One of the first new championships to be launched was the Bridgestone-backed IMSA Supercar Series where Corvette's, Lotus Esprit's, Nissan 300 ZX's and Mazda RX-7's typically ruled the roost. For the inaugural season in 1991, Porsche North America joined forces with the experienced Brumos team from Jacksonville, Florida and entered two 3.3-litre 964 Turbo's. IMSA's rules dictated that cars could only be lightly modified at first, both Brumos entries being enhanced with shorter and stiffer springs and dampers that lowered ride-height by 50mm. A short-range competition fuel tank with a reduced capacity of just 60.5-litres was installed because most races were typically sprints of no more the 100km. The normal Typ M30/69 flat six engine was retained and produced 320bhp at 5750rpm. An otherwise standard Turbo bodyshell was seam-welded for extra stiffness and also had no underseal, the interior being completely stripped of all superfluous trim. Porsche were initially required to bring the standard 964 Turbo's weight up to 1620kg which probably meant fitting around 400kg of ballast, however, this was soon reduced to 1485kg. The 1991 IMSA Supercar Series produced some high quality racing, Porsche's Hurley Haywood eventually taking the title with wins in three out of seven rounds.

For 1992, Porsche and Brumos continued their successful partnership and went into the new season knowing IMSA's revised rules would allow a little extra development. Further modifications were made to the suspension with stiffer springs and dampers whilst brake cooling was improved thanks to new ducts in the front spoiler. Increased turbo pressure, re-mapped engine management and better cooling helped output up to 380bhp at 6000rpm, a sintered competition clutch being fitted over the previously used standard item. Weight was reduced to a fixed 1430kg. Haywood was once again the main driver with Hans Stuck and Walter Rohrl alternating in the second seat. Between them, they knocked up numerous victories but Porsche eventually lost out to the works Lotus Esprit X180R of Doc Bundy. For the 1993 season, power was reduced to 370bhp as demanded by the new regulations, however, loads more torque minimised the performance deficit. That year, Hans Stuck blew the rest of the field away, the experienced German driver taking victory in seven out of nine rounds and ending up with 50% more points than his closest rival.