Powerful prototypes (effectively pure-bred two-seater racing cars with no real link to production vehicles) started to appear as the 1960s progressed, with worldwide battles between Ferrari, Ford, Porsche, Lotus, Alfa Romeo and Matra as well as other more specialist marques running on into the early 1970s. The competition at Le Mans even made it to the movie screens, with Steve McQueen's film Le Mans. This era was seen by many as the highpoint of sports car racing, with the technology and performance of the cars comfortably in excess of what was seen in Formula 1. Homologation saw many out-and-out racing cars produced in sufficient quantities to see them classed as production vehicles; the FIA responded by placing more restrictions on even the allegedly production-based cars and placed draconian limits on the power available to prototypes – these prototypes of the late 1960s/early 1970s were comfortably quicker than contemporary Grand Prix machinery and for 1972 they were constrained to run much smaller engines to F1 rules, often de-tuned for endurance. Group 4 Grand Touring Cars and Group 5 Special Production Cars became the premier form of "sports car" racing from 1976, with prototypes going into a general decline apart from Porsche 936 domination at Le Mans and a lower-key series of races for smaller two-litre Group 6 prototypes.