What is the difference in O2 readings before/after cats. - Z06Vette.com - Corvette Z06 Forum
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post #1 of 16 (permalink) Old 05-08-2002, 07:09 PM Thread Starter
FRISKY
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What is the difference in O2 readings before/after cats.

Does the exhaust going through the cats make much of a difference on the O2 readings? Does it add or subtract from the "real" O2 numbers? Most of the dyno shops use a tailpipe sniffer. How does that compare with using the sensor in front of the cats?

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post #2 of 16 (permalink) Old 05-08-2002, 10:17 PM
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MC Racing in Kansas City had me go to a nearby Muffler shop and have a 'bung' hole welded into pipe about 3 or 4 inches before the cats(didn't matter which side). They gave me a plug to take to the muff shop to put in the bung (it was threaded at muffler shop). The folks at MC don't like using the tailpipe clipon for the O2 wideband scan . They say that once the air has gone through the cats, you don't get nearly as acurate reading as you do if the sensor is inserted in the bung before the cats.


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post #3 of 16 (permalink) Old 05-09-2002, 01:35 AM
ZeeOhSix
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O2 levels obviously change when going through the cats, but I don't know the exact level. That's why there is an 02 sensor before and after each cat ... so the PCM can determine how well the cats are working.

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post #4 of 16 (permalink) Old 05-09-2002, 02:21 AM
c4c5specialist
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HI there,
Ok, o2 sensors are use to assist in fuel metering, as they react to oxygen content of the air flowing past the sensor, once the sensor has gotten to operating temperature.
Now, the primary o2 sensors are what the computers look at for 75% of the fuel trim calculations. These are in front of the converters.
The secondary o2 sensors are behind the converters, and are described as catalyst monitoring sensors. Now, the name is not exactly correct, for the PCM still uses these sensors for about 25% of the total fuel trim calculations.
The voltages will vary, from 100 millivolts, to about 800 millivolts.
However, the primary o2 sensors will always fluctuate in this range if correctly operating. And it changes fast, as the reaction to the oxygen around it.
The secondary o2 sensors will not fluctuate that much, due to the fact of the converters, as mentioned earlier. The will show almost a steady reading, as you see a consistant voltage, due to the converting process if the exhaust gas.
If the secondary o2 sensors were to fluctuate as quickly as the primary sensors, the PCM will set a .420 or a .430 code, for Low Efficiency Catalyst, meaning that the converters are not working.
This is why there are codes set when the converters are removed.
I hope this helps, and let us know what else you need.
Besttoyou, c4c5:eek2:

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post #5 of 16 (permalink) Old 05-09-2002, 03:34 AM
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This brings up an interesting point...
Are the AFR's we seek to achieve when tuning our cars on the dyno really accurate because of the effect of the cats?
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post #6 of 16 (permalink) Old 05-09-2002, 05:18 AM
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I'm with you, Vette! Anybody got wide band O2 readings and stock O2 values for the same pull?
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post #7 of 16 (permalink) Old 05-09-2002, 05:35 AM
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FWIW, my dynotune guy is putting the bung(s) ahead of the cats but behind the first O2 sensors - he sez this will give the most valid AFR readings. We will be doing this on 5/18, followed by an "untuned" dyno session. Even though the new LS1/LS6 edit software is not yet quite ready for release, we can at least check rearwheel h/p & Torques plus get AFR reads. I guess I am expecting a "pig rich" read, like so many others - but I wonder if the newly installed headers may reduce this situation somewhat?

Ultimately, with dyno-tuning, I hope to see at least 380 or so rearwheel translated to 435 crank using the 13% drivetrain loss estimate.




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post #8 of 16 (permalink) Old 05-09-2002, 09:35 AM
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380rwhp 371rwtq

Bleubyu, you should have no problem with getting 380rwhp. My car with just a donaldson filter,tpis headers,and maft. Timing wot @ 22 degrees total made 380rwhp and 371 rwtq.:D
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post #9 of 16 (permalink) Old 05-09-2002, 10:34 AM Thread Starter
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Quote:
Originally posted by VettTheRipper
Are the AFR's we seek to achieve when tuning our cars on the dyno really accurate because of the effect of the cats?
That is really what I wanted to know.

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post #10 of 16 (permalink) Old 05-09-2002, 10:55 AM
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Roger, I had 3 pulls with O2 readings done simultaneous on all three. I have the graphs on my computer at home. if you want me to e-mail these to you, let me know. By the way, this was untuned with rich readings (11.7 to 11.5)...369.2 rwhp..367?rwtq. Should receive my MAFT this week and will install, adjust and redyno. Hoping to be 380+..... (Also had just installed headers and X pipe and not sure if relearn had been accomplished)


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post #11 of 16 (permalink) Old 05-09-2002, 11:19 AM Thread Starter
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MikesZ06, Yes, please e-mail me the graphs.

Thanks, Carl ([email protected])

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post #12 of 16 (permalink) Old 05-09-2002, 12:03 PM
Roger Ramjet
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Mike -
Sent you a PM for the O2 data.
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post #13 of 16 (permalink) Old 07-06-2002, 06:49 PM
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O2 Sensor Bung

Greetings

Does anyone know where to get a titanium bung and plug to add port for dyno O2 sensor? I would not let shop add the SS one they had.

I just had my 02 dynoed (5500 mi, 357 RWHP and 344 RWTQ) with volant dual intake but no MAFT. (It is in the mail and I hope to install it soon.) Ran very rich (around 11) with tail pipe sniffer. Dyno shop said they have seen about 1/2 point of AFR change due to cats. I assume that the cats will remove some of the unburned fuel (therefore a reading of 11 was actually about 10.5).

Thanks to all for such a great forum!
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post #14 of 16 (permalink) Old 07-06-2002, 09:10 PM
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I resolved the need for a welded in wideband O2 bung by removing my cats all together.
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post #15 of 16 (permalink) Old 07-06-2002, 09:31 PM
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That's a good idea, Tamu, except in a state that requires annual inspections (like mine) :(


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