I think the Stealths were just untouched OOTB. My local race engine builder/machinist (Atlas Performance in Loveland, CO) has messed with those heads before, he said with a GOOD valve job (factory one kinda sucks) and valve heads back-cut it'll pick up 20+ cfm in the lower lift ranges especially just off the seat. Just what I need for a torquey truck engine that'll see towing duty with a very mild cam that only has ~.430" lift.
So in a nutshell regarding the Engine Masters test... factory iron heads were ported and flowed more than stock, Stealths were untouched but could've benefited from a valve job, and the Trick Flows were a tad too big for the engine to make good use of. IMO that explains the lower-than-expected HP increases for each head type.
