CATs and Cobbs, O2 sensors???

[RM1SepEx]

Looking for Exhaust help... I'm using a Cobb Accessport 3 and plan on running stage 2 tune.

I have an unequal length, ceramic coated header, has both O2 senser and exhaust temp probe in the up pipe. I've welded up the exhaust shown below: 3 inch SS bell mouth, down into high flow CAT, 90 degree 3 inch SS elbow into straight thru, center in, bottom out SS muffler. I'm not quite sure how much bend there will be in the exhaust tip and how far up the right rear corner it exits... FFR did one with the pipe coming out through a small aluminum panel...

The stock Subaru setup includes one more O2 sensor. I bought a bung, where do I put it? Do I use an OEM replacement? Pre Cat, post cat in the elbow?

Should I add a secondary O2 bung for a wide band for tuning? If so where would you put it?


35159[/ATTACH ] [ATTACH=CONFIG]35160 35161

[Goldwing]

Many aftermarket downpipes, including the crash damaged cobb I used, put the wideband bung in the bellmouth. My cobb had the oem o2 bung right after the high flow cat. It seems to vary how far back they put it. Don't forget to see how far the o2 sensor wire reaches when choosing a location, or whether it will interfere with anything once installed. I almost made the latter mistake.

[RM1SepEx]

Widebands don't like high temps... I'd expect it to be too hot if placed in the bellmouth. Can the Cobb Accessport actually use a wideband?

[longislandwrx]

I have mine in the bellmouth and it has held up well.

Some of the widebands have a narrowband simulator that you can use to feed the cobb/ecu. You can get tricky with the wiring and have the ECU monitor wideband signal, there are a few ways to do it, I think most people use the TGV wiring. sponaugle would be the guy to ask.


Edit 10/28/14

worth mentioning, i'm on my 3rd manifold o2 sensor, and still my original wideband.

[sponaugle]

Wideband AFR sensors are commonly placed in the downpipe behind the turbo. I have only seen a few cases where heat was a problem, and in both of those cases the sensor was just an inch past the back of the turbo and the exhaust was VERY restrictive (so there was measurable pressure build up). Wideband AFR sensors are more sensitive to pressure conditions then temperature. I have a wideband in the downpipe of all three of my turbocharged Subarus, as well as both downpipes in my TT GTO.

In the factory Subaru setup there are two oxygen sensors. There is a factory 'semi-wideband' sensor installed in the exhaust header before the turbocharger, and that sensor has internal compensations in the ECU for the additional pressure (back pressure ) from the turbocharger. After that 'semi-wideband' sensor there is a second normal 'narrowband' sensor that is installed after the cat. Most cars that are being tuned also install a third 'real' wideband sensor in the down pipe after the turbocharger and before the cat.

The factory 'semi-wideband' sensor (which is in the exhaust header) is used for closed loop fueling. I refer to it is a 'semi-wideband' because it does more then the normal narrow band 0-1V sensor. It provides a reasonably accurate AFR reading from about 12:1 to 16:1. Much richer and it gets very inaccurate, as well as much leaner. While it has compensations for the pressure environment in the header, those compensations make it very inaccurate at high load. As such, the Subaru ECU uses it only at cruise and medium load. You should have this sensor installed and working.

The 'rear' O2 sensor is used to detect the presence and operation of the catalytic converter. You can optionally delete this sensor but you will have to make a few tweaks in the ECU. It is not directly used for fueling compensation, although there are a few minor fueling corrections that can occur if it is connected but not working correctly ( see 'Corrections #3' ).

Most wideband O2 sensor also have a 'simulated' narrowband output, and this output can be used in place of that rear O2 sensor. Keep in mind that this simulated output will probably not be delayed enough to trick the ECU into thinking there is a CAT present, so you may have to adjust the P0420 code.

If you are looking for a surefire solution, I would put a bung in the downpipe for a real wideband sensor, and a second bung after the CAT for the factory rear O2 sensor. This means you will have three sensors installed in the car; The factory front sensor, your aftermarket downpipe wideband sensor, and the rear narrowband O2 sensor.

The Cobb software can interface with many of the common wideband sensors (PLX, Innovative, etc). It is done in the tuning software, not the accessport. It is possible to connect the output of the wideband sensor to one of the 0-5V TGV inputs so it can be logged in the Accessport, assuming you have deleted the TGV inputs.

I personally would never ever build a car without a real aftermarket wideband sensor. It would be akin to spending thousands of dollars building an engine and putting gasoline in it from a can you found in a barn that was unoccupied for 20 years. Do you want to find out that tune is a bit too lean by having a gauge tell you, or would you prefer to have a rod punch a hole in your block as an announcement. :)

Jeff

[RM1SepEx]

I didn't delete the TGVs yet :( tho I do plan on doing it... Your theory lines up with my dyno owning friend that is using several diff methods of tuning his fleet of turbo cars. He has had bad luck with heat and the widebands but as designed my exhaust has very little restriction, only the high flow flowmaster cat's 3/12 thick x 5 inch diameter core. The pipe is all 3 inch stainless and the muffler would allow you to roll a ball through it, the jog inside is only about 4 inches.

I'll go with the dual bungs, one in the bell mouth, the other between the cat and muffler. As usual, valuable help/expertise from this forum!

Thanks

[sponaugle]

Your theory lines up with my dyno owning friend that is using several diff methods of tuning his fleet of turbo cars. He has had bad luck with heat and the widebands but as designed my exhaust has very little restriction, only the high flow flowmaster cat's 3/12 thick x 5 inch diameter core.
Thanks


There were some particular problems that came to light with the first innovative LM1 wideband controller.

From the Innovative website:

Q: When do I need to make or install a heat sink?

A: The Bosch LSU4.2 wide-band O2 sensor (shipped as part of the LM-1 kit) is rated to operate at an exhaust gas temperature of < 1300 degrees (F), and a sensor housing temperature of < 900 degrees (measured at the bung) for maximum accuracy and control. When either of these operating temperature ranges is exceeded, the sensor can no longer be accurately controlled. Further, operating at or over these temperatures for any length of time can significantly reduce the lifetime of the sensor. The LM-1 is designed to display an error message under these conditions (currently 08- Sensor Timing Error) rather than provide inaccurate readings. For some turbo vehicles, rotary engines, and other setups, this error message can be encountered with annoying frequency.


They actually sell a heat sink:

http://i901.photobucket.com/albums/ac215/tradeworkz/tradeworks/HBX1.jpg

However, I would be highly suspect if the temperature in your downpipe was over 1300 degrees F. Exhaust temps right at the head/manifold junction are usually in the 1600F range, and post turbo are usually in the 1000F range.

The only case where you would have this much heat would be from a clogged CAT, or if this were a rotary engine. If for some reason you did a have a problem, it is easy make your own simple heatsink with with a piece of copper.

Jeff

[wleehendrick]

Jeff,

Thanks for the info. I'm running an EJ255 with stock up-pipe and turbo, plan to tune with APv3. I have a short custom exhaust with a bellmouth, no cat, and a small muffler. So I really don't need the OEM post turbo O2 sensor at all? I should be able to disable the CEL with the AP, and even if it were in place, without a cat, it would still throw a code, right? I have a bung to weld into my exhaust, so your recommendation would be to use that for a wideband and don't bother with any other O2 in the exhaust?

Thanks,

Lee

[CHOTIS BILL]

The Autometer wideband instructions say “Turbocharged applications should have sensor installed 4-5" after turbo on the down pipe.”

Bill Lomenick