In my application with a 23* SBC I have two main problems.
1. Aftermarket aluminum heads have thicker decks, larger ports and valves, and more material around the rocker arm stud areas.
This equates to "smaller" water jackets/passages if you will and less water in said head(s). After all, there's only so much external area a 23* SBC head can have.
2. SBC engines have this "flaw" of the siamesed exhaust ports in the center of the heads. Kind of like double trouble as far as heat being absorbed from this area.
Hot spots are common in this area of the head(s)... and the pressures generated by the steam from the coolant boiling are sometimes so high, that the system can't displace/remove the steam/air pockets.
Now when the thermostat opens coolant flows into the rad to be cooled. Most of the water released comes from the front portion of the heads. That's why I run additional corner lines... to get the hot, almost boiling coolant up front faster to be cooled. Same thing goes for the center exhaust port areas. After all, the thermostat won't close until it senses a cooler temp than its rating. So I use a line at the top center of heads to remove the hot coolant... but the plus side is i'm also forcing cooler fluid INTO the side of the heads, between the exhaust ports to remove/displace hot spot/boiling coolant. All of this flows to the thermostat area.
Now don't get me wrong, i'm not an engineer or some sort of fluid dynamics guy, but all of this makes sense to me, and has seemed to work quite well.
VERY open to other ideas/thoughts on this, as well as an answer to my original question(s) about where, if any I can add the other lines in heads.
Thanks a bunch guys