Citroën used four different (all aircooled) engines in the 'plain' GS and GSA types (1015cc - 1129cc - 1222cc - 1299cc).
Next to that there was the 1990cc wankelengine which was used in the GS Birotor only.
At the introduction in 1970 all cars where only equiped with the 1015cc engine. This engine, coming from the Ami,
was considered underpowered from it's introduction on. In 1972 Citroën took action and introduced a 1222cc engine.
This engine wasn't used in the Club only at first. The 1015cc lasted untill 1977, used only in the cheapest version.
In 1977 Citroën introduced a new developed 1130cc engine, which replaced the 1015cc. It was used in the Gspécial only.
The introduction of the GS X3 brought the fourth engine used in the GS. The 1299cc engine, which was later used in all GSA's,
except for the first versions of the GSA Spécial. The introduction of the GSA in 1979 is the end of the 1222cc engine.
In 1983 the production of the 1130cc engine is stopped, all cars now use the 1299cc engine.