SSpdDmon

iTrader: (10)

I got to thinking today....what's up with the stock O2 switchpoints??? Why do they move the rich/lean threshold for low airflow conditions more towards the lean side? Ever notice a difference between what the WBO2 said was stoich vs. what the STFT's say is stoich for the motor when it's consuming less than 32 grams/sec of airflow? I know a few people have...

If you look at the stock switchpoint settings, you'll see that the engineers created a relationship between airflow coming in the motor and what the sensor considers rich/lean using "Closed Loop Mode." Anything above 32 grams/second, the NBO2's switch at their midpoint (550mV). However, bellow 32grams/sec., they switch at 500mV (20~26 grams/sec.), 450mV (~14 grams/sec.) and 435mV (idle).

Why would the NBO2 rich/lean threshold be different for a motor consuming 100grams/sec vs a motor consuming 10grams/sec? My LC1 will report a 12.75 AFR (1.8mV) for cruise scenarios just the same as it would when I'm running the 1/4mi. Why should the stockers be any different?

My theory...maybe the engineers jig-rigged the NBO2's to provide some sort of "lean idle" or "lean cruise" for low airflow situations??? After all, it's not like you're making huge power sitting still, right? I think I'm going to run the experiment of changing all switchpoints to 550mV when I get the time. The more I think about it...the worst thing that could happen would be a richer idle/light cruise.

Thoughts?

possum22

Here's a post on HPTuners showing how the O2 switchpoints work.
http://www.hptuners.com/forum/showthread.php?t=4929

SSpdDmon

iTrader: (10)

That thread basically confirms what I suspected, which brings me to my next point. People who use the NB02's to tune their VE need to change the O2 switchpoint table to 550mV across the board. Otherwise, they'll be tuning the VE to a leaner than stoich AFR for lower airflow scenarios. Not exactly where you want to be when creating a foundation for your tune if you ask me...

SMOKINV8

iTrader: (6)

Good theory, but my personal experiences kind of contradict that. I've tuned a couple cars by narrowbands for the lower RPM's, only to have them verified as dead-nuts on 14.6-14.7 with a wideband when we went back for more tuning later. Granted, this is only a couple cars, but both had SVO 30lb injectors, so switching might have been theoretically off by a little.

white2001s10

iTrader: (4)

The rigging that you suggest is how they calibrated the sensor output for the changing airflow and temperature, since the sensor itself is a rather cheap, generic, and non-linear output device. I'd say they got it right for the sensor being used.

Narrowbands create voltage based on oxygen flowing by the sensor and are also a bit sensitive to heat. Air-flow through the system does change these factors.

GuitsBoy

iTrader: (4)

I believe that at low airflow, the sensors are running cooler, and simply produce a lower voltage. On the high end, I think they raised the points to offer some protection by fattening up at high airflow.

SSpdDmon

iTrader: (10)

But on a stock car where the sensor sits in the manifold with a cat right next to it? I find it hard to believe temps change so drasticly that they had to drop the rich/lean threshold 115mV. Yes it may be a non-linear O2 sensor, but isn't the midpoint where stoich is (ie 550mV)? What happens above or bellow that midpoint isn't really all that important. Either way, just thought I stumbled across something. Didn't know if anyone noticed anything funny...

white2001s10

iTrader: (4)

The sensitivity isn't linear.

There is several hundred degrees difference. You can easily test this with a K-type thermocouple in the exhaust.