SHELBY GT 350 Member
Joined: Sep 2010
Location: see user name
Rumor has it that the stock upper plenum will not accept a 75mm tb without some porting. As far as benefit to upgrading the tb by itself that's pretty minimal; does about as much good as a cai. Especially considering that the stock piece is 65mm, stepping up to a 70mm with the stock plenum seems pretty useless.
The choking point of the intake is the stock plenum; beyond that the next place is the cylinder heads. The PI intake is actually pretty decent and most would advise against replacing it on an engine with stock heads and cams; although with a supercharger there are decent gains to be had with an aftermarket manifold. As far as the 70 vs 75 debate (which apparently continues to rage on in some circles), we used to have a good sticky thread on that a couple years ago over in 4.6 Tech, which was accompanied with actual images of dyno sheets comparing the two. The winner was 75, every time, but only by like 1 to 2 rwhp, which is within a commonly accepted margin of error for dyno testing and at any rate nothing you would ever notice just driving your car.
FWIW, since 2007 I have had the ProProducts 75mm tb and plenum on my car. I figured since it doesn't cost any more than a 70mm then what the hell. Been very happy with it and doing this mod produced very noticeable gains per my in-house butt dyno.
2001 GT Convertible
All the usual bolt-on's /200 I6, C4
And a few Explorers too... 2008 4.0 XLT 4x2, 1996 XLT 5.0 4x2, 2003 Sport Trac XLT 4.0 4x4