Originally Posted by CDW6212R
I would stay with matching inlet pipe sizing to the MAF and TB. There isn't much to gain with making the pipe bigger than the MAF and TB, maybe for all out racing. There are many other things worth doing before that. The 70mm TB fits the intake opening well enough that no porting is really needed. Fitting the 75mm is not a huge change, it wouldn't take much to match that better.
Last week I purchased another ford stock throttle body (Pig in a poke) on eBay for $25 plus shipping. I thought it was a 75mm for a Mustang V8 but it turned out to be off a 97 Expedition. It has a side port below the TPS that bypasses the throttle plate to a base port. There's also a female hex head adjuster that controls the bypass flow. The port probably connects to the PCV. The throttle plate diameter is about 70mm but the inlet and outlet diameters are smaller (about 68mm). The inlet has an abrupt bore change just prior to the throttle plate.
Today I purchased another throttle body that hopefully will be more suitable for testing. It is a 75mm limited production (or no production) Ford racing unit that has been ported and polished. It comes with no TPS so I'll use the one on the Expedition throttle body. From the photos the inlet and the outlet port both look to be 75mm in diameter.
Rather than try to port the intake plenum to match I'll probably fabricate my own tapered adapter. That way I can switch back and forth between my stock and racing throttle bodies. I'll mount my unmodified stock gasket at the intake plenum and buy or make another gasket to fit between the throttle body outlet and the adapter.
I've read in other posts that converting to a larger throttle body usually results in more cable play. I had 3/8 inch cable play with my stock so I should be able to accomodate a 1/2 inch (or more) thick adapter. I won't know until I receive the racing throttle body if the stock air hose will fit it.