From what I've heard about the Explorer heads, the 96-97.5 (I think) were the same GT-40s as the Cobras had, but not decked so they had lower compression (not by much though 8.8-1 or something vs 9-1). The 97.5+ have the GT-40ps which have the "Turbo Swirl" chamber and the smaller valves. They also have a different spark plug mount that requires special headers. It's true that the Ps have better flow, but from the charts I've seen the difference is borderline negligible.
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