I have put sensors all over primary pipe, collector, before the merge, after and 8 inches down stream. I have never had a problem with reading jumping around. With that being said if there is high pressure hitting the O2 sensor the reading will change. On the dyno, when we do a turbo engine, I have a pressure sensor in the exhaust manifold that allows for a correction factor to be used.
this is from the Daytona sensor site, they have a chart that show the pressure relationship
The Bosch LSU 4 wide-band sensor is affected by exhaust pressure as shown on the graph below. The error (%) applies to the oxygen pump cell current. Note that 1 bar corresponds to normal sea level atmospheric pressure. For most performance applications, excessive exhaust back pressure is not a concern and the resulting small error can be disregarded. At high elevations, the error is also relatively small. At 10,000 feet elevation (about .68 bar), AFR values near 13.0 will be shifted up approximately +0.15 AFR.
hope this helps
joe
