89 Iroc Z

Why do narrow band O2 sensors become inaccurate at WOT? What is it about WOT that makes it become inaccurate? Can you still use it at WOT to tell rich or lean if not, why?

They reason I ask is everyone says you have to have a Wideband to tune WOT. I just want to know why a stock narrow band O2 can read air/fuel ratio at idle but not at WOT?

funstick

becuase a stock narrow band o2 sensor cant really read stioch. its more like a pendulum. it can read around it by a good bit it think ive confrimed that the bosch sensors are sort of accuarte from 12.9 - 15.8 or so. and even then its still sort of crappy. you can tune for wot with a narrow band its just a lot harder. the volate range is what hurts you as well as the fact that its desinged for swtichign not VOUT ouptut as AF ratio.typically i find this to be true with new sensors


.250 or so is 15.5 or so

.790 is roughly 12.5

this is not carved in stone but is sort of accurate. every o2 sensor will vary. good luck.

91L98Z28

a narrow band O2 sensor has two basic readings it can supply.

a reading of higher than 0.450mV, and a reading of lower than 0.450mV. The reading above 0.450mV means "richer than 14.7:1" and the reading below 0.450mV means "leaner than 14.7:1".

Now lets say it reads 900mV. What does this mean? "richer than 14.7:1". It could be 14.0:1, or 13.0:1, or 12.0:1, or 11.0:1, or really, anything below 14.7:1. you just don't know.

the same is true for, say, 100mV. Leaner than 14.7:1... could be 15:1, could be 16:1, etc..

now the way the stock computer makes this work is as follows. Say we start out rich (> 450mV). the computer pulls a tiny bit of fuel out at a time, until suddenly the O2 sensor swings to less than 450mV, then the computer adds a little, adds a little, and the O2 sensor swings back the other way. This is called a crosscount - the number of times the O2 sensor "swings" across the ~450mV range. The ALDL data stream even reports the number of crosscounts. The more crosscounts you get, the faster the O2 sensor is responding, and the more accurate the fuel ratio.

So if the O2 sensor can't read 14.7:1 (it really can't!!!) then what does the computer do about this? Simple...it keeps adding and subtracting fuel to constantly cross lean/rich over and over again. ANd if the motor goes from 14.65:1 to 14.75:1 and then back to 14.65:1 several times a second, then what you've got is an effective average of 14.7:1.

To more directly answer the question: the O2 sensor works perfectly fine at WOT. If you were at WOT and went leaner than 14.7:1, you can bet that the O2 sensor is going to indicate < 450mV. Guarnteed! The reason O2 sensors are not "accurate" at WOT is because all they can measure is "rich" or "lean" - they can't measure how rich or how lean. So if you want to run 12.5:1 WOT AFR, a narrowband O2 is useless. It will read basically the same at 12:1 as it will at 13:1. "Rich". That's all it knows and all it can accurately report.

RBMZ28

The o2 numbers generated by the ecu,read via scanner at wot
are they even close to accurate? Also the numbers you quoted as
being 12.50 to 1(.790 i believe you wrote) I have believed them to be .890 to acheive that ratio.Have also read and seen dyno charts to confirm .890 give or take .020 depending on your combination.You give very good info, but would you let me know
how you come to your conclusion.

91L98Z28

The following image demonstrates the relationship between O2 sensor voltage and AFR.



The AFR is measured in units of "lambda". The center of the chart has lambda = 1.0, which is 14.7:1. the verticle line immediately to the left, lambda = 0.94, is 13.8:1. The vertical line to the right, lambda=1.07, is 15.8:1.

If you analyze the graph, it's basically telling you that *anything* richer than 14.7:1 is going to read more than 800mV, and *anything* leaner than about 15.0:1 is going to read less than 200mV. Or in other words: as soon as there is oxygen in the exhaust, the output goes low (lean), and as soon as there is no oxygen in the gas, the output goes high (rich).

It has been tested and established that a narrow band O2 sensor can not accurately be used to measure AFR. They are really only good for one thing: answering the question "Is there oxygen in the exhaust?" If the answer is yes, then you know you are lean and the output goes low, and if the answer is no, then you are rich and the output goes high.

That is why wide band O2's exist. And it's why Honda used a wide band O2 sensor in their lean burn vehicles. A narrow band can't accurately measure HOW lean the engine is running, which Honda needed to do, so that's why they used a wide band O2 in that application. They didn't need to know "Am I leaner than 14.7:1" because that answer would always be yes. They needed to know "HOW MUCH leaner than 14.7:1 am I". A narrow band O2 answers the first question, a wide band O2 answers the second question.

More references, if you want them:

http://www.tragacs.com/wbo2.html
http://www.g-speed.com/pbh/afr-o2.html
http://my.engr.ucdavis.edu/~avsmith/o2sensor.html
http://www.syty.org/archives/syty/9607/msg00520.html

87_TA

Wow 91,
great explanation.