i have an 87 s4. the seals went about 2 months ago and its been sitting... i recently came across a 90 s5 and was wondering if i swapped everything including electronics, both harnesses, and ecu if it would work? ive seen posts about leaving the s4 electronics on an s5 motor but it also requires swapping the front covers and such... i just want to put it all together how it is and basically turn it into an s5...

 

If you swapped everything, including the body harness and some of the dash electronics, then yes, it is a "bolt in". However because of the numerous differences between S4 and S5, cross series swaps without a standalone are discouraged. If you want to go through with it, the easiest is to simply run the S5 engine using the S4 electronics already on the car.

 

Follow Mr Cake's advice.

 

This link may be helpful, too. -> Rotary Resurrection: Swapping an '89-'91 (S5) Engine into an '86-'88 (S4) Car

 

I would just use the S5 block. Less work and parts on motor itself are interchangeable but why go through the hassle of swapping harnesses and messing with factory wiring?