Porsche started racing with lightweight, tuned derivatives of the 356 road car, but rapidly moved on to campaigning dedicated racing cars, with the 550, 718, RS, and RSK models being the backbone of the company's racing programme through to the mid-1960s. The 90x series of cars in the 60s saw Porsche start to expand from class winners that stood a chance of overall wins in tougher races where endurance and handling mattered, to likely overall victors. Engines did not surpass the two litres mark until the rule makers limited the capacity of the prototype class to 3 litres after 1967, as the four-litre Ferrari P series and the seven-litre Ford GT40 became too fast. Porsche first expanded its 8-cyl flat engine to 2.2 litres in the 907, then developed the 908 with full three litres in 1968. Based on this 8-cyl flat engine and a loophole in the rules, the 4.5-litre flat 12 917 was introduced in 1969, eventually expanded to five litres, and later even to 5.4 and turbocharged. Within few years, Porsche with the 917 had grown from underdog to the supplier of the fastest (380 km/h at Le Mans) and most powerful (1580 hp in CanAm) race car in the world.