Gains would be from polishing the inside. The gains are minimal on the plenum since it is an expansion volume from the 6 intake runners to draw from. Polishing the lower plenum and intake runners would be much more beneficial, but still not a significant gain.i think i saw something like that on one of those G, Z forums.. it look pretty need.
Also how much or simply how could one polished the plenum.. i being reading on other forum. they polished the upper and lower plenum and mentioned they got some gain out of that..
As for polishing the outside of the plenum, that's tricky stuff. I bought a fully polished plenum and all I need to do is touch it up to get an amazing shine back on it. As for starting with a bare (or painted) plenum, I don't know how easy that is. I may take you up on that offer Brad... I just polished my stock intercooler pipes on the Evo X and it turned out pretty well... I also don't mind polishing, it's like my version of knitting
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