Complex sentence: This was competitive with the Audi Quattro in 1984 which also had but by the 1985 season Audi's S1 was homologated with plus meaning a turbo had to be added to the S4 engine design to remain competitive.
Split into simpler sentences: This was competitive with the Audi Quattro in 1984 which also had, but by the 1985 season Audi's S1 was homologated with. Consequently, a turbo had to be added to the S4 engine design to remain competitive.