M5 Guru (>2000 posts)
Join Date: Nov 2014
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In the FWIW department, Wiki indicates two benefits of secondary air injection. One is for helping burn the rich exhaust during open loop at cold startup. This is typically done with air injection upstream, at the exhaust ports, as is the case with the S62.
The other use is for aiding in the catalytic conversion process during normal running and is usually done with air injection downstream just in front of the cats (as with Sailor's reroute).
Thing is, the way the ECU is programmed, the pump only runs at cold startup. So injecting air downstream will likely only minimally aid in burning the rich exhaust at startup. The main benefit I see would be getting rid of the AA code.
Obviously it would be much easier to just code it out.