[steve25]

The very great thing about the 5 series heads and the 6 series heads ( other then the big 124 cc chamber of the 6H heads and some 5 series heads ) is that the change in the runner shape ( more rectangular) that the factory made of some reason that late in the game help to make for a head that is easier to port and not F up in many ways!

Note that I am talking about the runner hear and not the critical valve bowl and short turn!

Other then the 670 head casting a given amount of proper valve bowl rework and resizing in these castings will produce a bigger high flow gain then in any other D port head other then the late 62/63 421 SD 980 casting!

To all of you home porters allow me to repeat myself for your knowledge gain.
In terms of iron heads once Pontiac started to have to make the combustion chambers deeper to attain the needed compression this change also brought with it shallower valve bowls.

This fact means that good or bad flow changes take place faster with valve bowl reworking then in the high comp heads of 1970 and early, so you had better know what your doing with the guidance of someone who knows what to do and then also take it slow!

It's easy to loose air flow and some home porters have been lucky in that during there rebuild they went up big time in cid and with a increase in the amount of minimum port area these folks also got a gain in upper rpm HP over stock, but it was only due to luck!

Luck in the fact that the added cid of there motor and in turn gain in low speed port velocity covered up for the incorrect valve bowl porting work that lost them cfm and the velocity that the stock port had!