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Thread: OK so whats the deal with WB's after cats?

  1. #1
    Tuner in Training
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    OK so whats the deal with WB's after cats?

    Looking around for info it seems the cats will make the wideband read leaner than actual. Possibly up to 1-1.5points. Im thinking of installin dual WB's after the cats and just logging for a bit and see what they read. The cars on stock tune and Im trying to do all my homework before I buy. The exhaust has been upgraded with kooks headers and catted Y. It may take quite a bit of time but Perhaps I could Set up a WB as a NB in the primary slot and Have the other WB behind the cat and do some prelim logging. When it comes down to it it would just be really convienent to plug them WB's in at the post cat location. I really dont want any shop around my car for any reason as Im losing faith in the local shops. (my 4 day old ram received a large dent from a liner/exhaust shop.

    Well long story short any suggestions/experience with this? Im trying to shy away from extra bungs and messin up my pretty Kooks...

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    Erik
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  2. #2
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    honestly, i've heard all this too but my WB is after my cat and reads spot-on 14.7 and matches my commanded AFR as well. *shrug*
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  3. #3
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    Internet fib.. maybe .1 difference.

    A WB reads a o2 difference.. in a rich mixture (<14.7) there is no left over oxygen.. so the only burning that happens in the cat is with what is left over with combustion ineffeciency..
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  4. #4
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    Cool thanks guys. What WB options do I have? I hear LC1's are hit and miss. I would like 2. 1 for each bank. And one has to be portable so I can hook it up to the Ram when its supported.
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  5. #5
    Advanced Tuner Billf6531's Avatar
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    I'd be more interested in your results at WOT, or at least in PE. One w/b before the cat, and an identical one after the cat, would provide some useful data. Multiple runs, please, and test it on both banks to rule out an abnormal cat.

    Then again, it's not that difficult to replace one of the narrow bands with a w/b while tuning.
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  6. #6
    ya, the purpose of the cat is to burn "extra" fuel that comes down the pipe. that's why in PE where you're running richer you could get an invalid reading. PE is the most important reading you need for tuning. i wouldn't trust one after the cat. it costs next to nothing to have a bung put in. another thing is the closer to the engine the less lag in the readings. my .02.... hey, i gave you a buck. where's my change?
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  7. #7
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    im curious as to how you think a cat is going to burn extra fuel?

    You need o2.. not found in a rich mixture.. except what is due to combustion ineffeciency (less than a couple % max)
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  8. #8
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    Quote Originally Posted by Alvin
    im curious as to how you think a cat is going to burn extra fuel?

    You need o2.. not found in a rich mixture.. except what is due to combustion ineffeciency (less than a couple % max)

    Easy: The O2 sensor senses Oxygen, meaning if there is NO oxygen, the fuel won't burn, but since it DOES sense oxygen, the fuel in the cat will burn to the point it exhaust all the oxygen.

    In normal driving and in PE, there is enough oxygen to burn fuel in the cat. In COT, richening actually saturates the mixture to deprive the oxygen and then the unburnt fuel actually cools the cat.

  9. #9
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    You guys are missing the point.

    The wideband is referencing the atomoshpere. There is no (very very little/none to speak of) oxygen in the exhaust at wide open for that reason there is no significant "burning" in the cat.



    Also... theres a mass balance going on. So really it doesnt' matter if you measure AFR at the tailpipe, manifold, pre-cat, aftercat, or get this.... Intake! thats an old way of doing it! As long as there is no extra air being pulled into the exhaust you can measure it where ever you'd like. (temp and pressure does skew o2 and wideband readings)
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  10. #10
    At idle, the cats throw my WB off .2 to the rich side. 14.7 idle shows 14.5 on my wideband.

  11. #11
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    Quote Originally Posted by Country Boy
    At idle, the cats throw my WB off .2 to the rich side. 14.7 idle shows 14.5 on my wideband.
    I second that. 14.5 after the cat.

  12. #12
    Супер Модератор EC_Tune's Avatar
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    At cruise/decel the cats will affect your measurements since that is where they are most effective. They kind of act like a sponge. Your AFR won't vary much after the cat except at WOT or if you have a really rich/lean mixture.
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    If its generally accepted to meter O2 using a WBO2 at the tailpipe on the dyno, what's the diff with bringing it forward to the rear of the CAT? I can't see any...

    CATS dont work at WOT and work best at stoich. At WOT, the only thing they do is add restriction and heat. If a WBO2 is prone to bias as temps rise, then that's the ONLY thing I can think of that would put off an O2 reading measuring just after that CAT.

    It could also be argued that metering at the tailpipe is when the exhaust gases have cooled way down and hence will show an off reading too. if anything, I would suggest post-CAT would register leaner at cruise that pre.

  14. #14
    Tuner in Training Judge Smales's Avatar
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    So can we use them after the cat or is still advisable to use before the cat?
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  15. #15
    Супер Модератор EC_Tune's Avatar
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    While tuning (using the wideband) - pre-cat.
    WOT only, pre or post cat. However there can be issues with the tailpipe readings in general.
    One other issue with a post cat (tailpipe) reading is the time lag involved. The AFR will be skewed in time more at the low end than the top end.
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  16. #16
    Tuner alanderson1978's Avatar
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    I still think there's a problem with a WBO2 after the cat. The reactions that go on in a cat can release from NOx emmisions or use it to change CO to CO2.

    I put my WBO2 after my turbo. since it's an STS, the sensor is waaaay back there, and is after the cats. I have been having trouble getting the tune to act the way it should, the results of a change to the tune are not the expected results. My plan right now is to remove the cats and finish wrapping the exhaust to keep the exhaust as hot as possible for spooling, but also because the WBO2 may be affected by colder exhaust.

    I should also try a new sensor. Where's a good place to buy a Bosch WB sensor?


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    Last edited by alanderson1978; 09-01-2007 at 09:23 PM.
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  17. #17
    Супер Модератор EC_Tune's Avatar
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    In your case you HAVE to put it at the tailpipe. Transport delay is an issue with the STS systems.
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  18. #18
    Tuner alanderson1978's Avatar
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    I went to a local (75 miles) shop a few days ago, and they told me they figure about 3% error with the WB after the cats. They work on STS kits regularly.
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  19. #19
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    Quote Originally Posted by EC_Tune
    While tuning (using the wideband) - pre-cat.
    WOT only, pre or post cat. However there can be issues with the tailpipe readings in general.
    One other issue with a post cat (tailpipe) reading is the time lag involved. The AFR will be skewed in time more at the low end than the top end.
    even if you *should* put it "pre cat" while tuning, there's no *harm* in having it post-cat... as your reading would (theoretically) be leaner so you would be tuning slightly rich - no harm in that.
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  20. #20
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    Here's some comparison data from yesterday. LM1 is passenger post-cat, dynojet was driver pre-cat. Rounded to the closest logged sample.

    Code:
    Time	RPM	TPS	LM1	DynoJet	Diff	%
    18:00.2	2667	100	12.53	12.33	0.2	0.02
    18:00.5	2752	100	12.7	12.34	0.36	0.03
    18:00.7	2836	100	12.91	12.206	0.704	0.05
    18:00.9	2924	100	12.95	12.318	0.632	0.05
    18:01.0	3019	100	12.93	12.52	0.41	0.03
    18:01.2	3112	100	12.88	12.789	0.091	0.01
    18:01.5	3209	100	12.91	12.851	0.059	0.00
    18:01.7	3309	100	12.91	12.851	0.059	0.00
    18:01.8	3408	100	12.93	12.838	0.092	0.01
    18:02.0	3504	100	12.85	12.692	0.158	0.01
    18:02.3	3605	100	12.76	12.655	0.105	0.01
    18:02.4	3701	100	12.71	12.718	0.008	0.00
    18:02.6	3801	100	12.7	12.704	0.004	0.00
    18:02.8	3906	100	12.68	12.703	0.023	0.00
    18:03.0	4003	100	12.72	12.718	0.002	0.00
    18:03.2	4102	100	12.67	12.643	0.027	0.00
    18:03.4	4194	100	12.67	12.651	0.019	0.00
    18:03.6	4294	100	12.55	12.802	0.252	-0.02
    18:03.8	4407	100	12.6	12.764	0.164	-0.01
    18:04.0	4510	100	12.55	12.697	0.147	-0.01
    18:04.2	4618	100	12.63	12.629	0.001	0.00
    18:04.4	4717	100	12.74	12.68	0.06	0.00
    18:04.6	4828	100	12.82	12.794	0.026	0.00
    18:04.8	4932	100	12.85	12.959	0.109	-0.01
    18:05.0	5026	100	12.83	13.055	0.225	-0.02
    18:05.2	5132	100	12.88	13.122	0.242	-0.02
    18:05.4	5231	100	12.87	13.138	0.268	-0.02
    18:05.6	5329	100	12.99	13.09	0.1	-0.01
    18:05.8	5433	100	13.04	13.055	0.015	0.00
    18:06.0	5525	100	13.09	13.106	0.016	0.00
    18:06.2	5626	100	13.06	13.156	0.096	-0.01
    18:06.4	5723	100	12.99	13.185	0.195	-0.02
    18:06.6	5814	100	13.08	13.211	0.131	-0.01
    18:06.8	5908	100	13.11	13.206	0.096	-0.01
    18:07.0	6001	100	13.2	13.15	0.05	0.00
    18:07.2	6091	100	13.06	13.001	0.059	0.00
    18:07.4	6182	100	13.08	12.911	0.169	0.01
    18:07.6	6275	100	12.98	12.9	0.08	0.01
    18:07.8	6354	100	12.96	12.838	0.122	0.01
    18:08.0	6444	100	12.95	12.824	0.126	0.01
    18:08.2	6523	100	12.91	12.82	0.09	0.01
    18:08.4	6608	100	12.89	12.719	0.171	0.01
    18:08.6	6693	100	12.82	12.07	0.75	0.06
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