The S-Type was a major redevelopment of the Mark 2.  It used a mid-scale version of the Mark X independent rear suspension to replace the Mark 2's live rear axle and featured longer rear bodywork, among other styling and interior changes.  The S-Type was available with either a 3.4 or 3.8 litre XK engine but only in twin carburetor form because the triple carburetor setup would not fit into what was essentially still the Mark 2 engine bay.