GT40 vs GT40P heads, from wiki
"Ford offered a performance head that was a stock part on 1993 - 1995 Mustang Cobra models and pre- 1997-1/2 Ford Explorers and Mercury Mountaineers equipped with the 5.0L engine called the GT-40 head (casting id F3ZE-AA). In mid 1997, the Explorer and Mountaineer 5.0L heads were revised and re-named GT40P. The GT40P heads, unlike the GT40 heads, had a very well developed port shape/design which yielded ~200 cfm on the intake side and ~140 cfm on the exhaust side without increasing the size of the ports at all from standard E7TE castings, and without increasing the exhaust valve size. They also had smaller 59 - 61 cc combustion chambers for added compression, and the combustion chamber shape was revised to put the spark plug tip near the center of the chamber for a more even burn. These GT40P heads are extremely efficient and thus require only 28 degrees of total ignition timing to produce maximum power. Not only do GT40P heads flat out outflow regular GT40 heads, they also create more compression and a more even and more complete burn in comparison to regular GT40 heads"
http://en.wikipedia.org/wiki/Ford_Windsor_engine
Just doing some reading tonight....
