Theoretically there is no difference in what the sensor will see at the tailpipe versus the exhaust port as far as a/f ratio is concerned. There are factors that influence what that sensor sees. An exhaust leak is the big one and if a cat is in place that will obviously skew the results greatly. Exhaust leaks will generally show up as a leaner than actual reading. The sensor has to be in the tailpipe and not hanging half way out. And the sensor needs to be in good condition. Seems like everyone is using the Bosch sensors now and they are notorious for short life spans when they're exposed to leaded race fuel. They'll begin responding slowly and eventually default to a lean reading. If you are suspicious of the readings you are getting that Bosch sensor is the first place to look. For those of you that want to run a wideband in the car full time and run leaded fuel spend the extra money on an NTK sensor and the hardware to run it.
Again, in a theoretical sense, where using a tailpipe mounted sensor becomes an issue is in the delay of the reading and how that reading is logged versus actual engine RPM. It's possible to have a lean condition at 2500 rpm but depending on how quickly the engine is accelerating, exhaust velocity, exhaust length, etc the hardware might be seeing that lean condition at 3000 rpm and logging it as such.
It also sounds like there is some confusion about the hardware that is used for each location. The tailpipe "sniffers" should be the same hardware that would otherwise be installed closer to the exhaust port. Just with the sensor installed in the piece that bolts to the tailpipe or held up by a floor stand. For the purpose of this arguement there is no difference. A wideband is a wideband.
I've done several sessions at Automotion using my wideband installed in the header or downpipe and compared my results to their setup simultaneously installed in the tailpipe and havn't noted anything outside of an acceptable "noise" range. A ratio difference of around 2/10. Your results may vary. And I can see how it would be hard to trust someone until you've done multiple sessions with them and proven out their methods and equipment.
One of the other local dynos is notorious for giving poor results from failed sensors and problematic inductive rev pickups.
But honestly back to back there was a big difference between the two on my car. The tailpipe sniffer was reading much leaner than the broadband O2 sensor. Could this have been the operator? One was on a dyno jet the other was on my engine builders Mustang.
