Reply to "4V CC head porting with pressed bronze valve guides."

With a valve seat angle of 45° air flow increase shall be most linear as valve lift increases. Valve seat angles straying from 45° create a non-linearity. Valve seat angles greater than 45° shall favor high lift flow, valve seat angles less than 45° shall favor low lift flow.

The 351C 4V intake port, out of the box, has a valve pocket throat diameter of approximately 1.75" (2.40 square inches area), and will thus reach the limiting port velocity (sonic choke) at 7344 rpm (357 cubic inches). Whatever you do, do not "open-up" the valve pocket throat diameter more than 1.90 (2.83 square inches area). Opening up the throat larger than that will negatively impact the intake port's performance. At that throat diameter limiting port velocity is reached at 8660 rpm.

The quench chamber heads shroud both the intake and exhaust valves in places, and air flow (especially low lift air flow) can be improved with a bit of "un-shrouding" in the combustion chamber. As is true for any cylinder head, you want a nice, smooth, well blended transition from the valve pocket to the throat, a "rounded" throat, a nice, smooth, well blended transition from the throat to the valve seat, and from the valve seat into the combustion chamber. No recessed valve seats. Recessed valve seats must be blended to remove the sharp edges and smoothly blend the seat into the combustion chamber, or seat inserts can be installed to thus eliminate the recessed seats. This detail work will net you the most performance from the cylinder heads.

If your headers (or exhaust manifolds) are the type that turn-down and hug the engine then you want to leave the flat roof & the bump in the exhaust port, and also leave the exhaust port floor as-is. Those features in the exhaust port were put there to aid the port's performance when the headers/manifolds turn abruptly downward.

Intake valve seat width no less than 0.060", exhaust valve seat width no less than 0.080", and seat run-out no greater than 0.001".

Here's a picture of outstanding intake valve pocket & valve seat work. The combustion chamber could stand some un-shrouding at about 4 o'clock, and from 6 to 8 o'clock. However the un-shrouding from 6 to 8 o'clock should stay within the boundary set by the head gasket. The un-shrouding from 6 to 8 o'clock may have already been done to the cylinder head in the picture, its that tight.