Well, I have lightly glowing headers and lean numbers. I agree the sensor needs to be placed more downstream but I wouldn't go so far as to say totally useless. It does flicker back and forth too much but it still gives good indication. It's still somewhat useful.

Just the fact that the O2 sensor can fail and brake apart and throw debris into your turbo - expensive - scares me...

At idle there is little backpressure in the exhaust headers and when on full boost there is around twice as much exhaust back pressure in the headers as the air pressure the turbo is making in the induction system. Expecially if you're using an old reliable but obsolete and inefficiant (compared to current day Garret turbos) 3K K27 7200, and the backpressure in the headers will be even higher if you're using a K27 7200 hybrid with a big heavy oversized Holset compressor wheel that was never designed for a K27.

So if you're running up to 1 bar or 14.5psi boost, the exhaust backpressure in the headers is around 26-30 psi before the turbo. The increase in backpressure as revs and boost increase in the headers is not linear, it has a slope to it that changes with throttle position and rpms.

Your AFR's are almost backwards from where they should be and because your wideband O2 sensor is mounted before the turbo where it is under alot of exhaust pressure the readings are totally inaccurate.
That just adds to the confusion and because you don't know how far off these readings you posted are from reality it makes trying to diagnose whats wrong from your current AFR readings a total waste of time.

How do those sensors work and why does pressure affect the validity of the reading?

They measure oxygen the same way as o2 sensors but more accurate. I don't understand how pressure can effect the amount if oxygen but there must be something to it as its been discussed before. My afr gauge flickers back and fourth and I'm sure that's why but it gives enough for me to feel good about an average. Not perfect but some info.

OK, looks like it is based on diffusion and electrochemical reactions, so it would make sense that higher pressures would cause inaccuracy.

http://en.wikipedia.org/wiki/Oxygen_sensor

How much are we talking about?

I'm not a chemist, so I wouldn't know.

Doing a little reading it appears temperature is more important than pressure.

More reading... looks like a direct relationship between pressure and the reading. 2x atmospheric pressure means an AFR 2x actual. Edit: that is chemical activity. The Nernst Equation has a natural log of the activity in it.

So, it is a constant (RT/somethingrather) times 1/x times dx, where x is the exhaust pressure in multiples of atmospheric and dx is the difference between the two.

That should be the approximate error.

Anyway, its nonlinear.

I don't know what kind of AFM you have but on some you can adjust how often a sample is taken to minimize that jump. It can go from sampling every .10 seconds to every 1 second. At .10 my graphs are pretty jumpy, I run mine at .60 intervals during street use.

As far as sensor placement, this is off the WEGO site. I bought my WEGO IV from our host.


Can the exhaust system affect wide-band AFR readings?

Wide-band systems will give inaccurate AFR readings in certain situations:

· Excessive exhaust back pressure. Wide-band sensors are affected by back pressure. Excessive back pressure causes exaggerated AFR indications under rich and lean conditions, but has little effect at 14.7 AFR (stoichiometric). The WEGO is intended to be used with a free flowing performance exhaust. Overly restrictive stock mufflers may cause excessive back pressure under wide open throttle. When used with a turbo system, the sensor must be mounted downstream of the turbo. Motorcycle exhaust systems are relatively free flowing and problems with exhaust back pressure are not likely.

· Exhaust reversion. Reversion is the term for a negative pressure wave that can suck ambient air back into the exhaust and cause an erroneous lean AFR indication. Exhausts without mufflers, such as open headers or "drag pipes" on motorcycles, usually suffer from reversion effects and may not be suitable for use with the WEGO. Reversion effects will also occur with certain exhausts used on "bagger" style motorcycles, where two pipes split off near the rear cylinder. At part throttle, air is actually sucked into the left tailpipe. Reversion effects will be most noticeable at idle, part throttle low RPM cruise, and decel.

I ran 2 AFR meters on the red rocket, one before the turbo and one after. They were is sink accept when on boost. With sample smoothing they were very close on boost. Your problem is at cruise so the meter should be close enough for diagnosis.

Were they the same make and model wide band or narrow band O2 sensors and AFR gauges?

That's great info. Your very analytical and we are to benefit. Thanks for that.
Did you say that your numbers if smoothed looked similar?

That is great info Brian.

The gages/sensors I used were both Innovate but one was an older LM-1 and the other a new XD-16 gage. The reason for 2 gages was that originally I had a narrow band sensor/gage in the OEM spot on the J-pipe along with a wide band LM-1 in the muffler that was used for testing only. The narrow band was a permanant install and worked great for my purpose but did not have a numeric read-out because it is not accurate outside of telling you rich/lean/stoich. Folks like Jim had posted the caveates of wide band sensor location so I switched out the narrow band to the Innovate XD-16 and compared it to the the LM-1 located in the muffler neck. As Jim had stated the narrow band did a good job in the OEM spot before the turbo but the wide band got flaky on boost. I played with the smoothing to where they were close. Ultimately I put the narrow band back in the OEM spot and the wide band XD-16 in the muffler neck (the LM-1 is remote and used for tuning other cars).
As Jim stated it is optimal for full range use to locate the O2 sensor after the turbo. If you must use the OEM bung before the turbo this is what I observed.

Thanks again Brian, I still need to check my ignition advance to see what it's doing today. Happy new year guys.

Is it useful/necessary to have a wideband on a turbo-car that's close to stock(full exhaust/intercooler/turbo upgrade)? If so, where do you mount your gauges? Just curious..... I drive an STI daily with an AEM UEGO, but it's not stock(GT35R, bigger injectors, front mount, etc)....

Quattro, my tuner says it needs to be after turbo and before muffler & nice car you have there with some respectable power! Enjoy:)