Since Ford Motor Company introduced the Modular motor, they have produced many variations and modifications.  Here I will try to clear up some questions on the 4.6L.  This motor was originally introduced in the earlier 90s in the Lincoln Town Car.  Over the years many changes were made, but I won't go into that boring detail.  The first thing I want to address is the "SOHC" (Single Over Head Cam) and "DOHC" (Dual Over Head Cam).  Lets start with the SOHC, this indicates a single camshaft over each cylinder head, so if you have a 2.0L SOHC then you have a 4 cylinder witha single cam shaft, if you have a 4.6L SOHC, you have an 8 cylinder with 2 cam shafts, one over each head.  Now taking that into account, lets look at DOHC.  This is the same logic as the SOHC but instead of 1 cam you have 2 over each head, so if you have a 2.0L DOHC, you have a 4 cylinder with 2 cam shafts, and the same for the 4.6L DOHC, you have 2 over each head for a total of 4 cam shafts.  The 4.6L DOHC was widely used in the Mustang Cobra and Lincoln Mark VIII.   Ok, with that out of the way, lets look at 4.6L SOHC PI (Performance Improved).  This engine was first introduced in 1999 in the Mustang GT, the major differences can be found in the top end.  The heads were redesigned with larger valves, ports and cams.  With these changes came an improved intake manifold with larger runners and ports to allow greater air flow to feed the improved heads.  The horse power gain was considerably increase over the NON PI engines.  This engine was later release in the Crown Victoria, Grand Marquis and Town Car in 2001 and beyond.  Many individuals will tell you that the intake from this engine will not work on the NON PI engines, however, Custom Mustang of South Florida (ebay seller CMSF2003) has developed a kits to adapt this intake to the following vehicles.  96 97 98 Mustang GT, 96-00 Crown Vic, 96-00 Grand Marquis, 96-00 Town Car, 96-97 T-Bird and 96-97 Cougar.  The installation of this kit will increase your horse power by 15-20 and provide great throttle response.