where do i start? ok the Apexi AFC's are devices used to re-tune your car's ECU/ECM/PCM(engine computer). it does this by changing your car's MAF or MAP signal going to your car's computer like a EFIE, but unlike the EFIE the Apexi AFC's has multiple correction points. correction meaning fuel to air ratio. here is an example why a Apexi AFC is better in my opinion, a EFIE has ONE fixed setting, meaning when you set your EFIE gauge or knob that's it until you move it again so lets say your car idle at 700RPM and with your EFIE you can lean to a certain point and its good enough to idle low and not stall out but at a higher RPM or at a higher air flow it would stall out so you cant have that same setting for your whole driving, but with a Apexi AFC you have multiple correction points. so you can idle low like say you are at full stop at a light you can set your idle to very low just to keep the engine running and when you accelerate and your RPM goes above idle you can set it to run a little richer so you don't stall out.

check this site under support and you will find all the manuals for all the different types of AFC
http://www.apexi-usa.com/

you can check on eBay or craigslist in your area for all the older type AFC because Apexi only make one model now and its expensive. i prefer the VAFC's if the vehicle allows it. i buy 2 VAFC1 just 2weeks ago for $50 each so they are not that expensive.

read up on the manuals and get to know the AFC. the pro's use it to tune just keep that in mind.