It isn’t hard to visually tell the difference between the 934, 934 1/2 and the 935. While mechanically the 934 and 934 1/2 were vastly different (934 1/2 used parts from 935 except for its slantnose), the differences in weight changed the overall car’s performance enough to rewrite track use rules.
The Porsche 934 1/2 was a series of just 10 1977 Porsche race cars. Built specifically to contend with ever faster changing IMSA racing rules, both the 934 and 934 1/2 were being built and raced. With the overall evolution of spiraling attempts to produce quicker cars, there was this frenzy of interest. There were exactly ten cars originally sold and the private individuals that owned them wanted the fastest cars out there. Jim Busby (930 770 0951), Peter Gregg (930 770 0952), The Interscope Team (930 770 0953), George Dyer (930 770 0954), Vasak Polak for George Follmer to drive (930 770 0955), Ciro Nappi (930 770 0956* [known as factory extra car]), Bob Hagestad for Hurley Haywood to drive (930 770 0957), Ludwig Heimrath (930 770 0958) who was criticized for racing with 935 parts, Dick Barbour (930 770 0959), and lastly Clifford Kearns (930 770 0960).
The car came in one color, White and each were identical, as left from the factory. with widened fiberglass body kits and racing bred rule endorsed requirements. The turbo inter-cooler was probably the most significant part that gave these track warriors a sense of speed. At around 4 mpg, they unveiled themselves around tracks at head turning speeds.
Created in 1976 the FIA had developed six classes of prototype racers. Group 1 through 6, with Group 6 being a full bore example of modifiable speed. Group 4, has always been known to be Porsche’s dominant eligibility of a factory to create at least 400 road cars in a 2 years span. In this regulation, very few modifications compared to the road car were allowed. It was however allowed in that Group to increase safety like the installation of a fuel cell and a roll cages. To even out the competition between racers, displacement was tied to a minimum weight, from 495 kg with less than 500cc to 1270 kg for over 6000cc.
In just a few years prior to 1977, Group 4 allowed Porsche’s entry in GT-racing to be the 911 RS and RSR models. Powered by Naturally Aspirated engines of up to 3 litres were allowed on the track. However, these models were unlikely to be homologated, because of their limited production numbers, so a new car was needed. This “standardization” with racing vehicles, homologation, implied a series of sanctioned requirements. A matter of fact, when someone refers to G.T.O., it refers to Grand Touring Omologato, or for that matter “agreed requirements”. Therefore, meeting compliance to racing series technical guidelines refers to having to meet minimum levels of sales of the same model “production based” car to the public. The practicalities of having a race car on public roads might be allowed as long as allowed by the local motorist regulations. To put it bluntly, many of these race cars were allowed on public roads!
The 911 Turbo, or better known as the 930, was the new basis for Group 4 racing. The rule modifications allowed the 3.0L engine to grow to 4.0 and even 4.5 L. Stronger clutch and gearbox allowed these cars lots of room to build new bigger and faster engines. The 934 had to meet the 1120 kg weight, in which then, all Porsche did was offer bigger turbos from 480 bhp to 550 bhp.
It was then in 1975 the new 930 (911 Turbo) model was launched and this would form the base for the new Group 4 racer, aptly named 934. The displacement of Turbocharged engines are multiplied by 1.4 to get the normally aspirated equivalent displacement, this mean that the 3 litre Turbo engine placed the new Group 4 car in the 4000 – 4500 cc class. When Porsche designed the 930, the competition version was already anticipated and many components like the clutch and gearbox were made much stronger than needed for the 260 bhp 930, an ingenious way to evade the non-modification rule.
To comply with the regulations, the 934 would have to weigh at least 1120kg. This was easily achieved by removing most of the interior and weight was brought down to 1090kg even with the electric windows still in place. In race trim it was balasted to 1120 kg. Major modifications to the engine were not allowed, but due to the nature of Turbocharged engines, a higher output is fairly easily achieved. In Group 4 trim the 3 litre engine was good for a tarmac blistering 480 bhp and equipped with larger valves in 1977 over 550 bhp was reached.
In competition, the 934 1/2 allowed even more trimming the engine to its max allowing the 3.3L engine, a larger turbo, and car to closer meet the Group 4 regulations maxing it out power, minimizing all weight. It wasn’t until the 934 1/2 started seeing parts from the 935 did anyone started crying foul.