wiring harness the same?
 

i currently have an s4 but i want to do an s5 engine swap this is an NA. can i use the same s4 harness or do i need an s5 because i want to just swap in the s4 for smogging.

S5 and S4 engine harnesses and engine computers (ECU) are not the same and not compatible.

If you are attempting a swap between series, you need to decide on what ECU you are running and wire accordingly.

would it be best if i just saved my whole s4 motor for smogging and then completly build my s5 engine for racing and daily

as Icemark said, you'll need to have an S5 ECU and harness even to do that.

yeah i know that k thanks