My question is about the internals of S4 and S5 engines. I already know all of the external differences. Are there differences in S4 and S5 rotor housings. Or ecentric shafts? I know the rotors have slightly different compression rotors but besides that are their any differences with the engine internals as far as interchangability goes?

 

Counterweights are different, thats about as far as I know.