Why using the factory O2 sensor is less than reliable when it comes to A/F

[Pyro6]

I was in search of a WB that held up to race gas, Eric suggested Denso. I went with the Powerdex system and have been pleased. I agree with Dave, there are many advantages to monitoring both sensors. I've found at WOT the NB is a more repeatable reading. Like Steve's PL sample shows, at WOT it flatlines nice and that reading should stay consistent on every run.

[Steve Wood]

The actual a/f number is not something magic and may be different for every car to a slight degree...what we want is to run as lean as possible without detonation so the nb works well for that...the wise will add a bit more fuel back in to account for things that might trigger detonation.

At idle, all I want is a smooth idle and I want it to be consistent.  I prefer open loop altho it helps more on a car with more cam than stock.  Again, I lean it down until it starts to waver when the engine is cold, and add just enuf to smooth it out...when it warms up, it will idle over a wider range without a problem.  Both my cars like it somewhat richer than stoch so closed on a nb is not optimal.

I have had good results closed on a wide band...usually around 13.5/1 on a warm engine...but I am perfectly happy open loop all the way