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Thread: O2 sensor placement

  1. #1
    PF's #1 soft roader advocate Captiva Fan's Avatar
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    O2 sensor placement

    Big Red Car's dump-pipe is conical for the first 2/3 or so, it bell-shapes out to a fairly straight bit of pipe for the last third (where the wastegate output joins the exhaust).

    Now ... I'm fitting an O2 sensor soon, and I'm wondering whether it will read correctly if I fit it out where the pipe's cylindrical? If I fit it up closer to the turbo, it forces more gas to flow past the sensor; but then the sensor is more of an exhaust obstruction (plus I thought heat became potentially more of an issue closer to the turbo?).

    Can anyone tell me whether it's better or worse to fit the sensor after the conical section of the dump-pipe? Would my guess be true, in that there's only exhaust-gas in the dump-pipe anyway, so the sensor won't be reading too inaccurately even if it doesn't sample heaps of gases?
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    Registered User thechuckster's Avatar
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    not too close to the exit of the turbo otherwise the temps confuse or bork the sensor.

    the gasses are the same gasses as you move away from the turbo - they just cool down a bit.

    wideband sensors are self-heated so a few feet away from the turbo isn't going to be a problem (unless you have big leaks in the piping).
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    Non player character GTSBoy's Avatar
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    Wastegate is not always open, especially when on lightloads (ie when it should be running closed loop) so not a good idea to put it there.

    FWIW, on RB motors, the oxy sensor is only about 3" away from the turbine outlet. Same on my new fabricated replacement. Other engines they locate them as far away as the catalytic converter. So anywhere along your dump will be fine. The further up it is the faster it will warm up and give readings - but that only really matters to OEM engineers looking to make emissions targets.

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    Prisoner of Society Kaiser's Avatar
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    Isn't it dependent on the actual sensor?

    I think the Bosch LSU4.2 is supposed to be fairly tolerant of being a long way down the exhaust.
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  5. #5
    sack riding 10sec rx7's Avatar
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    you need to put the 4.2's a long way down the exhaust so they dont over heat and stop reading...
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    PF's #1 soft roader advocate Captiva Fan's Avatar
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    Ta for the comments; you spurred me into actually RTFM (how embarrassment!), and Tech Edge have actually got a pretty reasonable doc on where the best place is to fit the sensor.

    In short, they say 1m after the turbo outlet is the best place to try first on a turbo car; and on mine, that's just after the cat. So I called them to check whether after-the-cat is a good idea, and they said it'd be perfect (which is good - I'm fairly sure I had a hanger & flange installed there so I could fit a side-exit pipe for track-days if I ever got excited enough ... hence I may be able to just unbolt that part of exhaust to get the bung fitted).

    According to the Tech Edge website, a lot of factory sensors sit closer to the engine than the wideband styles they sell, but they're generally a different type of sensor. Also, narrowband sensors (which would be most of the RB's that GTSBoy is talking about?) prefer to be much closer to the engine than the 1m-from-the-turbo or 1m-from-the-exhaust-valve that's rule-of-thumb'd with aftermarket wideband sensor fitting.
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    Non player character GTSBoy's Avatar
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    Quote Originally Posted by Forg View Post
    Also, narrowband sensors (which would be most of the RB's that GTSBoy is talking about?) prefer to be much closer to the engine than the 1m-from-the-turbo or 1m-from-the-exhaust-valve that's rule-of-thumb'd with aftermarket wideband sensor fitting.
    Yuh, I should have guessed that you were talking about wideband stuff and not OEM style stuff.

    Personally, 'twere me, I would put your wideband in before the cat. Reason being is that the cat will burn awaysome of the excess fuel with some of the excess oxygen, thus changing the measured mixture compared to what came out of the engine. ie, the oxygen will read slightly lower than "real".....for what it's worth anyway.
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    PF's #1 soft roader advocate Captiva Fan's Avatar
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    Quote Originally Posted by GTSBoy View Post
    Personally, 'twere me, I would put your wideband in before the cat.
    Turns out I'd gotten a bit confused (old age does that), and there's a bit of flex-pipe before the cat; it's the back of this flex-pipe which is 1m from the turbo. So perfect!

    Well nearly. Tech Edge recommend mounting it on an angle between 15 degrees from vertical & 15 degrees from horizontal; the clearance (where the exhaust's a little tucked up under the body) may be a bit tight to mount it like that.

    Anyone know if I may get away with mounting it horizontally?
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  9. #9
    Ready... Take aim... oldcorollas's Avatar
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    the angle thing is about not allowing water to either drip into it from the exahust, or condense and pool in it when cooling...

    moisture and sensor = not happy...

    a little up from horizontal would be fine
    and yeah,, before cat = actual mixture coming out of exhaust.. seems odd for tech edge to say after cat is fine?
    Last edited by oldcorollas; 23-04-08 at 11:37 PM.
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  10. #10
    Registered User [RX3]'s Avatar
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    Quote Originally Posted by 10sec rx7 View Post
    you need to put the 4.2's a long way down the exhaust so they dont over heat and stop reading...
    Can the 4.9's be mounted closer to the turbine outlet on the dump pipe?

  11. #11
    sack riding 10sec rx7's Avatar
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    Quote Originally Posted by [RX3] View Post
    Can the 4.9's be mounted closer to the turbine outlet on the dump pipe?
    Itís no longer a issue with good controllers
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  12. #12
    Registered User [RX3]'s Avatar
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    I have the Haltech CAN WB01. Think it came with the ls 4.2 sensor - that one okay to use close to turbine ? Or just buy the 4.9?

  13. #13
    Purist, whats that? Jason Broadhurst's Avatar
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    Haltech CAN WB01 can't use the 4.9 sensor.
    Jason Broadhurst

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  14. #14
    sack riding 10sec rx7's Avatar
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    Quote Originally Posted by [RX3] View Post
    I have the Haltech CAN WB01. Think it came with the ls 4.2 sensor - that one okay to use close to turbine ? Or just buy the 4.9?
    i have run them 2in behind the turbo,... no you can only use 4.2
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  15. #15
    Registered User [RX3]'s Avatar
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    Cool, thanks.

  16. #16
    Registered User schnitzelburger's Avatar
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    just on this, my innovate MTXl wideband is about 300mm from the back of the turbo would this be an issue, should i move it further back?
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    Done.

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