Agreed with most of the above. the major difference. is the intake manifold attachment. different series of engines had different manifold and port designs.. while some are better then others, not all parts can bolt together. make sure the manifold you are using is the same as the Side housings you choose. also FC and FD RX7 engines have the motor mounts in different locations. so it's not a simple stupid modular slap together affair, but not particularly difficult either.

1989 is your answer <1989 is S4 >1989 is S5 manifold design and HW AFM vs. barn door AFM had a lot to do with it.. the S4 and S5 housings are 6 port if NA and S4T2 S5T2 are 4 port. (like was said earlier all Turbo's were 4 port)

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Tampa Rx7 repair, 1st place.