96-98 4.6 mod motors came with heads and intake mani that were less then desirable. a PI head swap essentially swaps these heads and intake with the heads/intake and cams from a 99-04 car.
The PI Swap is an upgrade for the SOHC 2V as an OEM alternative to the NPI factory components that were long ago determined to be teh ghey in stock form. Fortunately as the parts are OEM not FRPP and the stock MAF, EGR, and TB are retained, there is no worry about having an O/R classification or triggering the CEL- necessitating MIL eliminators. The upgrade produces great SOTP results at WOT. It is perhaps the best mod for a N/A NPI Mustang, aside from gears, F/I, or NOS/NAWS/DA JUICE.
This probably makes no sense, but I tried to use as many acronyms as I could.