In 1973 Benelli was acquired by Argentinian industrialist Alejandro de Tomaso together with its competitor Moto Guzzi which led to the development of new models featuring multi-cylinder engines, such as the "350 GTS" and the "500 Quattro", and the Sei powered by a 747cc (later 906cc) six-cylinder in-line engine, which put the firm briefly ahead of the Japanese competition. Although technically advanced, Benelli motorcycles of the 1980s were plagued by problems, and the production was finally stopped in 1988 when the company was merged into Moto Guzzi to create "Guzzi Benelli Moto S.p.A.", and the production plants in Pesaro were sold.