Effects of Cats on A/F Ratio?
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TECH Senior Member
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From: Bethesda, MD
Effects of Cats on A/F Ratio? If the wideband reading is from the post cat bung, is the reading distorted by the cat?
If yes, what are some solutions for tuning cars with stock manifolds and cats in place? Is their a correction factor that can be used? Should the STRIMs be used for closed loop tuning instead of a wideband and just use a pre-cat bung for WOT?
If yes, what are some solutions for tuning cars with stock manifolds and cats in place? Is their a correction factor that can be used? Should the STRIMs be used for closed loop tuning instead of a wideband and just use a pre-cat bung for WOT?
After the cat is the wrong place for a wideband sensor. The readings you get will be inaccurate and inconsistent. This is because of the chemistry change of the exhaust gas that takes place in the converter. Also, temperatures are pretty dynamic behind the cat as well.
Get a wideband sensor with two analog outputs. Use one to keep the ECM happy, and use the other output to log A/F in your logging system. These sensors can be used in place of the primary O2 sensor, and this will be more accurate.
Get a wideband sensor with two analog outputs. Use one to keep the ECM happy, and use the other output to log A/F in your logging system. These sensors can be used in place of the primary O2 sensor, and this will be more accurate.
https://ls1tech.com/forums/pcm-diagnostics-tuning/342988-wideband-accuracy-behind.html
But looks like the thread on ls1.com.au is gone now (2 years old).
But looks like the thread on ls1.com.au is gone now (2 years old).
Yeah I remember the thread over on the ls1 au site...basically someone did a test & at wot it was within a point so it was concluded it was accurate before or after the cat...im still not sure im completely convinced from one test.
Especially considering people like Greg Banish is quoted as saying "In order to properly measure this transition, you MUST have the wideband installed upstream of the catalyst. The cat stores oxygen and will continue to release it during WOT enrichment, making downstream O2 measurements look "lean" even though the engine out gases are on target. In short, tailpipe sensors suck."
And even Don(Slowhawk) says theres a difference between cats themselves(can't find his quote at the moment)
Bottom line is I wouldnt trust a post cat wideband too much.
Especially considering people like Greg Banish is quoted as saying "In order to properly measure this transition, you MUST have the wideband installed upstream of the catalyst. The cat stores oxygen and will continue to release it during WOT enrichment, making downstream O2 measurements look "lean" even though the engine out gases are on target. In short, tailpipe sensors suck."
And even Don(Slowhawk) says theres a difference between cats themselves(can't find his quote at the moment)
Bottom line is I wouldnt trust a post cat wideband too much.
I believe at higher gas flows the cats don't have enough
surface or time-over-target to do much with the chemistry.
I also disbelieve the idea that the catalyst holds significant
oxygen (certainly, a nit in comparison to WOT exhaust gas
being flowed).
Now, a set of hollow shells is a pretty brief swap-job to put
your mind at ease about the whole thing. For tuning and
diagnostic purposes only of course.
surface or time-over-target to do much with the chemistry.
I also disbelieve the idea that the catalyst holds significant
oxygen (certainly, a nit in comparison to WOT exhaust gas
being flowed).
Now, a set of hollow shells is a pretty brief swap-job to put
your mind at ease about the whole thing. For tuning and
diagnostic purposes only of course.
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Considering the cats can run hot at leaner conditions, and it is a fact that heat increases the thermal reaction ... Yeah, post cat results should not be used.
On F-bodies, it is pretty easy to install a wideband into one of the pre-cat O2 bungs. It can take me as long as 2 minutes to swap a pre-cat sensor out. If it is too hot, just place a decent sized fan on top of the engine bay, facing down, wait for a couple of minutes (depending on the ambient temp) and it should be cool to the touch. I much prefer this way over any post-cat wideband results.
On F-bodies, it is pretty easy to install a wideband into one of the pre-cat O2 bungs. It can take me as long as 2 minutes to swap a pre-cat sensor out. If it is too hot, just place a decent sized fan on top of the engine bay, facing down, wait for a couple of minutes (depending on the ambient temp) and it should be cool to the touch. I much prefer this way over any post-cat wideband results.
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I have checked them both ways many times. Unless the cats are hot they check the same. Idle and part throttle should be done with fuel trims anyway, as the PCM toggles the a/f rich,lean,rich,lean several times a second to store oxygen in the cats. All you will see on the wide band, other than initial cold start, dfco, PE mode, and COT protect mode will be averages. Might be close on some cars, not even close on others. Usually it will look lean with a wide band at idle and part throttle.
Launching!
Joined: Jan 2002
Posts: 299
Likes: 41
From: Detroit, Murder City
I have checked them both ways many times. Unless the cats are hot they check the same. Idle and part throttle should be done with fuel trims anyway, as the PCM toggles the a/f rich,lean,rich,lean several times a second to store oxygen in the cats. All you will see on the wide band, other than initial cold start, dfco, PE mode, and COT protect mode will be averages. Might be close on some cars, not even close on others. Usually it will look lean with a wide band at idle and part throttle.
At WOT, the "sponge" action of the cats is the primary concern for downstream measurements. It will take a second or two to fully saturate the catalyst to a rich condition. During this time, the downstream gases will show slightly leaner than actual as the NOx constituents are released from the substrate. Eventually, you reach a state of rich equilibrium where A/F in equals A/F out and downstream readings are reliable as long as there's no leaks or outside reversion affecting the UEGO sensor.
Again, my preference is to just measure upstream so that I don't have to wait for the cats to stabilize as the engine sweeps through the early part of the test. It is up to the tuner to decide which method he likes best.
Teching In
Joined: Apr 2003
Posts: 36
Likes: 0
From: Debary, Central FL
If you are just doing WOT tuning and/or relying on the stock front sensors for idle and part throttle, then it is possible to tune with the WBO2 after the cat, you just have to know the limitations. And on some really hard to dial in transfer functions (think big tube and low resolution) you may still want it before the cat anyways.
The cat skews the initial readings for the first few seconds, and adds a delay, which can be 100-500rpm in my experience. I can load up my dynojet 224xLC and get rid of alot of that delay by slowing the pull down, and get more datapoints at the same time.
I will say though, I tune just about everything with an AFM1000 w/NTK sensor, before the cat.
The cat skews the initial readings for the first few seconds, and adds a delay, which can be 100-500rpm in my experience. I can load up my dynojet 224xLC and get rid of alot of that delay by slowing the pull down, and get more datapoints at the same time.
I will say though, I tune just about everything with an AFM1000 w/NTK sensor, before the cat.
I almost regularly tune with both one in the front and the DJ's WB in the tail pipe. With cats, I always see the transition that Greg is talking about on the DJ's WB against my upstream one. I can see the delay too. Really I just use the dyno's WB as a sanity check and it is often nice to reference the two. It could make a guy paranoid tuning a positive displacement blower if all he had was a tailpipe or secondary O2 bung to measure from. If you factor out the initial portion of the pull and the minor delay, they read VERY close at WOT. The rear isn't much good as already noted for idle and part throttle. I use an LM1 in the front bung and change O2s very often given their cost and my peace of mind.
I usually try to do most of my work in open loop first. This avoids the problems associated with trying to hit a moving target as the PCM toggles either side of stoich at part load. Measuring with a good wideband (mine is lab grade and accurate to fractions of a percent) closer to the engine gives the best possible resolution, especially compared to stock HEGOs that may have +/-3% error to begin.
The PCM toggles the air/fuel even in open loop. Part of what it watches for (besides temp) to enter closed loop is how quick the 02s respond and how wide they swing. Initial cold start, PE mode and DFCO are the times it does not toggle. If you want acurate wide band data at idle and part throttle you need to zero the enable temps, delays, zero the P.E. mode enable throttle angles and set the P.E. vs RPM table to "1"s. Depending on your wide band, still may not agree with the fuel trims.
It's a shame GM did away with so many training centers, and cut off independent techs from attending. Lots of acurate information is now out of independant tech's reach. Best training, especially that applies to this, I've been lucky enough to recieve.
but is it the wideband or the NBO2s tendency to drift over time and their relative temp at that moment as well... Often their switchpoints don't agree with stoich on the wideband as you have noted when back in CL.
I have tested this over and over, same wideband, same vehicle, different locations. Saw the same numbers with cold cats as up front. All wide bands don't read the same. Ditto flow benches and dynos. As much as many people would like this stuff to all be absolute, in the real world it is not. They are tools for reference.
On The Tree
Joined: Jun 2015
Posts: 113
Likes: 12
From: Hopewell Junction NY
I know it’s an old thread but I can’t help but comment. This is real world scenario and I want to say this is exactly what happened with me. I trusted tuner and he told me it was at 13:1 wot with probe in tailpipe . I have cat converters and I had my doubts. I put in a AEM wideband in the header collector . Exactly a full point off at 11.8 - 12.0 (too rich) . Now I know why my ******* car is slower in 1/4 mile after his tune.
I know it’s an old thread but I can’t help but comment. This is real world scenario and I want to say this is exactly what happened with me. I trusted tuner and he told me it was at 13:1 wot with probe in tailpipe . I have cat converters and I had my doubts. I put in a AEM wideband in the header collector . Exactly a full point off at 11.8 - 12.0 (too rich) . Now I know why my ******* car is slower in 1/4 mile after his tune.
I know it’s an old thread but I can’t help but comment. This is real world scenario and I want to say this is exactly what happened with me. I trusted tuner and he told me it was at 13:1 wot with probe in tailpipe . I have cat converters and I had my doubts. I put in a AEM wideband in the header collector . Exactly a full point off at 11.8 - 12.0 (too rich) . Now I know why my ******* car is slower in 1/4 mile after his tune.
There's not much difference in how much HP an LS engine makes between 12 to 1 AF vs 13 to 1 AF. My 383 LS1 stroker made the same hp at 12.5 AF as 13 to 1 AF,
To lose ~35 hp you'd have to be at ~ 9 or 10 to AF
If the timing is off a couple of degrees that can hurt hp far more.
Last edited by 99 Black Bird T/A; Nov 8, 2022 at 05:50 AM.