Wide Band O2 sensors, must know info!

[scarceller]

This topic has come up several times when talking among tuners I respect. I want to share some important details about WBO2 sensors and using them properly. Your motor's health may depend on this information!

So, how much faith do you have in your WBO2 sensor?
You sure it's telling you the truth?

A WBO2 sensor's accuracy by design is heavily influenced by exhaust pressure. Bosch specs even state so very clearly.

Are you sure the bung/location you have put your WBO2 sensor in is not being influenced by exhaust pressure? You need to know, here's why?

Exhaust pressure will deviate the WBO2 lambda reading following this equation:

lambda|corrected = lambda|displayed - ((lambda|displayed -1)/a)*Pexh
Where:
a = 39.3 for lean mixtures, and 33.3 for rich mixtures
Pexh = exhaust pressure in "Hg above the pressure at which the sensor was calibrated.

This causes rich to read richer and lean to read leaner, the further we move from lambda 1.0 in either direction the worse the deviation becomes.

Here's real example:
Gauge reads 0.88 and we have 7PSI exhaust pressure

0.88 – ((0.88-1)/33.3)*7 = 0.91

The real lambda is 0.91 but the gauge reads 0.88 you are actually leaner than the gauge is telling you! You may be tempted to lean the fuel based on gauge reading but this will result in leaner mixture than desired and can have damaging results!

So, how much back pressure do you have in your exhaust?
A stock 3.2L with SSIs can have 5PSI pressure at WOT in the 5000-6500RPM range! Your AFRs will be leaner than the gauge is telling you!

The best WBO2 controller manufactures (AEM is one) know this and offer Exhaust pressure sensors that feedback pressure to the WBO2 controller so it can correct for pressure. But most cheaper sensor manufactures won't tell you this.

In summary: be aware of this limitation as I've seen it come up with our 3.2L cars often. Don't put all your hopes on WBO2 readings alone!

[JJ 911SC]

Sal,

Quite interesting. I went to their website but it seem to be related to Turbo user, but I may have miss it the NA function.

Exhaust Back Pressure Install Kit - 4 Channel Wideband

[tirwin]

So does the exhaust pressure change depending on how how far "down the line" the WBO2 sensor is? Meaning if it was placed closer to the exhaust tip would it lessen the chance of deviation?

[scarceller]

Originally Posted by tirwin

So does the exhaust pressure change depending on how how far "down the line" the WBO2 sensor is? Meaning if it was placed closer to the exhaust tip would it lessen the chance of deviation?

Correct, if possible weld the bung into the center of the muffler as back pressure will be less at that location than at the collector for cyl 1-3 plus side for in the muffler is it also sees mixture combined from all cyls 1-6! This is a issue that is not that well understood and not knowing about it can be dangerous. Many turbo folks know not to place WBO2 sensors in front of the turbo for this very reason.

Most tuners also place the dyno sensor at the tail pipe for this reason as well. And the better tuners actually monitor exhaust pressure!

[scarceller]

Originally Posted by JJ 911SC

Sal,

Quite interesting. I went to their website but it seem to be related to Turbo user, but I may have miss it the NA function.

Exhaust Back Pressure Install Kit - 4 Channel Wideband

Not just a turbo issue, it comes up more often in turbo exhaust but this issue can be a problem in any exhaust that has back pressure.