what major difrences are there, and how hard would it be to swap the keg.
to much rust and other issues on mine, found a 90 n/a with good body and blown engine, how hard would it be to swap.

 

intakes are different
different oil metering pump/front cover
center irons are different due to lower intake
other little things

rotaryresurrection.com has a good write up on this

If I were you, I would rebuild the S5 block.

 

ok well its going to be hard. if your taking everything from your s4 and putting it into your s5 your going to have to change out all and i mean all of the wiring. everything behind the dash to. other then that its simple everything bolts up. the wiring is the bitch of the swap. any other questions let me know i can help ya out

 

I have done this swap and drive my car everyday, its a 91 with an 87 13b, personally i dont like S5 13b's. But that is because i had one blow up on me, well at least let a apex seal go! But just shortblock the S4 motor and swap everything from the S5 to the S4, grind down the rear iron plate to let S5 intake manifold set flush with LIM!! oh yeah S5 compression ratio is 9.8:1 and S4 compression ratio is 9.4:1 that is NA specs of course!

 

cool. there are a bunch of othe things that i could do before all the wiring. my s4 has plenty of rust issues, but the motor pulls good and strong. and it has the 5 bolt, so i thinks me has me work cut out for me. thanks