In 2017, the Porsche Supercup celebrates its 25th anniversary. The one-make series has, alongside the various national Carrera Cup Championships, used its level playing field to allow the best up-and-coming racers to rise to the top. Two decades before the Supercup was even formed though, Porsche was involved in another one-make series, one with a unique aim: to find out, across the worlds of Formula 1, NASCAR, IndyCar and sports cars, who was the greatest driver of all. For North American readers, IROC will likely be more familiar in Chevrolet or Pontiac circles, while in Europe, the International Race of Champions enjoyed little publicity over its 30-year run.
However, before the succession of Camaros and Firebirds, the first series was actually run using 15 identical Porsche 911s: the IROC RSRs. Devised by legendary US team owner, Roger Penske, and the president of Riverside International Raceway, Les Richter, the IROC I championship would be contested over four races: three heats on the West Coast at Riverside and a grand finale at Daytona in the east four months later. A points structure would be applied at Riverside with only the six best racers making it to Florida. The whole thing would be televised on the popular Wide World Of Sports programme after TV marketer, Mike Phelps (working for the newly formed Penske Productions) struck a deal with broadcaster, ABC.
With a plan in place, Porsche agreed to build 15 identical Carrera RSRs for the inaugural series. Penske’s deal with Zuffenhausen came at a time when the RSR project was in a transitional phase, switching during the off-season from the 2.8-litre cars to the new 3.0-litre version scheduled for 1974. However, the 3.0 RS – the basis for the revered RSR – was not yet finished, so Porsche took 15 impact-bumpered Carrera 2.7 MFI chassis straight off the production line as the basis for the IROC 911 racers. Spec-wise, the IROC RSRs would be unique, blending key elements from the 2.8-litre RSR, upcoming 3.0-litre RS and the latter’s still-in development RSR brother.
The 2.7 Carrera shells (still being produced with the 2.7 RS’s tunnel reinforcement) were further beefed up with extra triangulation welded in around the rear shock towers and at the back of the engine bay; while the 2.8 RSR’s flared steel arches were fitted front and rear, along with glass fibre 3.0 RS bumpers. In order to reduce weight, the side windows and rear screen were replaced with Perspex, the doors were reconstructed in glass fibre and the pneumatic bonnet and decklid struts were removed (with both panels now using quick-release pin fasteners).
In the engine bay sat Porsche’s 911/74 fl at six, a 3.0-litre twin plug unit fitted with high butterfly throttles (rather than the slide mechanism that would later find its way onto the standard 3.0 RSR). Fed from a 110-litre plastic fuel tank in the front, the engine drove through the Type 915 gearbox (fitted with standard ratios) and a limited-slip differential, with 80 per cent locking on acceleration. On the suspension front, the Bilstein damper struts from the RSR were used alongside firmer torsion bars. Slowing everything down was the job of the four piston 917-style calipers and cross-drilled discs from the 3.0-litre RS, housed inside nine- and 11-inch wide Fuchs front and rear respectively.