This will probably be true for all the other ECU flavors but I can only verify on my ME 7.1.

Please read this as Spartan has been proven to provide false readouts when installed on ECU rear O2 inputs. There is a design flow in Spartan that causes it to drop voltages when connected to Me7 ECU. ECU/ME7 logger converts these to richer AFRs than the car is really running.

Please read starting from my post, the discussion is ongoing with several people verifying my finds:

http://nefariousmotorsports.com/forum/index.php?topic=3731.msg54790#msg54790