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joeycuccaro
08-01-2009, 08:12 AM
Piggie Pipes and O2 Sensors: How to avoid a CEL and Information

This guide is designed for individuals who gut ALL FOUR CATS / ONLY 2 CATS (PRE-CATS)

There have been numerous threads lately asking how one can avoid a CEL with the piggie pipe route. I will explain a method that will work for everyone if they do indeed go this route. I will also try and explain the logic behind the modification.

Let’s talk about what Piggie Pipes are. The stock exhaust has two CATS on each downpipes. The front most CAT is the pre-cat (closest to the motor) while the one further down is the main CAT. CATS are very restrictive components to an exhaust, but they help reduce emissions that are mandated by law. The cats in the exhaust act like sponges, slowing down the airflow in the exhaust. Think of it as placing a sponge under a stream of water from your faucet. The stream of water is absorbed and slowed by the sponge. This is exactly the job of a catalytic converter.

Now when we remove the cats, creating "Piggie Pipes," the air is less restricted and therefore flows quicker. In simple terms, the O2 sensors are designed to measure the speed of the airflow through the cats. If we remove the cats, the increased airflow will trigger the CEL, because the computer is sensing the CAT is not doing its job.

http://www.euroaddiction.net/forum/picture.php?albumid=89&pictureid=680

How can I get Piggie Pipes but avoid a CEL?
The trick to eliminate a CEL is to use O2 spacers also known as O2 foulers. To make this procedure more simple, I have decided to post this information based off the VAST O2 Spacers. I have found that you only need two total (1 per side) in order to avoid a CEL with either 2 CATS gutted or ALL 4 Cats gutted.

You will need to put the spacers in the main cat bungle on each downpipe. The reason we use spacers is to try and mimic the equation that was present with stock cats. The O2 sensor is set to read a certain air speed. By removing the CATS we greatly increase that speed, which causes a CEL that I explained before. By moving the O2 sensor farther away from the bungle with a spacer it will reduce the airflow to the sensor, therefore giving us a similar equation as if we had cats. One VAST spacer per side is enough to produce the same results as if we had cats.

http://vastperformance.com/shop/images/VAST_CELiminator_web.jpg

To solve the problem we simply screw in one spacer per side to the main cat bungle and then screw the O2 sensor into the end of the spacers.

You might also have to loosen the zip ties holding the O2 sensor wire to the transmission to free up some slack to properly insert the O2 sensors. I also recommend coating the threads of the O2 sensor and O2 spacer with Anti-Seize to prevent them from rusting in place.

http://www.expeditionexchange.com/rockware/antiseize%20006.jpg

That's it! Say good-bye to the CEL.

What will I gain from Piggie Pipes?
By doing the Piggie Pipes you will expect to gain around 10-15hp. The increased air-flow will allow the engine to rev faster. You will notice quite a difference during pulls. The sound increase is also another great product of piggie pipes.

Video of my exhaust setup
Labree Non-Resonated with Piggie Pipes (http://www.youtube.com/watch?v=-A5NYqpVn30)

Application with Aftermarket Downpipes
It is also to my knowledge that this modification will also work for gutted aftermarket downpipes. However, some aftermarket downpipes have different angles for the O2 bungles. For example, the design of FI downpipes makes it hard fit in normal O2 spacers. If you are having this problem you can purchase 90 degree foulers as shown below.

http://ep.yimg.com/ca/I/yhst-1918367471896_2087_17863191

Applied on FI downpipes thanks to Greg (mad70sx)

http://i586.photobucket.com/albums/ss305/mad70sx/IMG_3704.jpg
http://i586.photobucket.com/albums/ss305/mad70sx/IMG_3703.jpg
http://i586.photobucket.com/albums/ss305/mad70sx/IMG_3705.jpg


Any questions please feel free to ask. If there is something I missed, didn’t explain clearly or if you have another insight please do not hesitate to post. I am not an expert, just a person trying to help this community.

Enjoy!

mr. d
08-01-2009, 09:49 AM
Good write-up. Thanks for the effort.

I thought "piggie pipes" meant you only gutted the pre-cat. If I understand you correctly, piggy pipes means that all cats are gutted?

sear
08-01-2009, 10:04 AM
I'm considering the piggies with just the pre-cat removed. CEL possibilities is the main thing holding me back at this point.

Anyone running just the pre-cat removed? If so any CEL issues? Also, how complicated is the install. Will likely not DIY, what kind of shop would be best to install these?

UnstableOS
08-01-2009, 10:19 AM
What failure codes were you experiencing? I am still getting post cat codes that I cannot seem to eliminate..

sear
08-01-2009, 10:24 AM
Good write-up. Thanks for the effort.

I thought "piggie pipes" meant you only gutted the pre-cat. If I understand you correctly, piggy pipes means that all cats are gutted?

If I understand the term correctly, it can be either A) Just the pre-cat gutted or B) both. The OP's good write-up seems to address only option B.

FarmerBob
08-01-2009, 10:42 AM
Good write up Joey. Mine is gutted pre-cats, intake main cats, o2 spacers, JHM tune, everything ok.

Woodhead2k
08-01-2009, 11:49 AM
Great write up! So you have to put a spacer on top of a spacer then screw the sensor into the drilled upper spacer? Hope that makes sense.

Justincredible
08-01-2009, 11:56 AM
Yes but if you look at the pic he posted these Dp's are modified for the man cat and you wouldn't need to do any thing here. You can see the 02 was already moved.

If you had these Dp's (in the pic) with the main cat you would be all set. the 02 is already moved for you.

Justincredible
08-01-2009, 12:03 PM
here look and see the original locations and then the moved location. The pic below is actual piggie modified Dp's with main cats. these will not get a cel if done this way if you have the cats in.

Notice how the 02 bung location on these is behind the cat. This will pass Ca emissions and not get you a CEL if you have the cats in
Here is one of the kickers. You can make the Dp's obviously your elf if you move the bungs and not get a CEL. But placement will effect your mileage.

As joey said if you put your spacers in on catless Piggies you will be fine. This will only work real well on piggies.

Remember joey has laid this out VERY well. you want to create a spacing of the 02 sensor or mimic the speed/volume of air normally passed by the 02 with cats. This is why when you try this on aftermarket Dp's it will work but for only a little while. Using the stock Dp's helps keep every thing in its location all that's left for you to do is to get the spacing rite............well not anymore JOEY did that for you

http://www.audizine.com/gallery/data/500/VAST_Piggie_Pipes_B6-B7_S41.jpg

beemercer
08-01-2009, 12:30 PM
Since there was a bit of confusion in what exactly piggy pipes are I'll post my personal definition:

Anyways, "piggy pipes" essentially refers to the modification of factory downpipes for performance gain; the term migrated over from the B5 S4 section where it was a very common modification. To this end the modification (within the B6/B7 S4 performance scene) typically refers to the removal of only the pre-cat from each of the stock downpipes. This ensures continued emissions compliance, but still allows for a significant performance boost (due to lower exhaust restriction, and more advanced timing due to lower cylinder temperatures). As an option though it is possible to have both catalytic converters removed from each factory downpipe, in order to further reduce exhaust gas restriction, and increase performance at the cost of emissions non-compliance. The extra performance gained by going full catless when having piggies made is typically too slight for most consumers to risk the emissions non-compliance; resulting in the large number of pre-cat gutted piggie users you see on AZ.

UnstableOS
08-01-2009, 02:17 PM
So the CELs prior to the relocation were post-cat failure codes? Could someone please elaborate on this?? I am having a similar issue and need a bit of help correcting..

joeycuccaro
08-01-2009, 02:25 PM
Hey guys,

Sorry for the confusion but it seems the picture i used this morning was of the VAST piggies where they relocate the O2 bungles. I did not notice this at first but it is now correct for my purpose.

Please note that this information deals with STOCK piggie pipes, not the VAST ones. As it has been mentioned, VAST relocates the O2 bungles shown in Justin's post. My method will not work on VAST modified piggies since they are located farther down (past) the main cat. Please only use this guide if you have your O2 sensors located in the stock position on the main cat.

Woodhead2k
08-01-2009, 04:48 PM
Hey guys,

Sorry for the confusion but it seems the picture i used this morning was of the VAST piggies where they relocate the O2 bungles. I did not notice this at first but it is now correct for my purpose.

Please note that this information deals with STOCK piggie pipes, not the VAST ones. As it has been mentioned, VAST relocates the O2 bungles shown in Justin's post. My method will not work on VAST modified piggies since they are located farther down (past) the main cat. Please only use this guide if you have your O2 sensors located in the stock position on the main cat.

Explains everything haha, thanks for clearing it up I was beginning to think I was retarded.

audis4b6
08-02-2009, 09:25 AM
awesome write up joey!!!!!

Bravest NY S4
08-02-2009, 09:56 AM
Well done joey[up][up]

joeycuccaro
08-03-2009, 07:14 AM
Thanks guys! Im going to get pictures of the spacers on the main cat as soon as i jack the car up again.

ess_four
08-03-2009, 07:58 AM
I have straight dP's and 100 cell main cats. The main cat sensors are currently located before the main cats. I used to get the occasional CEL but I have not had one for a while. Car runs fine, although a bit rich.

Is it better to relocate the main cats sensors to the rear?

Thanks in advance

B6guy42
08-03-2009, 09:12 AM
Hey joey,

If I take my exhaust off completely and run nothing from the exhaust manifold back, will this work? KEEDING.

WinterRunner
08-03-2009, 10:02 AM
Just to let you know, I had NO LUCK with VAST spacers, and even Wayne's 90 degree spacers threw CEL's after 5K miles or so. I think it's more than that, but maybe my S4 was just picky....

joeycuccaro
08-03-2009, 10:05 AM
Just to let you know, I had NO LUCK with VAST spacers, and even Wayne's 90 degree spacers threw CEL's after 5K miles or so. I think it's more than that, but maybe my S4 was just picky....

Do you have VAST piggies or custom?

dextrek
09-01-2009, 02:21 PM
Do you have VAST piggies or custom?

It's the APR DP's. BTW, I have Winter's S now. But yea, it still throws the code. Here is it. I asked JHM and it might be the bad O2 sensor. They said I can get chipped and get rid of the CEL, but they still recommended me to change the O2 sensor.

5 Faults Found:
16523 - Oxygen (Lambda) Sensor B1 S2: Response too Slow
P0139 - 001 - Upper Limit Exceeded - Intermittent
18528 - Post-Catalyst Oxygen (Lambda) Sensor Correction; Bank 1: Too Lean
P2096 - 002 - Lower Limit Exceeded - MIL ON
16544 - Oxygen (Lambda) Sensor B2 S2: No Activity
P0160 - 004 - No Signal/Communication - Intermittent
16524 - Oxygen (Lambda) Sensor B1 S2: No Activity
P0140 - 004 - No Signal/Communication - Intermittent
16543 - Oxygen (Lambda) Sensor B2 S2: Response too Slow
P0159 - 004 - No Signal/Communication - Intermittent
Readiness: 0000 0000

brad65ford
09-08-2009, 07:38 PM
Anybody else do their own piggie pipe themselves with good results. I'm still wanting to try modifing the stock pipe myself. I can't see paying someone for removing the precats and welding a new bung. But if there's more to it than that, I'll send them in to jhm.

ngng
09-08-2009, 07:48 PM
You can also just get a chip.

hot2trots4
09-09-2009, 01:07 PM
I have seen that this only works on the 04s and some 05s.

I just had JHM turn it off in my tune. No catalyst efficiency codes since.

jfunkey
09-09-2009, 02:44 PM
I talked to a fella that has a 07 and this didn't work. any Ideas on what we can do.

justinperkins
09-09-2009, 07:44 PM
Thanks for this post Joey but I have one outstanding question after reading the other well-informed posts in this thread ...

If the pre-cats can be gutted but the car can still pass an emissions test (as Justincredible and beemercer have stated) then what is the function of the pre-cats? Has anyone reported difficulties passing a standard tailpipe sniff test with gutted pre-cats (CEL notwithstanding)?

jfunkey
09-09-2009, 07:46 PM
Go back and read justin's post again. he talks about it and explains it in there.

justinperkins
09-09-2009, 08:11 PM
Go back and read justin's post again. he talks about it and explains it in there.

I must be missing something here but all I see in his post is an explanation of how you would move the O2 to behind the main cats which would allow you to pass an emissions test.

I would have expected tailpipe emissions to increase with gutted precats, which would cause a test failure. Since that appears to not be the case (once you've relocated the O2 sensor), my only guess is the ECU alters fuel/air ratio to retain factory emissions with gutted precats.

If that is the case and as Justin says, MPG will be affected, how much are we talking about here?

beemercer
09-09-2009, 08:50 PM
Thanks for this post Joey but I have one outstanding question after reading the other well-informed posts in this thread ...

If the pre-cats can be gutted but the car can still pass an emissions test (as Justincredible and beemercer have stated) then what is the function of the pre-cats? Has anyone reported difficulties passing a standard tailpipe sniff test with gutted pre-cats (CEL notwithstanding)?

The pre-cats are there primarily for emissions compliance when the motor is running cold. Since they are closer to the motor they trap more heat allowing the catalytic reactions to occur, primarily the catalytic oxidation of excess hydrocarbons into water and CO2 and the oxidation of CO into CO2 (cold motors run ultra-rich and produce massive amounts of CO and unburnt HC's); the heat is necessary to reach the minimum activation energy of the various reactions. As soon as the motor reaches operating temperatures the main cat located further downstream will be at a great enough temperature for the catalysts to operate. So basically the pre-cats are there to solve an emissions issue that lasts for maybe 30 seconds in summer and 5-10 minutes in winter (my guesstimations on the times don't hold me to it scientifically).

The precat is just like a lot of emissions equipment (upstream SAI) that can be bypassed since it only really affects the motor at below operating temperatures. As long as you get your car nice and hot before going in for the sniffer you should be fine.

If you are interested I could do a writeup on how catalytic converters work, issues with AFR, issues with operating temps, etc. Basic emissions background information (I'm going to school for Chemical Engineering focusing on Environmental issues).

joeycuccaro
09-09-2009, 09:01 PM
The pre-cats are there primarily for emissions compliance when the motor is running cold. Since they are closer to the motor they trap more heat allowing the catalytic reactions to occur, primarily the catalytic oxidation of excess hydrocarbons into water and CO2 and the oxidation of CO into CO2 (cold motors run ultra-rich and produce massive amounts of CO and unburnt HC's); the heat is necessary to reach the minimum activation energy of the various reactions. As soon as the motor reaches operating temperatures the main cat located further downstream will be at a great enough temperature for the catalysts to operate. So basically the pre-cats are there to solve an emissions issue that lasts for maybe 30 seconds in summer and 5-10 minutes in winter (my guesstimations on the times don't hold me to it scientifically).

The precat is just like a lot of emissions equipment (upstream SAI) that can be bypassed since it only really affects the motor at below operating temperatures. As long as you get your car nice and hot before going in for the sniffer you should be fine.

If you are interested I could do a writeup on how catalytic converters work, issues with AFR, issues with operating temps, etc. Basic emissions background information (I'm going to school for Chemical Engineering focusing on Environmental issues).

Great information man. You seem to be really knowledgeable and interested in the exhaust set-ups for these cars. Very nicely put.

justinperkins
09-09-2009, 09:01 PM
Thanks beemercer, that's exactly what I wanted to know. I'm very interested in converting my DPs to piggies but don't want to increase my emissions in such a way that would cause me to fail an emissions test (in CA for example). As you said, as long as the engine is nice and hot, all will be fine. I generally get my engine warm before an emissions test anyway because I learned a long time ago that a cold cat won't do its job like a hot cat will. I've failed more than one test because I just replaced a cat and went straight to the emissions test without warming the car up.

I thought SAI on its own provided enough to get the main cats working properly at cold temps, but I guess auto manufacturers are always taking it a step further and had to give us precats too.

beemercer
09-09-2009, 09:51 PM
Thanks beemercer, that's exactly what I wanted to know. I'm very interested in converting my DPs to piggies but don't want to increase my emissions in such a way that would cause me to fail an emissions test (in CA for example). As you said, as long as the engine is nice and hot, all will be fine. I generally get my engine warm before an emissions test anyway because I learned a long time ago that a cold cat won't do its job like a hot cat will. I've failed more than one test because I just replaced a cat and went straight to the emissions test without warming the car up.

I thought SAI on its own provided enough to get the main cats working properly at cold temps, but I guess auto manufacturers are always taking it a step further and had to give us precats too.

Upstream SAI is in place to provide the extra oxygen necessary for the catalytic oxidation reactions to occur. Without the extra oxygen the reactions cannot occur. BUT the reactions will not even begin to occur until the minimum activation energy is reached, and the main determinant of U (internal energy) is the temperature and pressure of the exhaust gases along with their kinetic energy. Essentially though the temperature is the only factor. So you can keep pumping in the extra oxygen, but if the gases aren't hot enough you can't overcome the activation energy and begin the reaction. It is akin to pushing a boulder up a 5ft hill to let it fall off a 50ft cliff, you need a bit of energy initially to start the reaction up, but once you reach the top of the mound it goes (unfortunately these reactions are exothermic, otherwise they would act as a post combustion chemical heat pump).

beemercer
09-09-2009, 09:59 PM
Great information man. You seem to be really knowledgeable and interested in the exhaust set-ups for these cars. Very nicely put.

No problem, anything I can do to help. Emissions and gas flow are a great hybrid of my passion (cars) and my academic pursuit (Chemical Engineering). I'd love to work somewhere in the automotive field related to emissions or alternative fuels for ICE's. So really any questions you guys have regarding emissions, exhausts, chemistry, use of alternative oxidants and fuels, etc feel free to ask and I will do my best to give you an answer from my acedemic background (what I say may not be the standard thought in the automotive world though).

brad65ford
09-10-2009, 04:21 AM
The pre-cats are there primarily for emissions compliance when the motor is running cold. Since they are closer to the motor they trap more heat allowing the catalytic reactions to occur, primarily the catalytic oxidation of excess hydrocarbons into water and CO2 and the oxidation of CO into CO2 (cold motors run ultra-rich and produce massive amounts of CO and unburnt HC's); the heat is necessary to reach the minimum activation energy of the various reactions. As soon as the motor reaches operating temperatures the main cat located further downstream will be at a great enough temperature for the catalysts to operate. So basically the pre-cats are there to solve an emissions issue that lasts for maybe 30 seconds in summer and 5-10 minutes in winter (my guesstimations on the times don't hold me to it scientifically).

The precat is just like a lot of emissions equipment (upstream SAI) that can be bypassed since it only really affects the motor at below operating temperatures. As long as you get your car nice and hot before going in for the sniffer you should be fine.

If you are interested I could do a writeup on how catalytic converters work, issues with AFR, issues with operating temps, etc. Basic emissions background information (I'm going to school for Chemical Engineering focusing on Environmental issues).

Makes perfect sense, now I have the million dollar question. If I were to removed the material in the pre cats (gutting them out) where to I need to place the O2 sensor so I wont have a cel light? There is 4 o2 sensors correct? The picture shows the pre cat 02 stock location but doesn't show if they are relocated, are they? Or is just the main cat 02 sensors relocated?

joeycuccaro
09-10-2009, 06:48 AM
Makes perfect sense, now I have the million dollar question. If I were to removed the material in the pre cats (gutting them out) where to I need to place the O2 sensor so I wont have a cel light? There is 4 o2 sensors correct? The picture shows the pre cat 02 stock location but doesn't show if they are relocated, are they? Or is just the main cat 02 sensors relocated?

Correct, there are a total of 4 O2 sensors on the car. From what i understand VAST relocates the Main sensor bungle back to the latter position shown in the picture.

I figured out a setup, however, where if you gut all four cats you dont need to relocate and O2 sensor bungles.

brad65ford
09-10-2009, 06:56 AM
Correct, there are a total of 4 O2 sensors on the car. From what i understand VAST relocates the Main sensor bungle back to the latter position shown in the picture.

I figured out a setup, however, where if you gut all four cats you dont need to relocate and O2 sensor bungles.


Joey, I found this on youtube, sounds very nice![up]
http://www.youtube.com/watch?v=-A5NYqpVn30

joeycuccaro
09-10-2009, 07:22 AM
Joey, I found this on youtube, sounds very nice![up]
http://www.youtube.com/watch?v=-A5NYqpVn30

Haha, Thanks! Im dying to hear what it sounds like with Fast Intentions 2.5" downpipes.

avant1
09-16-2009, 01:09 PM
where did you get the 90 degree spacers? got pics?

joeycuccaro
09-16-2009, 02:10 PM
where did you get the 90 degree spacers? got pics?

I did a quick search and found them here...

90 Degree Spacer (http://yhst-1918367471896.stores.yahoo.net/oxsesp.html)

joeycuccaro
09-18-2009, 05:39 PM
Updated with more pics

Krueger
09-25-2009, 03:18 PM
I've got a Milltek non-res catback and APR downpipes. Like others, I didn't have any luck with the VAST O2 spacers. They would throw a CEL after ~50 miles. I've had much better luck with the Wayne angle block.

Incase somebody doesn't know what it looks like:

http://www.42draftdesigns.com/images/faq/wayne92slc.jpg

Email Wayne at wayne92SLC@yahoo.com and he'll hook you up.

I've only gotten a CAT efficiency CEL maybe a dozen times and I've had the angles blocks on for about 10,000 miles.

I invested in a code scanner when I had the VAST eliminators in order to delete the recurring CAT codes. It is absolutely worth it. I bought one from summitracing for $70 and I keep it in the bin under the driver's seat incase a code pops up. Here is the one I bought:

http://www.summitracing.com/parts/AXY-AX700/

Blackcoog
09-30-2009, 07:08 AM
Are the O2's in the S4 the same as any other Vehicle? On other cars I've worked with (Fords) you can wire a resistor and a capacitor into the O2 wiring which delays the signal back to the ECU as if the precats were still in place. I'd imagine that would work on the S4. Just a thought for you having issues with spacers. I can post up a schematic for those interested.

WinterRunner
09-30-2009, 09:36 AM
Wayne's are def better than VAST's but I would still get CEL's sometimes.

durrmcdanks
09-30-2009, 09:39 AM
Wayne's are def better than VAST's but I would still get CEL's sometimes.

How often compared to Vast? I would get a CEL every 20 to 30 miles with my Vast spacers.

WinterRunner
09-30-2009, 11:26 AM
How often compared to Vast? I would get a CEL every 20 to 30 miles with my Vast spacers.

I'd say 2-3 times a month so maybe every 500-1000 miles. Not really sure, but it def works better than VAST's.

Old Baldy
12-08-2009, 09:14 PM
OK, so I have a few Q's on this whole setup.

I have some (little!) experience with wide band O2 sensors (mainly the universal Bosch LSU4 pump series O2 sensors) and data logging of lamba, RPM, TPS and temp values, for my track Ducati motorcycles - used to build my own Magneti-Marelli ECU maps onto blank 512KB EEPROM chips, usuing a Moates chip burner. Data logging is via the tiny Zeitronix WBO2 controller with serial logging enabled to drive a Palm III PDA logging function of Lambda, RPM, TPS, EGT/Temps.

My questions related to this O2 sensor, CEL and piggy DPs are:

1. Are the Audi sensors not simply O2 Wide or narrow-band O2 sensors that measure the level of O2 in the exhaust?
2. If so...what on earth does this have to do with "air speed?"
3. If not...these sensors are actually not O2 sensors, but some tpe of "air speed" or velocity sensor? Is there any information or link to these sensors, which describes how they work? (My apologies, I have little car exhaust experience....!!!
4. If they are indeed simple, normal O2 sensors, then how does the "spacer" work in this context? USUALLY....an O2 spacer is used to isolate the delicate O2 sensor from the overly hot exhaust gasses and so reduce the heat input into the O2 sensor when it is expected to be subjected to overly hot exhaust gasses (either located very close to the exhaust valve, or from turbo/supercharger streams, etc.....and nothing to do with actual air flow volume.
5. If these are simple O2 sensors, are they wide or narrow band?
6. Do they feed back into the ECU for actual fuel/spark map updates (Closed loop mode?)
7. If wide band...are they in constant closed-loop mode, or partial closed loop for cruise and economy / light throttle feedback only?

My assumptions (if these sensors are indeed simply O2 sensors)....

1. A spacer is never going to be a permanent solution to avoid a CEL if the O2/lambda values are out of ECU expected range. The spacer MAY delay the signal from the sensor pump, but I would expect that after some time, the out-of-range O2 (due to missing or relocated cat) will be highlighted with a CEL. It's kinda like assuming that you could blow SLOWLY into a breathalyzer after 10 beers and not eventually blow a red light. If you keep on blowing, even less than full volume...you're gonna get caught with the sensor.

2. If there is ECU closed-loop feedback....which sensor provides this data? The second sensor at the main cat? If the 1st sensor simply triggers a CEL and does not provide lambda/AF feedback to the ECU, then could you not simply replicate the expected sensor output voltage and remove that first sensor entirely - and avoid the CEL? Can't recall the normal Bosch WB02 output voltages but as they are 0-5V range sensors, would expect that stoichiometric range is in the 2.4- 2.6V range, from a constant 5V input signal? That should be fairly easy to replicate, to avoid a CEL?


Excuse my ignorance on this topic....but I see one or two guys that seem to know this stuff on this thread, and would really like to understand how the Audi system works, and also how this "spacer" deal is supposed to work.

Appreciate the advice/info!

coltonk
12-13-2009, 03:01 PM
if you get a catback system does that attach to piggies or replace piggies?

joeycuccaro
12-13-2009, 03:26 PM
if you get a catback system does that attach to piggies or replace piggies?

Attaches to the downpipes. Baldy when i get a little bit of time i will comment on your questions.

Old Baldy
12-26-2009, 12:07 PM
Joey....just a friendly "bump" here, in case you've forgotten about the advice/follow-up (I know you're probably busy as hell, so excuse my little nudge here :) )

AJ88CAB
12-28-2009, 01:09 PM
Old Baldy - I think you are correct in your thinking. As I understand with the "dual O2" Sensor system...the first O2 sensor (before the pre-cat) does the majority (all?) of the exhaust gas sensing and signaling to the ECU for engine management. That is why it is ahead of the CAT.

The second O2 sensor located before the primary CAT is provided to check for a differential between the two O2 sensors. If it sees a differential it assumes the CATs are working. It is just checking for the proper function of the pre-cat...the assumtion being...if the pre-cat is working, the primary is probably still working too...I guess. Questionable engineering logic if you ask me but who am I to question the Audi engineers[rolleyes]

In any case, I believe the use of the spacers basically moves the second O2 sensors away from the exhaust flow enough so in some cases there is enough differential that the system thinks there is a functioning CAT between the sensors. It seem to work fine on some cars, but not on others. It probably has to do with the condition of the O2 sensors, but I am guessing a bit now.

The only 100% sure way to not throw a code with piggies is to relocate the primary CAT sensor bung to after the primary cat as shown on the image of the VAST piggies. I can't imagine a muffler shop charging much to do that if you are thinking about doing the piggies yourself. I have gutted my pre-cats that is what I am going to do with mine...Just haven't checked with any shops yet. I just don't want to take the chance of reinstalling and having problems.

The other 100% sure option is modified programming to remove the main CAT O2 sensors from the mix...since they do not really contribute to engine management anyway.

The disclaimer... my S4 is a B5 series and my rambling comments are based on that car's systems. I am assuimg it is similar for the B6 platform...