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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.

Last edited by Jon [in CT]; 11-03-2002 at 09:21 PM.
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