar to the new FIA Group C, were introduced in the World Endurance Championship. The main difference between the two categories was the former had no emphasis on fuel consumption which was highlighted by Derek Bell quoting "race fans do not come to races to watch an economy run!". Brian Redman was the first champion of the GTPs, driving a Lola T600. March also fielded prototypes, in which Al Holbert won the 1983 championship, and Randy Lanier a year later. 1984 also saw the introduction of the Porsche 962, which dominated the series from 1985 to 1987. Nissan then took control of the series in 1988, but faced challenges from Jaguar, Porsche, and Toyota throughout the proceeding three years. Toyota was quickest in 1992 and 1993 at the end of GTP era. Along with the GTP cars, the Camel Lights lightweight, an under three liter low horsepower prototype category was introduced in 1985. Argo were the first Lights champions, followed by Spice. Well known were also Tiga, Royale, Alba, Fabcar, and Kudzu.
Starting from the 1986 season, the GTP category had their own decal, which similar to the IMSA GT side decal, an extra P was added to denote their category Racing Sports Cars: Daytona 24 Hours 1986, Camel Lights cars also bore the same decal
By 1992, there were a number of factors that led to the decline of the GTP category. Porsche concentrated on its IndyCar program when critics stated that the Zuffenhausen marque should have built a followup of its 962. Back in 1988, Al Holbert realized that the 962 was beginning to feel dated. He proposed a follow up open top Porsche powered racer which would also be sold to customer teams. That project never got off the ground after Holbert's death in an aircraft accident later in the year. For some, much of the blame was on organization to allow the Japanese Works teams to dominate the series. Under Bishop's original vision, privateers and Works were able to race equally. Privateer teams to walked away, and the Japanese economy started to go downhill. These factors led Nissan and Mazda to walk away from the series. Critics say that the diminished variety of cars that would disappoint race fans which would finally kill the series off in 1993. GTP cars ran their last race on October 2, 1993 at Phoenix International Raceway.
The GTP category was credited for many innovations in the U.S. including antilock brakes, traction control, and active suspension. Dave Cowart and Kemper Miller's Red Lobster sponsored team of the early 1980s would innovate racing team hospitalities which became adopted by virtually every other teams in the future. . But for those who competed, it was credited for its camaraderie within drivers, especially rivals. Hans Stuck, commenting in the foreword of the book Prototypes: The History of the IMSA GTP Series, sarcastically compared the series' camaraderie to Formula One's lack of such.
With rising costs and factory teams walking away from the series which meant diminishing entries, IMSA introduced a new prototype category for in 1993 called World Sport Car (WSC). The WSC replaced the GTP and Lights closed-top cars for the following year. The WSC cars were open-top, flat-bottomed sports-prototypes with production engine as opposed to racing versions of production engines from GTP cars.
The WSC cars made their debut at the Miami Grand Prix with a sole entry of Brent O'Neill. The car finished last among the cars that were still running. After skipping the 12 Hours of Sebring, the category would compete for the remainder of the season as non-championship rounds, with no more than four cars entering. In 1994 Camel would be replaced by Exxon as the title sponsor. However, as the WSC cars took over as the leading category, their reliability would be tested at the opening round at the 24 Hours of Daytona. Two cars started on the front row, with eight WSC cars competing. Two cars finished the race, with the leading WSC car finishing ninth behind GT cars. The WSC cars would score its first podium finish at Sebring with a second and third place behind a Daytona winning GTS category Nissan 300ZX. That led to a rule change for the latter category as they would be barred from using engines that were originally for GTP cars. At the inaugural round for WSC cars at Road Atlanta, the new Ferrari 333 SP would make its debut in a mass media fanfare and win its debut race. The car regularly appearing on the podium on every rounds after that. Oldsmobile won the manufacturer's title over Ferrari by four points.
In 1995, a new rival for Ferrari appeared in the Riley & Scott Mk III. The car would make its debut at Daytona, but would retire after the eleventh lap after an engine failure. Ferrari would help the category to score an overall win at the 12 Hours of Sebring, and would take the title for both makes and driver. The Ferrari and the R&S cars were the dominant racers of the series from 1995 to the demise of IMSA at the end of 1998.
In 1996 Slater sold the organization to Roberto Muller (ex-CEO of Reebok) and Wall Street financier Andy Evans, who also was an IndyCar owner and owner/driver of the Scandia WSC team. Evans and VP of Marketing Kurtis Eide were responsible for the name change to Professional Sports Car Racing (PSCR).
In 1992, the long running category American Challenge would step into the GT series. It became known as the GTO category when the former GTO category was renamed to GTS (Grand Touring Supreme). The move was prompted by sponsor Exxon, who wanted the series named after its subbrand of fuel. In 1995, in a bid to move close to the European BPR Global GT Series, the GT category would undergo another major reformat. GTS became known as GTS-1, and GTU became known as GTS-2. In 1997, there was another category addition. GTS-2 became GTS-3, new GTS-2 category was announced to allow for the existing GT2 cars.
A breakaway series formed in 1998 involving the Sports Car Club of America and running under the name of the United States Road Racing Championship. It was headed by a group of competitors wanting to keep rules within the United States. After failing by 1999 the series was taken over by the Grand American Road Racing Association with the full support of NASCAR's France family and renamed the Grand American Road Racing Championship, later renamed the Rolex Sports Car Series. The series struggled early on, but has proven to be a formidable competitor to the ALMS in recent years with big name drivers, considerably larger fields, and much closer competition. Much like the split between Champ Car and the IRL critics say this split has been detrimental to the sport as a whole. Attendance, sponsorships and media coverage have dropped dramatically since the split in 1998.
After the series demise, a US based historical racing organisation, Historic Sportscar Racing (HSR) created a new series to put GTP and Group C cars that had been stored away to be put back onto the track, the series was called HSR ThunderSport, this would spark another similar revival series in Europe which then another UK based series would be formed called Group C/GTP Racing, of this date both series are still running as well as appearing in historical events.