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Old 11-03-2002, 08:20 PM   #1
Jon [in CT]
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Default An Important Note on WideBand A/F Sensor Placement

Bottom line: Do Not Install Wideband Sensors in Uppipe!

Yesterday I became aware that Garfield Willis, of EGOR fame, has returned from the dead, so to speak. This guy has more experience and knowledge about the NTK 5-wire sensor and its control in his big toe than all of us here in i-Club combined. He apparently started posting in the EFI332 newsletter this past July and here are some important tidbits that he has shared there.

If you hope for any accuracy from the NTK 5-wire sensor in the rich region, it needs to be mounted DOWNSTREAM from the turbo. Here's an excerpt from his post entitled EBP Correction of AFR:
Quote:
EBP [Exhaust Back Pressure] sensitivity in AFR sensors is one of the industry's "dirty little secrets" ... Testing on our pressurized sample-gas bench AND in situ comparison measurements, prove that at high boost levels especially, since the pre-turbo pressures can easily be equal to or well above the MAP levels, that the AFR results will be skewed as much as 30% with the sensor pre-turbo. At an actual AFR of 13, your uncorrected sensing equipment would be lying to you to the tune of about 20% lower/richer AFR. IOW, your uncorrected meter says you're at 10.4 AFR when you're actually at 13! Now I call that a truly *significant* difference.
And, if you're thinking your WRX's exhaust back pressure probably isn't too high given your aftermarket cat-free exhaust, guess again. In a post titled Turbo EBP expectations, Gar concludes after citing test data from a heavily instrumented Typhoon:
Quote:
OKaaay, so notice he got two ratios depending on his turbine housing selection: one had EBPs of 1.48 X MAP, and that was at a MAP of 16psig [psig = PSI-Gauge] (note he was using guage sensors even on manifold pressures because that's what he had available and calibrated; plus it makes comparison with EBP easier anyway). Sooo, that means if an AFR sensor was pre-turbo in that app, that sensor would be seeing a whopping 23.8psig (and note those pressure ARE guage). That's enough EBP to produce some prodigious AFR skews.

Fortunately, Frank has his AFR bungs all post-turbo, as I pointed out in a previous post where I quoted his later comments.

But notice that even in the 'best case' with a better install, you still have 1.09X manifold pressure for an EBP of 16.5psig. That is STILL, even in this very good ratio install, a ton of EBP compared to <1.5psig in a post-turbo or NA EBP.

That is why I've said repeatedly, no matter your install, IF you run these sensors pre-turbo (for per-bank or per-cyl tuning), you WILL see major AFR errors from EBP. Hence the absolute need for correction in these apps.
For the record, Garfield does sell EBP-compensating meters and sensors that can be installed in those situations that do require acurate A/F measurement before the turbo via http://www.egortech.com. It appears he hasn't spent much time on this site (the pic of the meter is of a prototype from at least two years ago). Also, to get EBP correction, you need to install an exhaust pressure transducer (sensor) near the A/F sensor. And I think he makes everything "to-order" (e.g. you must specify how many feet you need for the cable between the sensor(s) and the meter).

EDIT: Garfield is, deep down, a Subaru nut. He want's to use Subaru engines in experimental aircraft. And he want's to develop, ultimately, his own ECU for this purpose. Although his A/F detection (EGOR) and knock/misfire detection (IONEYES) projects would be useful in all areas of motorsport, his ECU will ultimately have greatly different (i.e. simpler) requirements than any car's.

But, I want to reiterate: Don't install your wideband sensor ahead of the turbo. Mount it at the top of the downpipe.
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Last edited by Jon [in CT]; 11-03-2002 at 09:21 PM.
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Old 11-03-2002, 08:33 PM   #2
WRX-Hellfire
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Default Re: An Important Note on WideBand A/F Sensor Placement

Quote:
Originally posted by Jon [in CT]
Bottom line: Do Not Install Wideband Sensors in Uppipe!

Yesterday I became aware that Garfield Willis, of EGOR fame, has returned from the dead, so to speak. This guy has more experience and knowledge about the NTK 5-wire sensor and its control in his big toe than all of us here in i-Club combined. He apparently started posting in the EFI332 newsletter this past July and here are some important tidbits that he has shared there.

If you hope for any accuracy from the NTK 5-wire sensor in the rich region, it needs to be mounted DOWNSTREAM from the turbo. Here's an excerpt from his post entitled EBP Correction of AFR:And, if you're thinking your WRX's exhaust back pressure probably isn't too high given your aftermarket cat-free exhaust, guess again. In a post titled Turbo EBP expectations, Gar concludes after citing test data from a heavily instrumented Typhoon:For the record, Garfield does sell EBP-compensating meters and sensors that can be installed in those situations that do require acurate A/F measurement before the turbo via http://www.egortech.com. It appears he hasn't spent much time on this site (the pic of the meter is of a prototype from at least two years ago). Also, to get EBP correction, you need to install an exhaust pressure transducer (sensor) near the A/F sensor. And I think he makes everything "to-order" (e.g. you must specify how many feet you need for the cable between the sensor(s) and the meter).

But, I want to reiterate: Don't install your wideband sensor ahead of the turbo. Mount it at the top of the uppipe.
Good info, But did you mean to say to install the WB02 in the uppipe? Or downpipe?

"Mount it at the top of the uppipe. "
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Old 11-03-2002, 08:39 PM   #3
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Thank you. I did, indeed, mean to say that the sensor should be mounted at the top of the downpipe (not the uppipe). I corrected my original post. (How the hell did I screw up my "bottom line" advice? ).
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Old 11-03-2002, 09:30 PM   #4
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Default very interesting ---- but !!!!

Wow, interesting implications to your comments. It would seem this would explain a few engines that detonated to death even though the tuners were sure they were running "pig rich".

I was wandering if one other option would actually work for the tuner who wanted to measure AFR in each cylinder.

If you ran a small bleed tube from up on the port of the manifold to a chamber with the heated wide band sensor. The small bleed tube would limit pressure rise in the test chamber due to its limited flow capability and yet you would be sampling directly off the port. All you would have to do would be to install some small tap bungs on each port and build the test unit.

Comments Ideas ??

Larry
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Old 11-03-2002, 10:01 PM   #5
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hotrod, is installing your "bleed tube" any less intrusive/onerous than installing the exhaust pressure transducer needed to allow the EGORmeter operate within EGOR's .1 AFR limits?
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Old 11-03-2002, 11:19 PM   #6
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Default Hmm not sure what your asking

If I understand the transducer you discribed, the install would allow you to read the integrated AFR off all the cylinders in the up pipe before the turbo.

As I see the use of the tubes, they would allow you to tap each individual cylinder, using the same sensor, for maximum repeatability. This would be for those who would want to trim or do diagnostics on the mixture balance on each cylinder. The other option would be a real time wide band sensor and pressure transducer in each exhaust runner, but that would be very expensive, not what I think most folks would want as a day to day configuration, and it would add the complication of equalizing the readings for multiple sensors so they can give meaningful comparisons.

In that sense they would have totally different applications. The bungs would be capped off when you were not doing precison tuning. It is possible that a person could leave the tubes in place all the time, needing only to tap each cylinder as needed or feed them all into a manifold to give a combined reading. Small bleed tubes like this are used when taking exhaust back pressure measurments to allow the exhaust gasses to cool enough so you can use an over the counter pressure gage to take the readings.


Pretty much a choice of evils I guess, but as I envision it the tubes would be an order of magnitude easier to impliment, cheaper and less prone to problems.

Larry
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Old 11-03-2002, 11:47 PM   #7
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Good post and good advice Jon. Just this week I was discussing checking pre turbine pressure with a fellow Icluber. I have done this before on other turbo cars. One car that was making 20 psi on the intake side was making 30 psi on the exhaust side pre turbine.

Installing a Bung, Stainless line, then barb with hose to a Boost gauge is a cheap and easy way to check Pre turbine pressure. Keep in mind that if the gauge leaks you will be sending HOT gases to the gauge and in the car with you.. If you use a long hose with a Stainless leeder you will be fine. The Cheap $40 VDO mechanical gauge works for this purpose.

Secondly I can also suggest a Turbine EGT probe in conjunction with a manifold EGT probe. A reading of say 800C at the manifold and a reading of 900C "in" the turbine housing would be an indication of pressure. If you compress a gas you make heat. 900C is the limit for "most" turbo companys in the turbine housing. Damage is being done at that level. Turbine or housing that is flows more is needed in this case.

I check AF in the downpipe at the base of the lower bend. Day by day I realize that the stock Wideband is not the answer to tuning like we all once thought. My question is this... How do we know that the stock ECU is not adjusting for such backpressure? At least a known figure on a stock car....?



CT
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Old 11-04-2002, 05:14 PM   #8
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Quote:
Originally posted by AZScoobie
I check AF in the downpipe at the base of the lower bend. Day by day I realize that the stock Wideband is not the answer to tuning like we all once thought. My question is this... How do we know that the stock ECU is not adjusting for such backpressure? At least a known figure on a stock car....?
It is very likely that the stock front A/F sensor from Denso is also pressure-sensitive. Here's a quote from the 4th page of Denso's 1997 SAE paper about their sensor (see http://jon.in.ct.home.att.net/SAE970843.PDF):
Quote:
According to Kamo et.al. [4] diffusion at the diffusion layer can be obtained with the character of "Temperature Independence" or "Pressure Independence" based on the radius of the micro pore. For the automotive application, the "Temperature Independent" type diffusion layer is used.
I expect that Subaru has made measurements with the stock exhaust so that they can predict exhaust back pressure using, perhaps, MAF, MAP and RPM. Since Subaru knows exactly how EBP affects the Denso sensor, Subaru's ECU likely makes corrections to account for it.

Last edited by Jon [in CT]; 11-04-2002 at 05:20 PM.
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Old 01-02-2003, 04:01 PM   #9
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Hmmm...very interesting...

I just posted a thread where I am going to get an ETAS Lambda Meter LA4 from work and do some datalogging with it. I might be able to get a second one and place one before the turbo, and one after and see how much they differ.
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Old 01-02-2003, 04:58 PM   #10
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Quote:
Originally posted by 2000vfr800
Hmmm...very interesting...

I just posted a thread where I am going to get an ETAS Lambda Meter LA4 from work and do some datalogging with it. I might be able to get a second one and place one before the turbo, and one after and see how much they differ.
Wow ... must be nice to have access to an ETAS LA4. I see they were "on sale" last month for only $3999.95 (see http://www.etasinc.com/html/la4promo...promo_main.htm).

Ideally, your test would have one sensor mounted in the uppipe and the other mounted in the downpipe. If what I have reported is correct, then when you're under boost and the A/F ratio is significantly richer than stoichiometric, the uppipe sensor will mistakenly report a richer A/F ratio than the downpipe sensor and as the A/F ratio moves farther from stoichiometric, the discrepancy becomes larger. At stoichiometric, you should notice very little difference.

Please do report your results.
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Old 01-02-2003, 06:00 PM   #11
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Yea...my job has some nice benefits. And hopefully when our AWD dyno gets installed, it will have even more benefits.

Anyway...I will try and get two for the comparison. We have like 10 for our 7 engine test cells, but some of the tests we are doing are requiring multiple meters. I will have to wait for another to become available for the weekend.

If that happens, and even though I would love to have one right before the turbo in the uppipe, and one right after the turbo in the downpipe, I am going to settle for the second one in my TXS testpipe right after the downpipe. It might not be the most accurate test, but, we should be able to at least see some of a difference if the theory holds true. I will install both sensors, record data, swap meters (not sensors) and record data, swap sensors and record data, then swap meters and record data. This will give me 4 sets of data points to make sure any differences between sensors and meters can be eliminated and only differences due to pressure are considered.

Should be fun.
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Old 01-02-2003, 06:02 PM   #12
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Or, I get a ride into work one day, remove my downpipe, have the welder in our shop weld in a bung, and then use that...

I like that idea better...
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Old 04-01-2003, 11:16 AM   #13
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Do you have results from your experiment?

Winslow
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Old 04-01-2003, 11:18 AM   #14
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Nope...I didn't get around to doing it before my #4 piston yielded. Maybe in July when my car is running again.
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Old 04-01-2003, 12:47 PM   #15
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Quote:
Originally posted by 2000vfr800
Nope...I didn't get around to doing it before my #4 piston yielded. Maybe in July when my car is running again.
What happend to your car?
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Old 04-01-2003, 12:52 PM   #16
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My #4 piston yielded and the engine is now in pieces in my garage...waiting to be reborn.
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Old 04-10-2010, 07:58 AM   #17
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nicee post.... n does anyone from this threadeven still exist..... lol thank for the search link
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