I have built some 3.5 inch boost tubes for a few 6.7s. This mod helped by getting rid of a restriction in air flow...the stock 6.7 cold side boost tube necks down quite a bit. I also do a bit of porting to the stock 6.7 manifold to give the engine a bit more air flow. Both mods will help gain some fuel economy and some extra torque. If one has a larger intercooler with the larger 3.5" inlet and outlet then these mods will work even better.