I already searched, but my apologies in advance if this is an easy question to answer.

There's a car that I'm looking to buy. It's an S4 vert. The guy has an S5 TII swap about ~90% done.
The things left are to install the S5 ECU and the S5 engine harness.

How hard is it to install these into the S4? Is there a guide around here or will I pretty much need to wing it? I've never done anything like this before, but I'm fairly well versed with electronics. Any sort of help is greatly appreciated.

 

you will need some additional S5 wiring and know how to re-pin the connectors because the S4 ECU plug in the dash and chassis harness is completely different.

only 2 of the 3 ecu plugs you will have on the engine harness, the third is going to be where your problems lie.

 

Thanks. Is there a diagram somewhere that shows which pins need to go where?

 

Actually, it looks like it's an S5 JDM TII engine. Is it easier to just keep the stock S4 harness and run an Rtek ECU?

 

FSM will have pictures of each of the third plugs to compare and see what needs to be moved where.

the S5 block has an electronic metering pump which can't be used with an S4 ECU, chipped or not. you would have to replace the S5 with an S4 front cover and the throttle body with an S4 one to work the mechanical metering pump.

you could also eliminate the metering pump, premix and use the S4 harness/ecu.