You should be just fine with new heads. Everyone says that the compression ratio goes up so much but remember that the newer modular Cobra's are around 9.8:1 and there are tons of people running tons of boost, NOS, etc and doing just fine. It's all in the tune!
Look at my mod list... I have 5.4/99PI heads and the car runs great!
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T. Teal
Founder,
Tealnet.com
Check out my
1998 Mustang GT
Mods: Vortech "S" trim supercharger, 5.4 heads (p&p), cams (reground), 99 intake, ATI 3 core intercooler, AFM Powerpipe, Steeda Lower Control Arms, Steeda Upper Control Arms, Steeda Moly Subframes w/cross-brace, 42 lb injectors, no FMU, Walbro 255 in-tank fuel pump, Walbro 255 in-line fuel pump, Ae