Under Group 5 rules, also known as "silhouette rules", several significant modifications were allowed (including bodywork modifications, larger wings, wider axles and water cooling), provided that the basic silhouette of the car remain unchanged when viewed from the front. The 935's engine was a (at 7,900 rpm) version of the regular 3.0 L flat-six, with 60 mkp (438lb-ft, 588Nm) torque at 5,400 rpm. Boost was between 1.35 and 1.55 and fuel consumption was 52 litre per 100 km. Capacity was reduced to 2.85 L, and with the turbo charging penalty factor of 1.4, it fit into the 4.0 L class which had a prescribed minimum weight of only . Porsche, having a lot of experience with lightweight cars, had managed to get the Carrera RSR prototype to under 800 kg. The empty 935 tipped the scales at 900 kg, and weight distribution could be balanced with an additional 70 kg. Preseason testing at the fast Paul Ricard circuit showed a top speed of . In addition to the naturally aspirated Carrera RSR introduced in 1974, Porsche offered a customer racing version of the 911 Turbo prepared for the more standard Group 4 rules, the Porsche 934. Some customer teams modified their 934s to Group 5 rules with body kits supplied by Porsche, these were often designated 934/5. Cologne-based Kremer Racing entered a 935 K1 built on a factory shell which in the first race, the six-hour Mugello, finished second behind the Martini Racing sponsored factory entry of Mass and Ickx. Porsches occupied the first seven places ahead of a BMW in eighth, which according to the points scheme meant that Porsche had now twenty points, and BMW three. The factory 935 also scored the pole, fastest lap and win at the six-hour Vallelunga, where a BMW was second and the best 934 finished fifth. After the second race, the CSI rule makers insisted that the "whale tail" hood of the road-going 930 must fit on the race car. The air-to-air intercooler setup under the rear hood had to be altered to a more compact air-to-water layout, which cost Porsche several weeks of testing and half a million Deutsche marks. The hastily modified 935 again won the pole and fastest lap at the six-hour Silverstone, but due to a clutch problem at the start, the Martini car could finish only tenth, with the second-placed Kremer 935 collecting valuable points for Zuffenhausen.