During the development and testing of the new M291 powerplant, it became apparent to Sauber Mercedes that there were problems in the engine design and construction. Thus the team started the 1991 season with a lone C291, while at the same time running an older C11.
While developing and testing the new M291 powerplant, Sauber Mercedes discovered problems in the engine's design and structure. Consequently, for the 1991 season, they started with just one C291 car, alongside their use of a more aged C11.