@NC Engine Builder currently has my ProMaxx 171 standard port heads and he's done a little work on them. He can elaborate more if he wants to but he made the comment that the ProMaxx intake SSR is nicer than the SpeedMaster head and he had both sitting on the table together. It didn't take too much for him to take them from 246@.550" (on his bench, which is 5-6 cfm lower than advertised) to 274@.550" with a max of 278@.600" which is where I believe he stopped. Not sure what if any difference there is on the exhaust side.