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-   -   Will stock O2 sensor read down to 11 AFR if moved after turbo? Any negatives? (http://legacygt.com/forums/showthread.php?t=184101)

caramall2 04-17-2012 01:55 PM

Will stock O2 sensor read down to 11 AFR if moved after turbo? Any negatives?
 
Heard the stock O2 sensor will read down to 11 AFR if moved after the turbo. True? I'd LOVE to not have to mess with my aftermarket wideband O2 sensor and just be able to log right from the stock sensor (it's a pain for me to hook up the wideband, plug in an extra cord into my computer, yada yada...).

Any drawbacks? I guess if it was it a place that didn't read correctly, you could get lean conditions in Closed Loop which could cause damage. I have 2 bungs in my downpipe so I guess I could hook up both and see what the reading are.

Seems like everyone would do this if it worked so I'm guessing I'm missing something. The info was on NASIOC and might be for STIs or something.

ClimberD@HexMods 04-17-2012 02:18 PM

On stock twinscroll cars the front O2 goes to the DP. Our USDM ECUs are not setup for the added delay in the exhaust gas hitting the DP O2 sensor. So closed loop AFRs I have heard can get a little less precise. It shouldn't make a difference in Open Loop.

As you suspect, main drawback is not being able to read below 11.2AFR or whatever the sensor floor is (I do not remember).

Often in the Subaru community (or any community) a flock mentality dominates. I have read isolated incidents of experienced tuners doing it this way, mostly due to running twinscroll headers, but one case on RomRaider of someone doing it for the heck of it. Being able to read AFR to the high 10s is a really nice benefit, since you gain a lot of safety for minimal loss in power.

You might want to research this more before jumping into it.

I'm going to run my car this way soon, will see how it goes...

caramall2 04-17-2012 02:47 PM

Thanks much. Good info on the delay. Let me know how it goes if you try running it that way. I need to just hook up a sensor permanently as it's a pain to put it in and out. Stock would be so much easier ;-)

Spec B 04-17-2012 02:50 PM

I think NFSW runs his o2 in the DP..no problems that I have heard of.

fahr_side 04-17-2012 08:27 PM

We had a discussion about this recently, and I've thought of doing the same thing. As ClimberD mentions, the stock sensor is a WBO2 but not scaled to read below 11.2, which isn't that low. The other issue with delay time seemed to stem from earlier ECU iterations... it came up on a 16bit car. We aren't sure if it's still a problem though guys like NSFW would have found it if was causing AFR fluctuations.

A friend of mine put the point to me in the best way. Why do you want to move the O2 sensor back there? To scale your intake? To be sure you have 11.2 or just >11.2 AFR at high revs / load? If this is the answer, and it was for me, it raises another question. How accurate is the stock O2 sensor at richer AFRs? How are you going to calibrate it? NSFW has a an aftermarket WBO2 installed right next to his stocker, so he knows how accurate his is, but how are you going to check?

Just do a permanent install with your aftermarket WBO2. The latest generation of Bosch LSU sensors are much more forgiving in pig-rich environments and last much longer than they used to.

NSFW 04-18-2012 12:50 AM

I have a scatter plot somewhere that shows the stock sensor and my PLX wideband reading within about 3% of each other, and I haven't noticed any drawbacks at all.

fahr_side 04-18-2012 12:53 AM

Quote:

Originally Posted by NSFW (Post 3851954)
I have a scatter plot somewhere that shows the stock sensor and my PLX wideband reading within about 3% of each other...

... down to 11.2AFR. Right? ;)

NSFW 04-18-2012 01:02 AM

Right. :)

caramall2 04-19-2012 09:37 AM

Thanks all. Good feedback.

IF moving it didn't affect it's originally designed functions (e.g. didn't cause delays or something due to moving it), it would still be nice to just quickly read down to 11.2 any old time, then hookup my other wideband only when needed.

ClimberD@HexMods 04-19-2012 09:55 AM

Anyone every look into using a wideband's 0-5V output and splicing that into the ECU?

Wideband's controller would control the O2 sensor, and you would need to scale the sensor in the ECU, which wouldn't be all that difficult. Unless you have something like AEM's programmable output, in which case no ECU programming required. Even then, is there a programmed AFR floor, regardless of sensor output voltage, and can it be over-ridden? I have not seen that table...

fahr_side 04-19-2012 10:09 AM

Quote:

Originally Posted by ClimberD@HexMods (Post 3854822)
Anyone every look into using a wideband's 0-5V output and splicing that into the ECU?

To what end? If you're going to install an aftermarket WBO2, just leave the stock one in place to carry on controlling CL fueling, and use your WBO2 to scale the intake for OL, plus the usual monitoring / logging duties.
Quote:

Originally Posted by ClimberD@HexMods (Post 3854822)
...is there a programmed AFR floor, regardless of sensor output voltage, and can it be over-ridden? I have not seen that table...

There is a table that sets the rich limit for the O2 sensor, and it can be edited, but again... to what end?

ClimberD@HexMods 04-19-2012 10:17 AM

[quote=fahr_side;3854859][quote=ClimberD@HexMods;3854822]Anyone every look into using a wideband's 0-5V output and splicing that into the ECU?
Quote:

To what end? If you're going to install an aftermarket WBO2, just leave the stock one in place to carry on controlling CL fueling, and use your WBO2 to scale the intake for OL, plus the usual monitoring / logging duties.

There is a table that sets the rich limit for the O2 sensor, and it can be edited, but again... to what end?
Logging wideband O2 is an inconvenience to me, and this is such an easy fix...
Pros:
-Cheaper Wideband O2 replacement than Front O2.
-Read accurately to 10.8. Maybe stock can do this???
Cons:
-Installation and setup.

I'm not saying YOU or ANYONE should do it, I'm simply wondering if it's doable. Sounds like it is.

If you're in my situation (tuning via Cobb AP and AccessTuner, JDM twinscroll, want easy logging without laptop) then it makes total sense. I only have one bung in my DP. Why add a second... I would strongly consider it if my Front O2 came up for replacement. I already own the AEM wideband, so it would be a no cost wide-er band replacement.

fahr_side 04-19-2012 10:22 AM

Sure... if the stock front O2 were to go out and I couldn't get one as cheap as a Bosch replacement... I'd think about this approach.
I think there is a reason they limit the rich reading to 11.2 AFR. Perhaps NSFW tried lowering that and checking it against his PLX? I'm just doubtful the reading is useful below that point.

ClimberD@HexMods 04-19-2012 10:46 AM

Quote:

Originally Posted by fahr_side (Post 3854894)
Sure... if the stock front O2 were to go out and I couldn't get one as cheap as a Bosch replacement... I'd think about this approach.
I think there is a reason they limit the rich reading to 11.2 AFR. Perhaps NSFW tried lowering that and checking it against his PLX? I'm just doubtful the reading is useful below that point.

Conversely, 11.2 is TOO Low for the sensor's intended function, which is to do low backpressure closed loop near-stochiometric AFR sensing. It may have the capacity to go well below 11.2, accurately, but what good would that do in a up pipe? None, because it would be pointlessly incorrect in an up pipe. But not in a down pipe. If it's a linear sensor with room to spare down low on its sensing range and linear voltage output, would be a great software solution to the 11.2 AFR, which is a touch too rich for where I want to be.

NSFW did you ever mess with the stock sensor's floor? Any desire to?

WRteXan 04-19-2012 11:02 AM

I'm in for more info. Having 4 subarus (3 forced induction), I hate moving my WB02 from car to car. I have bungs on the turbo models in the DP, immediately after the turbo, so this would literally be a PNP setup for me.

Mr. D,
I may be wrong about this, but the older (pre-06 impreza, pre-05 legacy) front O2 sensors read in mA. I could probably convert a 0-5V signal to a mA signal with the required speed and accuracy. The problem with my LGT is that in 05, they went to a uA signal. Converting that might get tricky.

edit:

Just pulled up ROMS for each.
02 WRX reads -13 to 10.9mA for AFR of 11.36 to 25.34:1. 24mA conversion is simple enough... offset for negative current... not as simple, but possible.
05 LGT reads -1.3 to .74mA for AFR of 11.15 to 20.28:1. 2mA is getting tight to control. It's not the uA I thought, but a very small current change represents a large change in AFR.


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