I have mine right after the joint btw dump and mid pipe. I just memorise the number on dyno at idle and wot, if that number changes then there would sth wrong. Not perfect but works so far.

The number is out (lean) by .2 on idle and .3 on wot, and sensor is perpendicular to the pipe.

using the Perth-WRX mobile app

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MY08 STI 422whp at 1.5bar on E85, built by Diversion Garage, assembled and tuned by EvoR

Update from new owner: 375whp at 1.4bar on 98, at Tokyo Motorsport