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Old 10-21-2012, 09:41 PM   #1
cowboy_Rob
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Just swapped my intake to a kstech mega maf and while I'm working on scaling it I noticed that at idle when my stock o2 is reading 14.7 on the logger my AEM uego reads 15.2. Which one is right or is there any explanation for them not matching?
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Old 10-21-2012, 10:11 PM   #2
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14.7 is right. my car always read 14.5-14.7 idle but it's an NA. if anything turbo cars should be richer no leaner.
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Old 10-21-2012, 10:16 PM   #3
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Well the ecu self adjusts the fueling until the stock sensor reads 14.7 but if the stock sensor is crapping out it can bias either way. Or my uego could be off. I don't know which one to blame
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Old 10-22-2012, 07:46 AM   #4
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Quote:
Originally Posted by Pickler View Post
if anything turbo cars should be richer no leaner.
That makes absolutely no sense. Turbo or not at idle or cloosedloop crusing, the ECU will target 14.7:1. It's not like you are seeing positive pressure at idle.
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Old 10-22-2012, 07:48 AM   #5
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Quote:
Originally Posted by cowboy_Rob View Post
Well the ecu self adjusts the fueling until the stock sensor reads 14.7 but if the stock sensor is crapping out it can bias either way. Or my uego could be off. I don't know which one to blame
Where is your Uego sensor placed?
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Old 10-22-2012, 07:50 AM   #6
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Uego is at the bottom of the downpipe about 3 foot from the turbo like recommended.
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Old 10-22-2012, 12:30 PM   #7
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Quote:
Originally Posted by cowboy_Rob
Uego is at the bottom of the downpipe about 3 foot from the turbo like recommended.
I bet you misread that recommendation. It usually reads, "36" from the head". Which would put the WBO2 up near the turbo outlet.
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Old 10-22-2012, 02:35 PM   #8
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If you continue reading it says on turbocharged applications to mount it 36 inches from the turbo outlet
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Old 10-22-2012, 02:51 PM   #9
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Don't be stupid friend... Would companies such as Cobb, Perrin, and others put a WBO2 hung in the bellmouth if it was a terrible idea to do so?


Further more, this is copied directly from the AEM instructions.

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A weld-in M18 X 1.5 boss is supplied for sensor installation. Mount the O2 sensor in the exhaust system at least 18 inches downstream from the exhaust port. If you anticipate high EGT's (over 800C), run a turbocharger, run at high RPM for extended periods of time or plan on running leaded race fuel then you must mount the sensor at least 36 inches or more downstream of the exhaust port
Show me where it says 36" from the turbo.

Reading comprehension owns you.

Last edited by Black94Snake; 10-22-2012 at 03:08 PM.
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Old 10-22-2012, 09:58 PM   #10
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Well turboxs put theirs at the bottom of the downpipe. Either way the directions say at least 36 inches they don't say no more then xx. Have you ever seen one read differently by where it was mounted? I've never seen one read differently by where it was mounted unless there was an exhaust leak before the sensor. But this is the first car I've used one and had a factory wideband also. My other experience is all with dsm's and factory cars
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Old 10-23-2012, 01:22 AM   #11
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Quote:
Originally Posted by cowboy_Rob View Post
Just swapped my intake to a kstech mega maf and while I'm working on scaling it I noticed that at idle when my stock o2 is reading 14.7 on the logger my AEM uego reads 15.2. Which one is right or is there any explanation for them not matching?
At idle the ECU is generally going to aim for 14.7 or a hair richer. If you want to know exactly what it's shooting for, you can log the closed-loop fueling target AFR and/or the "final fueling base" parameter. The latter includes compensations that can sometimes be surprising. (Those are the names in RomRaider's logger; the names in Cobb's logger might be different, I wouldn't know.)

So one theory is that your stock O2 sensor is a bit messed up and it reads 14.7 when the actual AFR is 15.2. The ECU hasn't recognized the problem, so it believes the sensor. If it gets bad enough you'll get a Check Engine light.

The other theory, of course, is that your stock O2 is working perfectly and your AEM is off by 0.5.

You might be able to use the rear O2 sensor to break the tie. I have never really paid attention to my rear O2 sensor readings, so I don't know if this will work, but it seems plausible...

Keep in mind that the rear sensor is not very accurate unless you're close to 14.7. But if it says 14.7 I'd believe whichever wideband also says 14.7. Try logging the stock O2 sensor, the AEM and the rear O2 sensor, and see what happens.

If you're cruising at a steady speed, the ECU will start oscillating the AFR a little bit rich and a little bit lean. All three sensors should bounce above and below 14.7 together. If only two of them actually do, then you know which sensor is hosed.

Also, I moved my stock sensor into my DP bellmouth and nothing changed, so I wouldn't worry about where yours is located. It reads within about 3% of my PLX wideband, except that it won't read richer than 11.1 or 11.2.
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Old 10-23-2012, 05:24 PM   #12
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Mystery solved. I found a tiny crack partway up the downpipe where I peened in the side of it to squeeze the external gate by it
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Old 10-23-2012, 09:49 PM   #13
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cool... your widband helped you find an issue.
and yes, mine is mounted about 3ft from the turbo (suggested by my tuner) and it reads about .4 lower than the stock sensor.
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Old 10-24-2012, 12:29 AM   #14
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Quote:
Originally Posted by hutch959 View Post
cool... your widband helped you find an issue.
and yes, mine is mounted about 3ft from the turbo (suggested by my tuner) and it reads about .4 lower than the stock sensor.
If your stock sensor is in the stock location (below the up-pipe) then the discrepancy may be due to the high pressure in that area. That's the main reason the stocker is insufficient for tuning.

Although I thought that was supposed to make the stocker read too rich... not really sure though.
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Old 10-24-2012, 09:02 PM   #15
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uhhhh... i was talking about the stock sensor that is at the back of the downpipe, right before it goes into the cat.
i have a cobb dp, and i mounted the wideband o2 sensor about 6 inches back from the bend. there is a indention in the body that allows you to roll it up about 15 degrees. it has been working great.
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Old 11-14-2012, 08:35 AM   #16
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My wideband reads very close to the stock O2 sensor near the stoich area, but as the AFR gets richer (below 13), the reading between the two becomes further and further apart.
The wideband always reads leaner and in the low 12's, the wb is leaner by as much as 0.8 compared to stock .. is this normal? (my stock O2 doesn't go below 11.36 so not sure how big the gap is below that).

I'm about to scale MAF based on the wideband readings, but I'm worried I could be making it too rich, if my wb's reading is wrong. Should I trust the wb?
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Old 11-14-2012, 11:48 AM   #17
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There is a reason you have a wideband and don 't necessarily use the stock AFR sensor for tuning. Mainly because the stock sensor is in a high pressure area being before the turbo.
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Old 11-14-2012, 06:50 PM   #18
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If you're catless, the issue with wideband placement is not so much the reading (it should be essentially the same where-ever you put it post-turbo) but the lag between the commanded cell / scaling and the reading on the wideband.
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Old 11-14-2012, 07:36 PM   #19
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I've seen multiple AEM's read low 15's when they are supposed to read 14.7 at idle. I've learned to deal with it.

If Subaru thought the best way to monitor CL AFR was in the downpipe, wouldnt they have done that? A wideband should be used for in boost only IMO
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Old 11-14-2012, 10:43 PM   #20
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they put it where it is, because the first turbo motor had a cat pre turbo, which would alter the afr if they had the sensor in the downpipe

and they haven't changed the casting since
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Old 11-14-2012, 10:47 PM   #21
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Quote:
Originally Posted by rocketperson7 View Post
they put it where it is, because the first turbo motor had a cat pre turbo, which would alter the afr if they had the sensor in the downpipe

and they haven't changed the casting since
It would cost subaru nearly nothing to change the location to the downpipe if they wanted to. Its in the uppipe for a reason.
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Old 11-14-2012, 11:03 PM   #22
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Originally Posted by BlazeRex View Post
It would cost subaru nearly nothing to change the location to the downpipe if they wanted to. Its in the uppipe for a reason.
Probably for rapid closed loop changes. The farther the sensor is from the heads, the more delay you get between combustion and measurement. This will effect the behavior of the closed loop system, enough delay and you can start to get oscillation. This has nothing to do with whether or not the measurements are correct though, just the time delay in getting them.
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Old 11-14-2012, 11:05 PM   #23
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Quote:
Originally Posted by shin1 View Post
My wideband reads very close to the stock O2 sensor near the stoich area, but as the AFR gets richer (below 13), the reading between the two becomes further and further apart.
The wideband always reads leaner and in the low 12's, the wb is leaner by as much as 0.8 compared to stock .. is this normal? (my stock O2 doesn't go below 11.36 so not sure how big the gap is below that).

I'm about to scale MAF based on the wideband readings, but I'm worried I could be making it too rich, if my wb's reading is wrong. Should I trust the wb?
That's due to the pressure in the uppipe, as has been said a few times in this thread. The way the wideband sensors work depends heavily on the pressure of the air that they're measuring. Raise the pressure, and it alters the readings. The pressure in the uppipe pre-turbo is very high, IIRC it's roughly twice as high as the intake manifold pressure, and this causes the stock sensor to read rich when in boost. Install your wideband post-turbo, or move the stock sensor post-turbo, and the problem disappears because the pressure there is more or less atmo.
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Old 11-15-2012, 04:52 AM   #24
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Quote:
Originally Posted by BlazeRex View Post
I've seen multiple AEM's read low 15's when they are supposed to read 14.7 at idle. I've learned to deal with it.

If Subaru thought the best way to monitor CL AFR was in the downpipe, wouldnt they have done that? A wideband should be used for in boost only IMO
For the twin-scroll cars, Subaru actually did put the front O2 in the downpipe.
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Old 11-15-2012, 07:43 AM   #25
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The pressure thing makes sense and it gives me confidence that the wb reading must be right. The location of my wb is as per the Innovate's instructions on my stock downpipe, after the turbo but before the cat.

At first I thought it might be a calibration issue but then that wouldn't explain the fact that it reads the same as stock O2 around 14.7. The pressure thing explains it.
Thanks!
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