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Mobile AFR Capability and Tailpipe Sniffer

Practical Reflash Tuning

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Discussion and questions related to the course Practical Reflash Tuning

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I own an AEM X series Wide Band AFR system and plan to use as a portable capability for tuning. I acknowledge permanently mounted at least 18 inches from the head is optimum. The length of my entire exhaust system to include straight thru muffler is 46 inches (117cm) (mid-engine car).

Question1 : When using a tail pipe sniffer with the WB sensor, how much error is actually induced?

Question 2. Will it improve idle low REPM readings to attach an extension on the tail pipe mounting probe and go deeper into exhaust to minimize/prevent any O2 reversal contamination?

Question 3. During tuning if we shut down LTFT and STFT during MAF calibration, do the OEM narrow band sensors work? If not operating, could one be removed, and wide body inserted for MAF/VE tuning only?

RE: Question #3, I remove one of the rear O2s given its only job is to monitor its catalytic converter, and as such, having it temporarily removed will not be impactful to tuning. When done, be sure to clear any codes that may have popped up while you were running it while removed.

RE: Q1, I do not believe any accuracy error is introduced unless there is a "leak" in the exhaust system at a Venturi point that would suck fresh air into the exhaust system (unlikely). I am assuming the car does not have catalytic converters.

The "error" you may run into is a time sync issue between engine RPM and lambda reading. Consider doing a ramp run on the dyno. It takes time for the exhaust gasses to flow through the exhaust system and make it to the sensor. By the time that happens the engine has already increased RPM which can result in the lambda reading at 4,000 RPM on the graph actually applying to 3,800 RPM in reality. Those are just made up numbers but hopefully you understand the point.

The factors that determine the error are variable so you cannot apply a fixes RPM shift to the entire graph. Error will be greatest at low RPM low load and least at high RPM high load. The work around is steady state tuning or a very slow ramp run.

RE: Q2, The probe that came with my kit is 30". I do insert it as far as practical for the specific car but honestly think anything beyond 6" is unnecessary. I have not done any testing too back up that figure though.

re:q3 - Not sure if narrow band sensors work if you shut down fuel trim. I’d expect you will still get a log of mVolts from these sensors.

Just to share my experience here - I have had very good results using the factory narrow band sensors to calibrate MAF from 4000 RPM and below. I have had some poor results using wideband in the tailpipe for tuning low RPM areas, so I don’t bother with that anymore. The factory narrow bands work really well here. I find the wideband to be unreliable at low load and low RPM, even when I pull the post cat O2 sensor and install it there.

I leave LTFT and STFT on, I turn off DFCO, cat overtemp protection, and make temporary changes to set power enrichment to 14.7 at all times. No wide open throttle runs! With these changes to the tune, you can rough-in the MAF curve (sometimes you can dial it in completely) just by revving the engine/holding steady RPM in the parking lot in park/neutral. Then go for a drive and slowly accelerate from 2000-4000 RPM for a few sweeps. Also hold RPMs at 2000, 2500, and so-on for each 500 RPM increment for about 10-15 seconds. This is harder than it sounds when driving on the street. I log a histogram of LTFT vs MAF Frequency (counts, volts, whatever) and apply the percentages to the curve as correction factors. Smooth the curve and retest. Then set PE, COT, and DFCO back to normal. I routinely get LTFT within +/- 5% after one round with this method.

I use the wideband in the tailpipe to dial in MAF for anything over 4000 RPM, and it works well.

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