One of the highlights of IMSA GT racing was the GTP (Grand Touring Prototype) Era from 1981 to 1993.
The class was created in 1981 when IMSA GT co-founder John Bishop decided to not follow FIA's newly introduced Group C rules and instead, introduced the GTP class for sports prototypes. GTP cars were similar to the new FIA Group C cars with the main difference was that prototype cars had an emphasis on fuel consumption. GTP was a class for roofed prototypes with certain dimensional restrictions, but instead of the more usual limits on engine capacity, it placed limits on fuel consumption. The class is highlighted by Derek Bell saying "Race fans do not come to races to watch an economy run."
Brian Redman was the first GTP champion, driving a Lola T600 with a Chevrolet engine. Al Holbert won the 1983 championship with a Chevrolet powered car, changing to Porsche power later in the season. Randy Lanier a year later won with Chevrolet power.
1984 also saw the introduction of the Porsche 962, which dominated the series from '85 to '87. Nissan then took control of the series in 1988, but faced challenges from Jaguar, Porsche, and Toyota throughout the next three years. Toyota was quickest in 1992 and 1993, at the end of the GTP era, as Dan Gurney's All American Racers team campaigned the Eagle Mk III, a car so dominant that it has been blamed for the demise of the class. There were many other manufacturers in the GTP class, such as URD Rennsport, Spice, Intrepid or Gebhardt, and in the early 1990s, Mazda.
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 race team hospitality, practices which were subsequently adopted by virtually every other team. For those that competed, GTP was recognized for its camaraderie among drivers, especially rivals. But 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.
Following a successful heart surgery in 1987, Bishop began to rethink his priorities. He was approached by Mike Cone and Jeff Parker, owners of Tampa Race Circuit. In January 1989, Bishop and France sold the series to Cone and Parker. The new owners relocated the IMSA headquarters from Connecticut to Tampa Bay. Bishop would stand down as president in favor of Mark Raffauf, who was his deputy, and its representative on the ACCUS board. Cone and Parker sold it to businessman Charles Slater.
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 to its 962. Back in 1988, Al Holbert realized that the 962 was beginning to feel dated. He proposed a followup open-top Porsche powered racer which would also be sold to customer teams. That project never got off the ground due to Holbert's death in an aircraft accident later in the year.
For some, much of the blame was on the organization for allowing the Japanese teams to dominate the series. Under Bishop's original vision, privateers and "works teams" were able to race equally. Privateer teams walked away, while the Japanese economy started to go downhill. These factors led Nissan and Mazda to leave the series. Critics predicted that the decreased variety of cars would disappoint race fans, and in fact, it did finally kill the series in 1993.
The sports car category was introduced in 1993 to replace the GTP category in 1994. After a period of multiple ownerships, the organization was eventually renamed Professional Sports Car Racing (PSCR). In 1999, PSCR decided to drop their own championship series in order to sanction a new series: the American Le Mans Series. Despite having various official names, the GT series was known commonly as the "IMSA series," as it had been the organization's dominant series.