Porsche 956/962 Engines – Group C vs IMSA

In the early 1980’s there was a divergence in the rules for sports car racing world-wide. IMSA (International Motor Sports Association) in the USA proceeded with the GTP concept. The GTP (Grand Touring Prototype) was a prototype car using street car based engines. By 1982, the FIA was moving to a fuel based formula for sports cars. Porsche knew the 935 was no longer competitive and so they developed the 956 Group C car. It was highly successful, finishing 1-2-3 at its second race, the 1982 Le Mans. It was a ground effects prototype car with an engine type of 935/76. A flat six-cylinder, which was based on the earlier 936-engine (which in turn used the still-born Indianapolis race car engine). It featured a combination of water-cooled cylinder heads and air-cooled cylinders. The engine specifications were as follows:

Type 935/76 (1982)

The factory ran these cars in Rothmans colours in 1982 but they only sold customer versions of the 956 in 1983. IMSA refused to allow the 956 in the Camel GT series as it did not meet the rules in several areas: (1) It was deemed to be unsafe with the driver’s feet in front of the front axle. For safety reasons, IMSA GTP (Grand Touring Prototype) rules specified the driver’s feet had to be behind the front axle centre line; (2) The 956 also used a ‘non-production’ based engine, which was not IMSA legal; (3) The 956 had an aluminium roll cage which IMSA was not happy with, and in fact banned completely in 1983; and (4) The 956 only had a 100-litre fuel tank, whereas IMSA GTP regulations called for 120 litres.