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 Post subject: Air/Fuel Sensor Location
PostPosted: Tue Feb 26, 2013 1:15 am 
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On the ST205 the narrowband oxygen (A/F) sensor is located on the exhaust collector before the cat. Some time ago I had an A/F (lambda) probe thread installed in the front exhaust pipe post cat. Assuming the cat is at working temperature I assume that it will consume oxygen so the A/F ratio measured post-cat will differ from the reading pre-cat? I don't particularly want to remove the narrowband oxygen sensor as I want to retain the economic benefits of running OEM closed loop at lower rpm. What do people think of locating the wideband sensor post-cat?

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PostPosted: Tue Feb 26, 2013 2:50 am 
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It will read wrong, worse in the areas that matter I suspect when all the nasty NOx and co is split into dolphin friendly O2
If your talking about stormy then the MoTeC should use the wideband for closed loop. If it doesn't I'd demand a refund of the £1000000 wideband upgrade fee
On snowy use the stock location and invest in a wideband that has simulated narrowband out for the ECU - most do this


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PostPosted: Tue Feb 26, 2013 9:30 am 
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two_OH_five wrote:
On snowy use the stock location and invest in a wideband that has simulated narrowband out for the ECU - most do this
Does that mean it can output narrowband O2 to keep the OEM ECU happy in closed loop AND simultaneously display wideband to my Blitz AFR gauge?

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PostPosted: Tue Feb 26, 2013 9:53 am 
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It's good practice to use separate sensors for control and monitoring, otherwise calibration drift will pass unnoticed.

I was lead to believe the cat has no effect on lambda. Bear in mind mots and rolling roads just shove a pipe up the exhaust.

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PostPosted: Tue Feb 26, 2013 10:05 am 
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Tricky subject, the Wikipedia article is quite confusing.

On the one had, looking at the reactions that take place in the cat there doesn't seem to be free O2 involved in them.

On the other hand Toyota and virtually all other manufacturers now used pre and post cat O2 sensors so logically you would have thought it must make a difference.

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PostPosted: Tue Feb 26, 2013 11:43 am 
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http://en.wikipedia.org/wiki/Oxygen_sensor

Quote:
Location of the probe in a system

The probe is typically screwed into a threaded hole in the exhaust system, located after the branch manifold of the exhaust system combines, and before the catalytic converter. New vehicles are required to have a sensor before and after the exhaust catalyst to meet U.S. regulations requiring that all emissions components be monitored for failure. Pre and post-catalyst signals are monitored to determine catalyst efficiency.


http://www.dtec.net.au/Lambda%20Testing ... verter.htm

Quote:
Summary points:



When engine conditions are stable there was no noticeable (significant) difference in readings before and after the catalyst.

When mixtures were altered rapidly or/and conditions changed there was a delay until the post reading stabilised.



Exhaust is best sampled before the converter if possible, this avoids delays, gains a slight increase in accuracy, faster sensor heating (closed loop), less moisture condensation exposure and less gas transport delays.



But, from this testing, the tail pipe measurements are shown to be acceptable for performance testing/tunning, especially since much of this work is done at Lambda’s that render the catalytic converter ineffective. Bear in mind though that rapid mixture changes may ‘dampen’ out the readings so when testing perform ‘ramp’ tests slowly and avoid sudden mixture changes.

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PostPosted: Tue Feb 26, 2013 12:39 pm 
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:)

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PostPosted: Tue Feb 26, 2013 2:51 pm 
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I still stand by my post.

I wouldn't map with a post cat sensor because the variables are clearly unknown. Google throws a wealth of conflicting info on this subject, some of which is more reliable than others. Well I would but you wouldn't make an aggressive fuel map for sure
I based my original post on the fact Mike pointed out - new cars require two sensors pure and post to meet EU emission regs. It allows the monitoring of CAT health which infers a distinct change in readings pure and post

Don, yes it will do both wideband AFR display and narrowband volts out to the ECU simultaneously


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PostPosted: Tue Feb 26, 2013 3:33 pm 
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On some (my old vems for one) you can put an offset on the narrowband output thereby tweaking the mixture while in closed loop mode. Useful if you want to gain a bit more ecconomy and let the dolphins worry about themselves.

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PostPosted: Tue Feb 26, 2013 10:27 pm 
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I agree with Steve.
I did read up on this a while back and there is a difference in measured results pre & post cat both in absolute terms and time domain.

Cats are different and will operate with varying efficiency at different temperatures so it is far from a linear correction.

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PostPosted: Wed Feb 27, 2013 12:37 am 
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Thanks for all the feedback guys.

It would seem that the preferred location, particularly for mapping is pre-cat although post-cat will still yield useful information.

Quote:
yes it will do both wideband AFR display and narrowband volts out to the ECU simultaneously

Any suggestions on what sensor(s) will do this?

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PostPosted: Thu Feb 28, 2013 11:35 am 
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No sensor directly does this that I am aware of. You programme the wideband controller to output a psuedo narrow band voltage.

Innovate LM-1 & LC-1 do as I believe does the Zeitronix.

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PostPosted: Thu Feb 28, 2013 9:46 pm 
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My tuner has OEM Autronic gear that clamps a probe in the muffler. Although I don't have cats, it apparently is a regular method for Autronic mapping. After the clamp worked itself loose during a few WOT runs, I mounted the sensor in the downpipe though.

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PostPosted: Fri Mar 01, 2013 8:17 am 
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Diceman wrote:
No sensor directly does this that I am aware of. You programme the wideband controller to output a psuedo narrow band voltage.

Innovate LM-1 & LC-1 do as I believe does the Zeitronix.


Yes, the Zeitronix does that too.

But: The Zeitronix fitting instructions says that the sensor will have to be located 20 inch away from the turbo as the wideband sensor seems to be less heat resistant so it wouln´t be a good idea to use the position of the stock sensor.

So I kept the stock sensor to feed the ECU and put the wideband O2 sensor in the additional hole my Aussi downpipe have therefor near the lower flange:

Image

The reading will be the same as the cat (a 100 CPI steelcat) was located to the Aussi frontpipe:

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PostPosted: Fri Mar 01, 2013 9:51 am 
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Useful info and photos. I see your cat is in the same position as my track build. The options are more limited for my standard ST205 with the OEM cat in the downpipe. If there is a way of configuring the readings of the wideband sensor to simulate narrowband input to the OEM ECU as well as sending a wideband signal to a gauge that would be ideal.

From what I've read so far the cat is only active under certain conditions of temperature so placing a wideband sensor after the cat will still give a reasonable idea of AFR's. Clearly with a standard car there is no way of adjusting anything its purely for info. Based on rolling road readings over the years the standard car massively overfuels on high boost so that sort of thing should be very obvious

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