I had a set of heads like that on a 273 and would barely start when I was done putting them on or any time there after. It was a teribly slow even for a 273. I eventually pulled them off when it was backfiring like crazy, from burned valves , that was when I saw how bad the valve job really was. I slapped on some 318 open chamber heads I had layin around and got tons more power from them than the closed small chamber 273s even though it should have been the other way around