Unlike the earlier cast iron head, the x-flow alloy head from 1980 to 1993 was a very thin casting with small ports. You can't go above 40 mm on a circular port enlagement without air, oil or water leaks. Ace engine builders weld the long turn radius on the out side of the inlet tracts, and create oval or D port heads where the total area is greater than a 42 mm circle with some ease. Rectangular or oval ports or even apple ports work better than round ports because the flow is biased to the top of the port. CUTTING THE BASE OF THE PORT more deeply is the worst you can do tothe head. The Falcon X-flow head is just an updated 351c C head in alloy, scaled down to fit the Ford I-6. Ever since the first Cleveland and big block Lima heads were minted in the late 60's, all SVO improvements have had smaller ports with hig-ported intakes and exhasts.
On a turbo, just clean up the inlet ports without any polishing, and run the best valves and look at duplicating the XE combustion chamber profile. This will take the cahbers up from 49 cc to 53 to 56 cc depending on how you grind out the chamber. The XF heads have bigger inlet valves, but loose intake velocoty becasue they are high swirl heads with a funny shroud around the intake valve.Great on a stock carb or EFI, makes them very economical, but a waste of time on a modifed engine.
A smaller valve XE head flows just as much horpspower or air as an XF, but the secret is in the combustion chamber, not the ports. That's the good oil from an ex Repco and Ford Special Vehicles engineer in a 1990 Street Machine article.
Deano
