kinda dumb

I dont wanna sound like a moron but waht the hell thats how ya learn.

what is the difference between a 12b and a 13b?
and a fc and a fd?

12A housings 70 mm, and 13B housings 80 mm wide.
FC 2nd gen RX7 (86-91), FD 3rd gen RX7 (93+)

thanks

with the width of the housing being different, 12as are 1147cc(70ci) and 13b are 1308cc(80ci)

12as last production year was 1985
13b have been arround since 74-75, and have been introduced in rx7s in 84