The 934/5 – Porsche’s answer to IMSA in 1977

Sports car racing changed drastically in the early- to mid-1970s. The rules for the World Championship shifted away from the 5-litre cars such as the 917 and Ferrari 512, to the 3-litre versions such as the Ferrari 312PB, Porsche 908 and Alfa Romeo T33 in 1972. While most Porsche customers ran the RSR in GT racing, a few also ran 908s, but Porsche knew the 908 was not competitive for overall victory anymore. It was under powered and with the raising of the minimum weight, it basically lost any advantage it had against the Ferrari and Alfa.

Porsche started working on the adaptation of their turbo technology (taken from the 917 Can-Am cars) to their 911-based production cars, which saw the introduction of the Martini RSR turbo cars. These ran effectively in 1974 and 1975 as prototype class cars, but still were not quick enough to win overall against the 3-litre cars. Porsche knew that the World Manufacturers Championship in 1976 would switch to production based cars in Groups 4 and 5. They therefore spent 1974 and 1975 developing the Carrera RSR turbo to prepare for the introduction of the 934 and 935 in 1976. Group 4 required a minimum 400 cars to be built, but Group 5 had no minimum number, and allowed more modifications to the production base model.

For the 1976 season this resulted in the introduction of the 934 and the 935. Both these cars were derivatives of the street 930 turbo, where the 930 Group 4 became the 934, and the 930 Group 5 became the 935. The factory ran the 935s in their official works Martini team, and sold customer 934s to teams such as Kremer, Loos and Max Moritz.

IMSA (International Motorsports Association) had been started by John Bishop in the USA in 1969. By 1972 he was sanctioning sports car races, and had secured sponsorship from Camel cigarettes for the series. Early races in 1973-1975 were dominated by the Porsche Carrera RSR. More powerful American cars such as the Corvette and Camaro also ran in the series, and while sometimes faster than the RSR, never had the reliability. For the 1976 season, IMSA did not allow any turbocharged cars in its series. Porsche customers continued with the RSR, but were outclassed by both Al Holbert in a Chevrolet powered Monza, and occasionally by Peter Gregg who had switched to BMW CSLs for the 1976 season. Al Holbert won the championship that year in the Monza. In 1976, Porsche’s customers in Europe had started to modify the 934s to be more competitive, and to run under Group 5 regulations. Several American customers bought 934s and ran them in the Trans-Am Championship, notably Al Holbert and Vasek Polak, but it was George Follmer who won the Trans-Am Championship in Vasek Polak’s 934.

For the 1977 season, Porsche lobbied heavily for the 935 to be allowed into IMSA. Porsche knew they would sell customer single turbo 935-77s for the 1977 season, while they would run twin turbo cars in their Martini team. The US market was large for Porsche, and they wanted to showcase their technology in IMSA which was the better known of the American series. However, in IMSA, John Bishop would not allow the 935-77 to run. He was afraid it would dominate the series (probably rightfully so). And so, after many negotiations between Jo Hoppen (Porsche’s US racing director) and IMSA, the 934½ was born. [NOTE: the car was referred to as the 934 1/2 in the US while the Porsche factory used 934/5 – both of these monikers are correct].

The 934½ was basically a hybrid model that was a 934 with some permitted modifications, as specified by IMSA. It was made for IMSA only, and the major changes from the 934 included: wider wheels, wider fender flares, a 935 rear wing and Bosch mechanical injection instead of the K-Jetronic system on the 934 (see table below). European teams did not want them, as they were not competitive against a 935-77. In the end, one was sold to a small Italian team to run in Italian races. These cars (the official 934½ from Porsche) would not be available until the Sebring race in 1977. Ted Field (Interscope) wanted a car for the Daytona 24-hour race, so had Porsche build him one on a 1976 934 chassis in late 1976 (a 934 with the allowed modifications). A few others, like Gary Belcher bought used 934s and modified them.

The original 1977 934½ chassis numbers (from Porsche), with original buyers, were as follows:

The basic differences between the 934, 934½, and 935-77 are listed here:

The cars were reasonably successful, but were hampered by late arrival in the USA, as the first ones did not arrive until Sebring. They qualified 1-2 but had issues in the race. The Gregg/Busby car (chassis #930 770 0951) was leading late in the race when a wheel fell off causing damage. After repairs they could only get back as high as third, being beaten by the two RSRs of George Dyer and Diego Febles. By Atlanta more cars were available, however, Peter Gregg’s car was sent home from the Atlanta race as he was deemed to have made ‘too many unauthorised modifications’ to the car. The rest of the IMSA runners had mixed results because for the teams who were used to the RSR, these cars were hard to drive due to the turbo lag. They were also much heavier than the RSR, but of course had a lot more horsepower, but they also tended to use up tyres rapidly. Hurley Haywood, George Dyer and Danny Ongais won races during the year, however, none of them could maintain a consistency of finishes to stop Al Holbert from winning the IMSA title for the second year running in a Chevrolet Monza.

After being sent home from the Atlanta IMSA race, Peter Gregg did not come back. He took his car and ran in the SCCA Trans-Am series which allowed the 934½ as well. There, over the course of the season, he won the championship on points from Ludwig Heimrath. However, Heimrath protested at the Mosport event late in the season, and after the season, the FIA ruled Gregg’s car had been illegal at Mosport due to some bumper brackets or some such part. The FIA declared then that Heimrath had won the title due to the points change from that race. Porsche however, did not hold with this, and printed the SCCA Trans-Am winning poster with Peter Gregg’s car on it.

By the end of the 1977 IMSA season, these cars were obsolete, as IMSA had decided to allow the 935 customer cars to run in 1978. And so, the owners of the 934½ started the process of ‘upgrading’ these cars to 935 specifications but this required a fair amount of money, as new bodywork, engines and wheels were required. Subsequently, several of these ran for many years in IMSA in 935 specifications.

Written by: Martin Raffauf
Images by: (as credited)