The "PI swap" is a very common upgrade among the Mustang crowd. But here's the basics: There are 2 versions of the PI heads: Windsor and Romeo. The Windsors came out in 1999/2000, found mostly on Mustangs and Crown Vics. The Romeos came out in '01, and are the more common and popular version.
While there are physical differences between the W and R heads, there are no major differences between the 2 heads performance-wise. The Romeos can be identified by their 11-bolt valve covers, while Windsors are 13/14 bolt covers. Dropping the PI head onto a non-PI (NPI) bottom end raises compression to around 10:1 - 10.4:1(a few tenths higher than a stock PI motor), depending on head work etc. and this along with the stock PI intake and cams, is good for anywhere from a 35-45 hp improvement, without even pulling out your tuner. Now raising compression WILL send your torque up the power band, but the 4.6 loves rpms anyway. The trick is to make other changes to make the upgrade useful to trucks.
I'm planning on doing the swap on my truck this fall/winter, in conjunction with an upgrade to 35s and 4.10 gear swap. I've already started researching combinations that'll be "truck friendly".
There is so much info on these swaps on the Mustang forums, I'd suggest trying Corral.net or SVT Performance. I could write a novel, as I've done the swap a couple times on my Stangs. I can assure you it'll wake up ur 98 like it's born again.
Last edited by BossRacing71; 07-15-2014 at 01:13 PM.