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Old 05-10-2005, 03:29 PM   #1
Scooby Newbie
Member#: 78987
Join Date: Jan 2005
Default Ok to Move Air/Fuel Sensor Further Downstream?

Hello All,
I'm doing a vanagon/subaru conversion with a 2003 WRX engine and I'd like to move my front air/fuel sensor further downstream into the uppipe for clearance reasons. Does anyone have any experience with a change like that? Do you think the exhaust temp will drop too much and effect my readings if the sensor is 8 inches or so further downstream?

Thanks for any ideas,
2003 USDM ej20t
stock headers
stock ecu
catless uppipe
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Old 05-10-2005, 03:37 PM   #2
Performance Driven
Member#: 75855
Join Date: Nov 2004
Chapter/Region: South East
Location: Jacksonville, Fla
04 Dynapack


have seen up-pipes with o2 or tuning bungs.
heat wrap wouldn't hurt..

sometimes you do what you have to..
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Old 05-10-2005, 04:12 PM   #3
Scooby Guru
Member#: 32792
Join Date: Feb 2003
Chapter/Region: NESIC
Location: lincoln, ri
2003 GGA MBP
12.9 / 105+


iirc, there is a jdm variation that puts the MAIN o2 sensor in the downpipe. someone who knows more than i do about that may pipe in...

moving it a few more inches downstream won't make a difference, imo.
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Old 05-10-2005, 04:44 PM   #4
Scooby Specialist
Member#: 22127
Join Date: Jul 2002
Chapter/Region: South East
Location: Oviedo, FL
2013 BMW 335i Sedan
Estoril Blue


the gp-moto 4-2-1 headers move the o2 bung to right before the turbo, where all the exhaust is finally merged. It really doesnt make a big difference where you put it, its for idle control and light-load conditions anyways.

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Old 05-10-2005, 04:49 PM   #5
Scooby Specialist
Member#: 24140
Join Date: Sep 2002
Location: SFBA
02 Impreza WRX


Usually professional dyno tunes are done with a sniffer in the tail pipe. I have read about comparison tests of sensors immediately after the turbo, to mid-stream, to tailpipe with little to no difference. Also, I have never read any O2 manufacturer instructions that say anything about sensor placement other than that it must be after the turbo and within a low enough heat range so as to not damage the sensor. I have my WB very close to the stock read sensor. It is incredibly consistent with the stock sensor (within the reasonable use of the stock sensor).

I would give a strong vote for not worrying about placing it down stream.


p.s. You will hear people say that down-stream sensors are skewed because gas continues to burn in the pipe. Tests I've read about show this not to be the case. However, I do suspect there is some truth to it, as it makes a lot of sense. I do not think, however, that this happens during consistent throttle but under on-to-off throttle conditions when you get very rich and often have after-fire.
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Old 05-10-2005, 05:42 PM   #6
Scooby Newbie
Member#: 78987
Join Date: Jan 2005

Thanks for the knowledge, folks. This will really help me out.
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Old 05-10-2005, 06:16 PM   #7
Uncle Scotty
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Join Date: Mar 2002
OK buy Nates beans


...as long as it is before any cat(s) and you have no leaks, it should be all good

...and the twin scroll JDM ones have the o2 in the dp 'cuz the exhaust dosen't 'come together' (1&3, 2&4) until the turbo inlet....
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