differnce between pi and npi
 

whats the difference in the heads and intake manifold from a 99 and up and 96 - 98 i no the 99 and up are pi but whats the difference?

The intake ports are different on the heads and intake. The NPI are teardrop shaped and the PI's are more round squarish looking. Also the cams on the PI are alittle better than the NPI. The PI flows better on intake and exhaust than the NPI, but after .500" Lift, the NPI start to flow better on the intake side than the PI, and the PI still flows way better than the NPI on the exhaust side after .500" lift.

+1. to that. Perfect.

PI heads have smaller combustion chambers (42cc as opposed to the NPI 51cc) and slightly larger valves as well.

PI intakes and NPI intakes are of course slightly different and while you can adapt a PI intake to work on a NPI head for the most part, I don't know of anyone that's done the inverse since doing so would be plum silly.

In early PI and all NPI intakes the plastic coolant crossover was known to fracture and Ford lost a class action lawsuit about it. The problem was fixed relatively early in the PI series intakes and the coolant crossover was change to aluminum.