A Big Chief style cylinder head has a "rolled" valve angle which minimizes the combustion chamber volume which in turn allows a very small dome to dispurse volume to achieve maximum compression. The shallow chamber combined with the minimal dome and velocity of the ports allow for an effencient combustion. On a real good 11 degree Big Chief with 2X4 carbs, I have went as little as 29.5 degrees. It depends on what the head is, but if this is an 18-14 degree style big chief I would never exceed 32-33 degrees.