its a pre programmed piggy back, its tunable, there is a dial that goes from 0 which is stock to 9 which is full rich. its been a long time, but each click is about ~.5 an AFR point. they used to offer two chips, one for stock turbo, and one for the HKS T04 kit (JDM tuners could burn custom chips too), and then if you really wanted to go bananas they have a GCC which makes the AFR adjustable in RPM increments.

Fcons basically require a good running fairly stock car, and the units are really old, i think you could do better these days

 

thats interesting. theres a guy local sellingone with a S4 wiring harness adapter. i have a stock s4 and kind of want to keep it that way, nothing extreme so i wonder if that would support what im after for a more or less "plug and play" kind of tune