From what I hear they are fairly good heads, but a good port job really helps them breathe, and will help performance. The FRPP s/c should make your 4.6 scream!
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'94 Cobra
308ci, ported GT-40 heads, Ford cam .512" lift and 284° duration, 70mm TB, Custom 80mm MAF, JMS chip, Vortech S-trim 8#, MSD, TREX fuel pump, Accel coil, 3.55's, FMS 1 5/8 headers, BBK Offroad H-pipe, and 2-chamber flows.
1994 Mustang Cobra (UPDATED!)
Check out the pics of my Motor Buildup!