O2 sensor

[Darrell N.]

Bruce:

I checked with our research chemist here, who is somewhat familiar 
with O2 sensors.  The probable reason for the shift in the O2 sensor 
reading by moving it to after the turbo is the difference in absolute 
pressure.  Ahead of the turbo location would be at a slightly higher 
pressure than after, because of the backpressure from the turbo.  The 
O2 sensor is somewhat pressure sensitive, as it works on the partial 
pressure of oxygen passing through a zirconia membrane.  Since the 
sensor has a reference port to atmosphere, and actually measures the 
difference between exhaust gas O2 and atmospheric O2, a decrease in 
pressure would result in an downward shift of the sensor reading across 
the board.  This may have a slight effect on your AFR, but not much, 
since it oscillates around the stoich point anyway.

Also, the turbo would have a smoothing or mixing effect on the exhaust 
gasses, averaging out cylinder to cylinder variations in both O2 content 
and pressure pulses  This may have the effect of lowering the apparent 
"noise" on the O2 sensor which could be seen as a slight shift in 
readings.

regards
darrell n
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