The V6 S Series cars used Ford's Cologne V6 in 2.8 L, 160 hp (S) and 2.9 L, 170 hp (S2-S4) forms. The early "S" cars are often referred to as "S1"s, to distinguish them from later iterations. The S3 and S4 received longer doors, although some late S2's were also thus equipped.
Vehicle models ending with "C" were used to denote vehicles which were fitted with a catalytic converter. Only the S3 and S4 were fitted with catalysts. Cat standard in the UK was only introduced in August 1992, at "K" registration, catalyzed earlier cars were intended for export to markets with tighter emissions standards.
Source: Wikipedia, 2012