Hey guys, as long as you have that new plenum in your hand, take a peek inside. Most of them have casting goobers and minor defects in the surface that impede flow. If you have a dremel or handheld die grinder remove the bumps. I was easily able to access all of mine with a hand-held die grinder. After you do that, spend $10. on a 3M scotch brite/emory wheel (they are about 2.5" diameter and easily fit in a 75mm plenum. Polish the S**T out of the inside! This setup yields a smooth surface and enhances flow, better flow equals HP, every little bit helps on a NA 2V 4.6. Also, if you dislike the "whistle" on part throttle, (mine sounds like a belt chirp) carefully break the edges on the IAC ports (Idle air Contoller) and EGR port* (especially here, because the incoming air has most velocity at this point) I used a ball grinding end of about 3/4" on my die grinder. When finished, wash thouroghly in a pail of warm suds (clean completely with a rag and toothbrush through all ports. I noticed alot of you are installing TB/Plenum setups, I figured I'd through out this tip to y'all.
John