Hey guys, I'm rebuilding an S5 engine and have found a ton of S4 front irons but not a cheap S5 one yet! I know there is a difference in the port size and the oil o-ring, but I'll be porting the engine so the port size difference doesn't matter to me and I can special order the gasket kit from Atkins. Are there any major differences that would keep me from using a S4 front iron with my S5 engine? I also have a S4 front cover if I have to use one of those. Anyone with experience doing this? I'm weighing the dificulty of modifying the S4 iron vs finding an S5 one. Thanks!

 

NA or turbo?

If NA, an S5 front iron will be missing one of the port-air passages, which will mean passing a smog test will be pretty unlikely. I don't know if this applies in your area. Pic here of the manifold side: http://homepage.mac.com/carldavis/intake/mod.html

If turbo, I don't think that passage is on the front iron for S4 or S5. The 4-port manifold pictured here has an EGR flange, which makes it S4 : http://www.aaroncake.net/rx-7/4PortLIMTo6Port.htm