IMO it is probably because that is where the original wiring harness reaches to. The most appropriate position would be in the 4-1 collector, that way the sensor gets to sample the average A/F of all cylinders. Although for an unheated sensor it will take longer for the sensor to reach operating temperature the further you have it from the engine.
There has been a previous thread on this subject. Could be worth a look.
Wed, 20 August 2003 05:47
