I'm looking at putting an actuated exhaust cutout in between the upstream O2 sensor and cat. Now I'm concerned about fuel mapping on my EcoBoost. If the cutout is closed 90% of the time, will it still hurt my tune?
Shaun from AED posted this in another forum I visit. It was meant for 2011-2014 Mustangs but it still applies to newer Stangs as well.
Most people know the Rear o2's are used to detect proper catalyst functions (o2 saturation rates from the catalyst), but many do not know they are used for something far more important for the performance crowd.
The Front O2 sensors are Wideband O2's which have an output range of 0 to 5 volts. Based on O2 content the UEGO sensor targets a given voltage, which is translated by the software to Lambda. They are VERY accurate at detecting o2 content.
The Rear O2 sensors on these vehicles are Narrow band type O2's which have a 0-1 volt range and are not accurate enough to read actual O2 content but rather they oscillate from full rich to full lean and the average value over these oscillations is accurate but only when near Stoich values. (IE at richer WOT A/F Narrow bands are not accurate)
Wideband O2's tend to 'drift' over time and they do need calibration from time to time to stay accurate. Some tuners replace their wideband o2's every 6 months to keep their O2 data accurate, others (like me) use high end ECM equipment that can be re-calibrated at any time. Innovate give this option with their aftermarket wideband o2's, as does NGK.
The amount of drift in the Wideband depends greatly on the temperatures and contaminants the o2 sensor sees, so it is difficult to speculate the frequency of necessary re-calibrations.
Ford in their wisdom took this into account and uses the Rear o2's to keep the Front O2's properly calibrated.
Narrow band O2's do not need to be calibrated as they are continually oscillating from full rich to full lean and the average is very accurate, but only when near Stoich, so Ford uses the Rears to re-calibrate Stoich of the Front O2's.
So what does this mean for the aftermarket?
This means *if* you remove your rear O2's and have them disabled in the tune the ECU will NEVER re-calibrate the Front Wideband O2 sensors and over time (impossible to say how long) they will start to drift and become inaccurate.
This also means if you KEEP the rear O2's installed and turned ON in the tune but have exhaust leaks between the Front and Rear O2's the ECU will incorrectly re-calibrate the front O2's and the car will end up running rich.
So the answer to the question 'Are Rear O2s necessary?' is Yes, *if* you wish to keep the front O2's in proper calibration. The other option is to replace your front O2's regularly to keep them in proper calibration. (like we do with race cars running stand alone systems).
My advise is to KEEP the Rear O2's properly installed and functioning in the tune while ensuring NO exhaust leaks are present up stream from the Rear O2 sensors.
