Engine potential
 

I was wondering which car's engine has more potential in performance, the FC or the FD?

Well, there are many differences between the two. Most important being that the rd rotors are a little lighter and supposedly weaker due to thinner casting. Also, the ports on the fd engine are a bit more agressive i believe.

Series 5 rotors (FC '89-'91) are identical in weight to Series 6 (FD) rotors. They have a different part number but are interchangable...my FD is running series 5 rotors. FD side housings have more aggressive porting, stock, and the manifold angle into the port is a more gentle 45 degrees as opposed to the FC side housings' 90 degree entry from manifold into engine, IIRC.

The REW engine in the FD has more potential for performance but that's not to say you can't get good power numbers out of a FC engine. The REW has higher oil pressure, better bearings, and better dowel pin casting than its predecessors...however the series 5 FC engine had improved dowel pin area casting as well. I'm not sure if the REW simply copied that or if it was improved further.