I ordered a jspec S4 engine and got an S5. What would I need to do to get it to run with my S4 harness and ecu. The injetor clips are diffrent, so, could I use the S5 clips and splice them into my S4 harness? Also, I'm gonna run premix.

It's not quite that easy. You'll need an s5 ECU. OR go stand alone!!! Some of the sensors have dfferent impedences and stuff.

its a lil easyer then ya think. just swap the frount cover , water pump , metering oil pump tb and linkage over from the s4 to the s5 aswell as injectors. the turbo on a s5 is far better then the s4 anyway. use all sensors from the s4 on the s5 and u shouldnt have any trouble. as for premix u wont need to if u use the frount cover of the s4. but if u must its 200 to 250/1 inthe gas tank with a good 2 stroke oil. but idealy just use the short block and turbo and intake manifold of the s5 and all the other parts of an s4 and u should have no trubble