Then you really don't understand what the OEM O2 sensors are capable of reporting - which is really nothing more than a narrow band light show. And the indicated A/F ratio at cruise has no meaning whatsoever. You're throwing boost at a pedestrian engine with hypereutectic pistons and sintered iron rods (both of which are bad). Knowing A/F ratio in boost as you approach max rpm, or while under heavy load, is critical to not breaking off a ring land or worse.
All of this.
Under part throttle operation you're always going to be seeing something in the area of stoch, if you dont, there is a SERIOUS problem. It's under boost and high loads were an actual wideband is required. The OEM ECM should be able to maintain part throttle A/F ratios through a TON of crazy scenarios, but once you go full throttle and pull out of closed loop operation, that's where it gets involved. I'm assuming the OEM O2s are not wideband here, although a few cars do come with them.
Although, the second part of that is both knowing what the numbers mean, and how to do something about it if there needs to be something done. If you don't have ECM access to the tables to adjust power enrichment settings, you really are just watching the parade. Further more, if you dont know the parameters to the engine, in terms of intake/exhaust temps, what your A/F should be under full load, etc, etc. Then you have the potential for multiple layers of places to adjust enrichment under boost, intake temp multipliers, cat over temp multipliers, etc, etc.
I've a bit of experience in tuning supercharged engines, but those were in sports cars with a pretty large set of data to pull from before even looking at a specific car. For the vast majority of people, if the hardware and tune don't put the engine in the right state, there's just not much they will be able to do with it to fix it. If anything they might make it worse. I've seen more than a few blown engines after someone snagged HP Tuners and a laptop and figured 'those OEM noobs left 8 HP on the table by not leaning out the top end of the RPM band, let me just fix that right quick and head to the track'.
This makes me think back to the highschool days and my DSM (always breaking haha)
