of Ingolstadt, and by 1966 were using the spare capacity of the Ingolstadt plant to assemble an additional 60,000 Volkswagen Beetles per year. Two-stroke engines became less popular during the 1960s as customers were more attracted to the smoother four-stroke engines. In September 1965, the DKW F102 was fitted with a four-stroke engine and a facelift for the car's front and rear. Volkswagen dumped the DKW brand because of its associations with two-stroke technology, and having classified the model internally as the F103, sold it simply as the "Audi". Later developments of the model were named after their horsepower ratings and sold as the Audi 60, 75, 80, and Super 90, selling until 1972. Initially, Volkswagen was hostile to the idea of Auto Union as a standalone entity producing its own models having acquired the company merely to boost its own production capacity through the Ingolstadt assembly plant – to the point where Volkswagen executives ordered that the Auto Union name and flags bearing the four rings were removed from the factory buildings. Then VW chief Heinz Nordhoff explicitly forbade Auto Union from any further product development. Fearing that Volkswagen had no long-term ambition for the Audi brand, Auto Union engineers under the leadership of Ludwig Kraus developed the first Audi 100 in secret, without Nordhoff's knowledge. When presented with a finished prototype, Nordhoff was so impressed he authorised the car for production, which when launched in 1968, went on to be a huge success. With this, the resurrection of the Audi brand was now complete, this being followed by the first generation Audi 80 in 1972, which would in turn provide a template for VW's new front-wheel-drive water-cooled range which debuted from the mid-1970s onward. In 1969, Auto Union merged with NSU, based in Neckarsulm, near Stuttgart. In the 1950s, NSU had been the world's largest manufacturer of motorcycles, but had moved on to produce small cars like the NSU Prinz, the TT and TTS versions of which are still popular as vintage race cars. NSU then focused on new rotary engines based on the ideas of Felix Wankel. In 1967, the new NSU Ro 80 was a car well ahead of its time in technical details such as aerodynamics, light weight, and safety. However, teething problems with the rotary engines put an end to the independence of NSU. The Neckarsulm plant is now used to produce the larger