One of the key elements on this is where the O2 bung is located. In the collector, carb car, guaranteed crazy reading (on 2-step) - in a primary tube - it might be ok or will straighten out much more quickly. The reason for this the back-pulses of clear air coming into the collector at lower RPM's.
Big cube bracket cars can get away with it in the collector. Stock & SS - put it in the primary.
Also, EFI cars seem to have more stable O2 readings on the 2-step.
Here is a log of a car showing pretty stable O2 readings bungs are in primary tubes. BUT it is an EFI car.
