So what does that do to the ecu?
Nothing, as o2 sensors sit before cats?
Good point, but it would change the airflow in that area of the pipes, so maybe the ECU adapts to it.
The ECU has the capability to separately adapt both the timing and the fuelling. As standard both functions are on and the adaptive range will cope with a stage 1 tune of air filter and derestricted exhaust (baffelectomy, MMC, G56 etc).
If you can get an unlocked ECU and are able to get a dyno tune (eg at Krazy Horse) then the timing adaptive would be left on but the fuel adaptive switched off and the fuel map optimised on the dyno.
For stage 2 (569 cams) the adaptive fuel will go out of limits at higher revs/wider throttle and throw a check engine light. This is why you'll need an unlocked ECU and a modified fuel map. The S&S ECU has a selectable default map which will cope with the higher lift cams or companies such as KH and G56 provide a custom map. Again, a session on the dyno will allow this default map to be tweaked to your individual engine and set-up.
