AFR accuracy with a tail pipe sensor in a dual mode (NPP) exhaust

[hawk02]

Someone over at C5 posted dyno results of his bone stock ZL1. He asked about his car running lean since the dyno measured an AFR from 12.5 to almost 13. A well known tuner responded that the dyno operator most likely used a tail pipe sensor to measure the AFR and that the dual outlet mufflers on the ZL1 allow fresh air to reach the sensor and throw the AFR way off.

I'm curious if others have seen inaccurate AFR measurements with the use of a tail pipe sensor in a dual mode (NPP) exhaust. I've had my ZL1 dyno tuned by two different tuners and both used a tail pipe sensor to measure the AFR. Both tuners richened the AFR at WOT because they said it was running too lean. Now I'm wondering if it's been richened up too much because of the use of a tail pipe sensor.

[8850]

I don't have a ZL1 but all I have read about placement of wideband sensors to get accurate readings is to locate it 12" to 40" from the head exhaust port and before the cat converter. If this is the case then I don't know why Innovate even makes a tail pipe adaptor. The tail pipe adaptor is the easy quick way but probably not accurate. If there are no cats on the vehicle I believe you would be closer to accurate but still not the recommended placement.

[BMWMD82]

I haven't noticed any major AFR changes with the C6 NPP style mufflers, but it's possible the Camaro version is different. When my car was stock, it showed AFRs in the 11.7 range at the tailpipe when on the dyno. This was with the wideband installed in the NPP mufflers.

I can say that I usually see a little leaner AFR with a tailpipe mounted wideband on a dyno vs. my onboard wideband that is installed in the X-pipe before my cats. Is there room to have an extra 02 bung added before your cats? For a while I was running LS9 exhaust manifolds and cats, and I found a way to mount my wideband before the cat. It was tight, but I was able to do it.