AFR Readings Off?

[Philbar715]

I have a 05 GTO with a ported TB and intake manifold. Axle back mufflers. Stock everything else. I have HPtuners standard. The fueling and spark side of the tune is 100% stock. I did a light tune on the VE table using LTFTs a few weeks ago. No MAF tuning yet. Innovate LC-2.

Now Since my exhaust is all stock, I put the wideband sensor in the bank 1 rear O2 bung. So it is after the cat. Readings at idle and cruise and close to stoich (14.7). They usually sit somewhere around 14.8 at idle and cruise. I did calibrate the sensor in free air (not in exhaust) twice per the innovate instructions.

I went out to log a few quick pulls and my AFR at WOT seems a bit off. It was in the 13.5-13.8 range at WOT. Now I didnt get to make many pulls because I was short on time so I need to get out there and do some more logging.

I verified the AFR by the gauge in the car, and I logged it via HPTuners through the serial port using the built channel and transform settings.

Is this right? 13.5 seems pretty lean at WOT for a stock tune. For those of you without access to HPtuners I am attaching a picture of my stock PE table. Commanded AFR should be around 11.5-12.5.

Attached my logs and tune.

[5FDP]

I wouldn't trust the wideband with it being after the cat converter, most if not all widebands say they must be installed pre-cat to prevent skewed readings.

[Philbar715]

I wouldn't trust the wideband with it being after the cat converter, most if not all widebands say they must be installed pre-cat to prevent skewed readings.

I agree that it should be throwing the readings off, but by that much? Most tuner shops around here stick a wideband up the tail pipe of the cars they are tuning if they dont have a wideband already installed. My dads Corvette and Silverado were both tuned using that method by Carolina Auto Masters.

Also forgot to mention that i did ground it to the ECU ground wire since I am feeding one of the analog outputs in to the fuel tank pressure input on the ECU.

[IARLLC]

There is no chance that a wide-band placed after a cat can give accurate air/fuel information! No chance! Anybody who has ever compared pre-cat O2 info to Post-cat O2 info knows there is a massive difference after the cat....and it cannot be accurately predicted or expected to be the same from car-to-car. Differences in condition, temperature, efficiency, type, etc hugely affect how much and in what way a cat affects the mix of chemicals that escape out the back.

It is possible that people have compared pre-cat O2 sensor readings (at/near stoich) to the post-cat readings of the wide-band to try to make some conclusion about how much the cats affect the ratio.....but as mentioned above, from car-to-car it would vary a lot.

[SultanHassanMasTuning]

looking at your O2 readings its borderline far from your commanding afr between 11.5-12.5

[Philbar715]

There is no chance that a wide-band placed after a cat can give accurate air/fuel information! No chance! Anybody who has ever compared pre-cat O2 info to Post-cat O2 info knows there is a massive difference after the cat....and it cannot be accurately predicted or expected to be the same from car-to-car. Differences in condition, temperature, efficiency, type, etc hugely affect how much and in what way a cat affects the mix of chemicals that escape out the back.

It is possible that people have compared pre-cat O2 sensor readings (at/near stoich) to the post-cat readings of the wide-band to try to make some conclusion about how much the cats affect the ratio.....but as mentioned above, from car-to-car it would vary a lot.

It seems at WOT there is at most a 5-6% difference. For my purposes and light mods I can live with a 6% error. See this thread below,

http://www.hptuners.com/forum/showth...B-s-after-cats

looking at your O2 readings its borderline far from your commanding afr between 11.5-12.5

(stupid questions incoming) Were you looking at the narrowband voltages to determine that? Last night I was looking at the logs closer and I noticed that the narrowbands were at ~850MV during WOT. That voltage seemed to correlate to 13.5-13.8AFR from what ive seen. I was also looking to try and determine if there was a way to "calculate" the AFR readings at WOT based on the stock sensors in the car so i can have something to compare my WB to but other than narrowband voltage there doesnt seem to be anything.

I am going to clean the MAF and finish up the VE table and tune the MAF.

I also found this thread with some good info in it - If you go to post 23 you'll see that he is running around 13 at WOT. Is that normal for our cars?!
http://www.ls1gto.com/forums/showthread.php?t=193871

Thanks for the help so far everyone.

[SultanHassanMasTuning]

set your target in PE say 1.175 which equals to 12.5 AFR target

now you need to adjust your MAF curve from say 6500hrtz and up increase total by say 7%

its not the best going by Narrow bands especially on older sensors.

but i would aim for 890-900 for a nice fat AFR

[Philbar715]

set your target in PE say 1.175 which equals to 12.5 AFR target

now you need to adjust your MAF curve from say 6500hrtz and up increase total by say 7%

its not the best going by Narrow bands especially on older sensors.

but i would aim for 890-900 for a nice fat AFR

I dont want to tune WOT by the narrowband voltage, I was just comparing my NB voltage to the AFR of the wideband. Why do you say the MAF needs to be increased by 7%? I was going to tune the MAF by using LTFT trims today, but I cant seem to find a good guide.

[gottogo]

Philbar, what is your commanded afr at wot?

[sunny]

Just multiply your MAF tables by 1.07 from 6500 up would be the quick fix. Then drive it a while to make sure you're long term fuel trims are not going positive when you go wide open. If they are and most likely they will be you can tune that out buy fine tuning the MAF or just turn off your long term fuel trims until you have time. That is the quick fix and then you can take your time and learn to tune.

[Philbar715]

This morning I took it to go get some MAF cleaner. On the way I did a WOT run or two, and what do ya know. The readings were almost spot on to my stock PE table! Nothing changed since last night and this morning other than I shut the car down and started it back up.

I only have HPTuners standard. I am logging the WB two ways right now. I am using the serial output of the WB and logging that in HPT. I also have one of the analog outputs of the WB going to the fuel tank pressure input on the ECU. I set that up with the correct formula. I have the WB grounded to the ECU ground also. The serial readings and readings from the ECU are very close but slightly off.

I cleaned them maf, then I tuned the MAF table and it was off by almost 8% in some spots. I checked the VE table and thats nearly dead on. After tuning both I went out and did some PE tuning, I set it to 12.5 AFR and logged some runs. It hits the AFR almost dead on and holds it great.

I am still battling some KR (bad in some spots), does anyone else get lots of KR around 1500 RPM (in 4th in my case A4 trans)under heavy throttle? Ive seen up to 10KR there. WOT I might see a blip of 2KR but it goes away quick. To start to tackle that I am going to run some seafoam or GM top end cleaner through it to hopefully remove any carbon build up.

I attached my logs if anyone wants to see them. Cats dont affect AFR much at all apparently.

Philbar, what is your commanded afr at wot?

Before the PE table was stock, so anywhere from ~11.5-12.5. I have now set it to 12.5 across the board.

[IARLLC]

Thanks for bringing up the HPTuners blog. Looking at the graph posted by Jim Mueller, though the car was at full throttle the whole time the wide-band after the cat was off by as much as .75 lean and by .268 rich...and varied back and forth. So its error varied by more than a whole point (1.13/1). This is not % but the difference of 14.7/1 and 13.57/1 AFR for instance. They also seem to suggest that the error would be less at full throttle so it could be off even worse at light throttle...and that they get .5 leaner AFR readings in the tailpipe (even without cats).

So tuning at the tailpipe will lose considerable accuracy (fuel efficiency and power). And tuning with a post-cat wide-band will net more than twice that error varying in either direction without predictability...or at least this is what the dyno tests showed.

Some of us spend a decent penny to get some power. Almost all of us spend a bit on gas. I cannot imagine why welding in a bung (or using the pre-cat O2 bung) in order to tune accurately is not worth the time or money it would take. Of course I have clients that think it is not worth it but they lose power and fuel in the long run. I am grateful for the research in these threads as it gives me a more accurate way to educate clients about their choices.

[Philbar715]

Thanks for bringing up the HPTuners blog. Looking at the graph posted by Jim Mueller, though the car was at full throttle the whole time the wide-band after the cat was off by as much as .75 lean and by .268 rich...and varied back and forth. So its error varied by more than a whole point (1.13/1). This is not % but the difference of 14.7/1 and 13.57/1 AFR for instance. They also seem to suggest that the error would be less at full throttle so it could be off even worse at light throttle...and that they get .5 leaner AFR readings in the tailpipe (even without cats).

So tuning at the tailpipe will lose considerable accuracy (fuel efficiency and power). And tuning with a post-cat wide-band will net more than twice that error varying in either direction without predictability...or at least this is what the dyno tests showed.

Some of us spend a decent penny to get some power. Almost all of us spend a bit on gas. I cannot imagine why welding in a bung (or using the pre-cat O2 bung) in order to tune accurately is not worth the time or money it would take. Of course I have clients that think it is not worth it but they lose power and fuel in the long run. I am grateful for the research in these threads as it gives me a more accurate way to educate clients about their choices.

I am not sure where you see in that chart it is off by a whole point? The most I see it off at is at 2800-2900 RPM, and that is literally a 5% difference. Everywhere else is a 1% difference.

Where am I going to weld in an additional bung in the stock 2005-2006 GTO exhaust before the cats? There is literally zero room unless I take one of the stock narrowband sensors out or I put the sensor literally in front of the stock sensor. See the pics below of what I have to work with. I would gladly spend $20 to have a shop weld me a bung in but there is no room, because the angle of the pipe and how closely it runs to the frame.

http://i277.photobucket.com/albums/k...le/StockEx.jpg

https://encrypted-tbn1.gstatic.com/i...klFGf-WTcWUrnK

http://www.ls1gto.com/forums/attachm...1&d=1276024452

[IARLLC]

Please reread what I wrote. I did not write that it was off by a whole point. I wrote how much it was off lean at one point (actually two points) and how much it was off rich at another point....that the error varied by over a point. And that according to them the error would be less at full throttle (even worse to tune part throttle with post-cat wide-band). If it was just .5 or .6 lean all of the time a person could figure this in and make improvements but with it varying in both directions...during a full throttle run...

As I wrote "or using the pre-cat bung". It seems to me that the best case would be to do as most of the "Getting started/Read me/How to" articles suggest tuning light/part throttle VE the MAF with factory O2 sensors then pull one or both O2 and install wide-bands for accurate full throttle tuning.