I agree with Beater, I think too much is being made from the siamese port issue. In a Flathead V-style engine with siamese exhaust ports, it has been shown and proven that dividing the ports helps dilution and reversion of neighboring cylinders that have firing sequences closer together in those type of engines(90 degree firing)! In the 6 cylinders, the firing is farther apart(120 degrees), and doesn't create the same dilution and reversion issues,even on adjoining cylinders! There have been many high horsepower siamese port engines built and raced,both N/A(over 600 HP) and Forced induction(over 1000 HP), that haven't had the ports divided. Not to say there might not be some gains to be achieved from dividing the ports. But as already has been said, you make the port window smaller, which will diminish the flow to some degree and possibly create other issues as well. Also, by design, the siamese port has a very poor short side radius, and adding a "lump" to even a stock engine, will yield a very noticable increase in performance. Like Beater said,a 12 port would be nice, but i've seen a "lump" siamese head flow over 330 CFM on the intake on quite a few heads.In comparison,I doubt a 12 port could obtain that because of their thin casting design.
