I was wondering... if I am rebuildng a s5 2nd gen engine can i use the rotor housing from a s4 engine?
The s5 engine I a reblding is from a 91' non turbo and the s4 engine is in a 86' non turbo. Would it work?

 

IT would, but I wouldnt recommend it really. For one, there is a minor plug timing/position change between the two. Also, the 88 and prior housings seem to wear quite a bit more than the 89+, so you're bound to have mismatched wear.

But yes, it could be done.