I tried to use the search engine for "s5 engine to s4", but the only word it used was engine. I didnt really want to look at every post from the last 3 years to find the answer, so I ask:

I'm going to be helping a friend swap everything from his 91, into a s4 he's going to purchase. I know we can swap the ecu/wiring harness for the engine, but is there any reason to swap CPU or any other harnesses to keep everything working properly? We'll be swapping dashes anyhow but I just want to make sure, thanks

 

If you choose to swap the front harness/dash harness/cluster/warning lights, you'll need to take the S5 CPU with it. With the dash out, just swap of all of it. Running down all the little changes needed to mix S4 & S5 wiring can lead to some major headaches. The route that would require the least swapping (but most rewiring) would be to change only the ECU/emissions harness/engine and adapt the connections at the front harness to emissions harness connector. The FSMs will be your friend.