Of the 1,580 original RS's built, only 55 became the RSR 2.8 race cars. For racing, the Porsche factory introduced code M491, the Renn Sport Rennen (RSR).

Porsche took 55 RSH body shells off the line before the engines and transmissions had been ﬁtted and moved them down to a separate factory called Werk 1. This was Porsche’s racing shop and they chose this line to ensure an extremely high standard of preparation for their racing clients.

The 2.8 RSR looks different from a standard 2.7 RS because of its massive fender flares, central oil-cooler air intake, 'duck-tail' wing and ultra-wide Fuchs wheels. While the wider wheels and arches combined with the low air dam to give the RSR a much more aggressive look, the conversion process was much more than just a bunch of tweaks, it was a serious engineering exercise.

The car was wider by two inches and the arches were ﬂared to the maximum permitted by the sporting rules, all to allow big 11 inch rear and 9 inch front wheels. The brakes were based on 917 components, with huge axially and radially drilled discs and ﬁnned four-piston calipers. Porsche engineers also developed revised suspension settings and added more adjustability to help with race setup. They lowered the car and all the rubber was taken out of the suspension joints.

Weight-saving was a key focus and Porsche was able to cut 176 lbs from the already light standard RS Lightweight. That was harder than it sounds because Porsche also strengthened the RSR’s chassis in three key areas at the rear of the car to cope with the increased demands and speeds placed upon it. Inside, weight was taken away from everything. The interior had a bare metal ﬂoor, minor bits of felt trim and a rubber mat, framed by a lightened roll cage. About the only notable things inside were a leather cord to open the door and a 10,000 rpm rev counter.

'When it was done, the RSR stood as the lightest, fastest, most powerful and most agile 911 ever to enter GT racing and the model immediately proved its worth in racing. The car won the ﬁrst round of the 1973 Work Championship for Makes. It then followed up by capturing wins at the 24 Hours of Daytona, 12 Hours of Sebring, Targa Florio, and six of nine rounds in the 1973 European GT Championship.

Many 2.8 RSRs, were later uprated with RSR 3.0 liter engines, wich is also the case in this mod.