thanks and good question, the green box we are tapping for last few pages is infact two units built into one, those units will be emulator and advancer, emulator fools the ECU while on CNG that car is still on petrol by generating some fake MAP/MAF signals, ok we remove the emulator then, check engine light will show up thats all as far as i know, second unit is advancer, its job is to advance the timing to match with the input fuel that will be CNG, ok this device will do it electronically and i dont want to do it that way so i remove the advancer unit and set the timing advance by clocking the distributor and i am good to go.
now why would you need these? because you need to run your car flawless and fuel economic as its original setting on petrol. if you dont have advancer emulator you are compromising efficiency on both petrol and gas as your car will not be optimized for any of them, if you have advancer your car will be adjusted for petrol from ECU and for CNG you will tune timing advance from advancer. hopes that my limited knowledge hepls in clearing the confusion in your mind.