Mostly from research, testing and trial and error.
No quite sure what "system" you are referring to, please email to hho@hydrogen-generators-usa.com or respond here and we will answer your question better.
Your BMW's Oxygen Sensors; You have 2 narrow band sensors upstream of your catalytic converter and 2 narrow band sensors downstream of your catalytic converter. We usually recommend the ?Quad Digital EFIE? in these cases because it has the superior digital circuitry and it?s the most flexible if ever used on another vehicle.