Flowing the bigport head

[oldeskewltoy]

What are you doing with this engine??? What is the rpm range you desire? What power levels do you need? What will be your principle use for it??

90+% of the time porting done to 4AG heads does more damage than good.

IF you plan on using this as a well tuned street engine, producing a moderate boost over original(20%) while retaining the OEM intake... then there are 3 things of importance... (in my opinion)

#1) each port(16) at the transition point from horizontal flow to vertical flow has a minor casting flaw. The 4AG head was cast in 2 stages(from what I can see...) The two casting points meet at the port transition point. In all likelyness you will NOT see this, but once the valves are removed you CAN feel it... just run your finger up the port edge and it will FEEL obvious
#2) The TVIS plate adds low end torque, and as long as peak rpm doesn't go beyond the factory ECU rev cut, the TVIS plates add more in the bottom end then they rob in the top end(being in the air stream....) BUT there is a flaw in how the intake/TVIS and head assemble... or rather where the intake and TVIS assemble. If you have your largeport head bare you will see that two of the intake bolt holes are actually larger than the bolts... because they have dowels to make sure there is an EXACT fit between the head and the TVIS plate. Now look at the other side of the TVIS plate... where the manifold meets the TVIS plate. There are NO dowels! Although the differences in the stud as to the bolt holes in the manifold is small, it still can lead to manifold mismatching which can cause eddy points in the intake stream. What is needed is a dowel, or roll pins to lock the intake to the TVIS plate so all the air passages can be tapered to minimize eddy points.
#3) Once the valves have been lapped in and a final valve to valve seat depth has been determined (how far down inside the seat the valve face is from the combustion chamber) then and only then should you begin to measure and adjust combustion chamber volumes. Using a pipette you fill each chamber and measure its volume... once you have found the largest... you then CAREFULLY begin to remove material in the areas noted by TRD bible. Another note here is to do a MINIMAL TRD polish to the LARGEST chamber first... remeasure it and then proceed with the other chambers till they all have the same volume.

The most significant rule of thumb with largeport 4AG heads is minimalism... Don't do anything too much. The main idea is to imcrease port velocity, NOT to screw it up