Nope, AFR's don't remain fine. I've tested this since M18 swaps were not even known to be common. One of the first swifts ever swapped m18 back in 2010-11 and it's been a decade I've had my hands on a bunch of them, that car was owned by a very close friend and perhaps the highest hp M18 NA build (allmotor) pk has ever seen, from a basic assembly swap till a fully forged internals build with a piggyback, i've seen it all. Let me give you a very basic scenario, let's suppose X ecu has Y injectors to be controlled from factory, they have Z injector size in cc. At any specific rpm and specific load condition, let's suppose ecu has a duty cycle of 30%. VS "A" ecu which has "B" injectors and "C" injector size in cc that lets "C" injectors have a duty cycle of 50% on similar load conditions and rpm. Would you imagine both having same AFR? This is just from a very basic stand point for the sake of basic understanding and logical reasoning for a common reader. We have not even taken other physical geometry into consideration like headflow CFM, combustion chamber dynamics, lift type (or it's presence/ absence) etc etc. So yeah, thinking that being something ideal for fuel saving is a lie to self.
