oval-like epitrochoid-shaped rotor housing that sealed the combustion chambers. A feature of the engine was its willingness to rev quickly and quietly to very high engine speeds, but it was precisely at these high speeds that damage to key engine components occurred: all Ro 80s came with a rev counter, but cars produced after 1971 also came with an "acoustical signal" that warned the driver when the engine was rotating too fast. The Ro 80 remained largely unchanged over its ten-year production. From September 1969 the rectangular headlights were replaced with twin halogen units, and air extractor vents appeared on the C-pillar behind the doors. In August 1970 a slightly reshaped plastic grill replaced the metal grill of the early cars, and a minimal facelift in May 1975 saw the final cars getting enlarged rear lights and rubber inserts in the bumpers which increased the car's overall length by 15 mm to 4,795 mm. The placement of the rear license plate was also changed from below the bumper to above it. This resulted in the boot lid lock being repositioned to the rear lip of the boot lid itself, instead of just below it. Unreliability The car developed an early reputation for unreliability. The Ro 80 engine in particular suffered from construction faults, among many other problems, and some early cars required a rebuilt engine before , with problems arising as early as . Originally, the rotor tip seals were made in three pieces, out of the same material. The motor's design caused the center section to wear more quickly at cold starts compared to the other pieces; the worn centerpieces allowed the two other parts of the seal to move, which in turn allowed combustion products to escape the seals. The tip seal centerpiece was then redesigned using ferrotic material, and the problem was entirely resolved. The fact that the rotary engine design had inherently poor fuel economy (typically 13-16 L/100 km) and a poor understanding of the Wankel engine by dealers and mechanics did not help this situation. By the 1970 model year, most of the reliability issues had been resolved, but a necessarily generous warranty policy and damage to the car's reputation had undermined NSU's financial situation irreparably. NSU was acquired by Volkswagen in 1969 and merged with Auto Union to create the modern-day Audi company. Production Series production began in October 1967 and the last examples