The primary drawbacks to the nPI heads as compared to the PI heads are the intake ridge, smaller intake port volume, and smaller exhaust valve. The intake ridge’s purpose is to induce in-cylinder mixture motion. However, it also acts a shroud to a valve that already is shrouded by its close proximity to the cylinder head wall. The intake port diameter is adequate for most naturally aspirated and mildly blown applications; however, the increased intake port diameter of the PI heads will support higher horsepower applications. Finally, the small exhaust valve acts as a hindrance to exhaust flow and it is a major reason why these heads wheeze out before the PI heads or the SVO heads.
The PI heads utilize a quench pad instead of a ridge to induce mixture motion. This quench pad does not provide as much of a restriction as the intake ridge in the nPI heads. The intake port volume is increased to 159cc and the exhaust valve got an upgrade from 34mm to 36mm. The PI heads also include PI cams that have a higher lift than the cams provided in the nPI heads. The cams also include a slightly smaller duration to enhance low end torque.
A popular upgrade for nPI equipped Mustangs is the PI swap. This includes swapping PI heads and the PI intake on an nPI 96 – 98 Mustang resulting in an approximate gain of 45 RWHP, but with an increase in the compression ratio to approximately 10.5-1 making these PI head swapped engines supercharger unfriendly.
Another common swap that involves less labor at significantly less cost is the PI cam and intake swap. The swap provides a gain of approximately 30 - 35 RWHP while maintaining the stock compression ratio. Because used PI cams and used PI intakes can usually be purchased for approximately $100 each, this mod is proving to be one of the best dollar for horsepower mods available for the nPI equipped Mustang.
