I added the ¾ lip or “flare” on the outside of my cowling exit to increase cooling (Texas summers).  While doing that, I also riveted a half round aluminum lip on the inside, about 1.5 inch radius.  So the air rises over the hump and bends downward before it “sees” the exit.  This smooths and energizes the air (can’t remember where I first saw this, but used it on my Kitfox firewall).  Result, my chts went down 20 deg. Or more.  Ed Gray