"Head Case....."  

Mikes heads (MBE) sells DIRT Spec heads that ARE UNDER COVER PORTED and they claim they flow a greater amount of air than Brodix as cast heads and they also claim the Port work WILL PASS TECH. If you want a set they will set you back about 3500 bucks for a pair of heads (compared to the price from brodix 1400 bucks a set) with out any valves and springs (bare head). There is a fair amount of gain when the chamber is cleaned up by a valve as it lets air flow be less turbulent. There is also power to be gained or lost by the valve job it self without touching the casting. The rule is pretty straight forward as to what you can do to the Spec Head...