Originally Posted by cowanpp
That's what I would suggest. Either that or get some weak ass ECU that'll supply less fuel.
That's the only way you can do it. In closed loop mode, the ECU will just adjust the fuel trims to get the a/f back to what it thinks it needs if you adjust the fuel pressure regulator or take fuel away using a afc. What ECU are you using, you can just have someone write new fuel maps for it.