well, tuning the a/f ratio in general CAN net you more power, however... what exactly the "idle a/f ratio" pot does is somewhat of a mystery to me. I tried adjusting mine according to the shop manual and could never get it right.. however, experimenting with it did definately vary my performance results, it just seemd to effect the a/f ratio in general, not just at idle. I've discussed this pot with some other FC fanatics and it seems likely that it's just a pot that offsets the AFM which is just another pot hooked up to a flapper on the S4. If this is true adjusting the idle a/f ratio pot effects the readings the ECU gets from the AFM pot throughout it's range, so you could potentially use it to 'tune' the a/f ratio but it's not very effective as there isnt fine enough grained level of control... which is probably why it would be called the idle a/f ratio adjustment, you just tune it at idle to a specified value in the shop manual and supposedly are done with it, it's like setting a reference point for the rest of the values read from the AFM.



However, I could never get mine quite right for other reasons, I think I just had a number of vacuum leaks that made my idle all over the place so whenever I tried to adjust the a/f ratio pot it was inaccurate due to intermittent vacuum leaks. There are probably > 10 things you could repair, upgrade, or eliminate on your car with better results and less headaches right now... start with weight reductions, first step in that direction could be ditching that big assed iron bath tub of an exhaust manifold and replacing it with the RB header. Weight reduction and power gains. If you really want to tune the engine drop the stock ECU and pickup a programmable one, the stock ECU runs quite rich.