The passats are using VW's CR version 2, and are getting slight better MPGs. The engine code is different, as well as the emissions system. I think it'd take quite a bit to get the two systems to blend.
with the time and cost, a DPF delete would be the best investment, maybe not so for the environment though.
His: 2010 Sedan, cog swapper, build date 07/09
Hers: 2012 Passat, Roof+Nav, build date 05/12
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