I cant say i understand the whole piston ported, cowl pressure change thing, but i do have an ESM Corsair that has a G-38 in it that has the carb just barely out of the side of the cowl. My first dozen flights or so were done with no stack and i never could get it tuned. Always ran perfect on the ground but would never run right in the air. Finally decided to try a stack on the suggestion of a fellow flyer. Within 3 flights had it tuned great. Also noticed a whole lot less fuel residue on the side of the fuse. I tried the stack that came with my G-20.