Complex sentence: In automobiles, the four-cyclinder boxer powerplant goes as far back as the start of the 20th century; and in the early 1970s flat-four engines were being manufactured by Subaru, Lancia, Alfa Romeo, VW and Porsche as well as Citroën.
Split into simpler sentences: In automobiles, four-cyclinder boxers go as far back as the start of the 20th century. By the early 1970s flat-four engines were being manufactured by Subaru, Lancia, Alfa Romeo, VW and Porsche as well as Citroën.