When I put together my stock YS 400 auto I removed any casting nastiness from the head exhaust ports, gasket matched the log inlet ports, opened up the log exit as much as was safe. I used 2 1/4" mandrel bent pipes ( 2.25" offers a 26% increase in cross section over the OEM 2" ) and figure between those various changes I've got to be flowing reasonably more than an OEM 2" system. Engine is all stock except for the .040" over bore ... so while probably not optimum, has got to be better than OEM.
Still using the OEM single snorkel air cleaner though.
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