Head selection, like cam selection, is based largely on the RPM range that you want to operate in. That RPM range will be dictated primarily by your intended usage of the truck.
Here are some of my thoughts on the topic. I am working on writing a program to assist me in selecting heads for my applications. One is my boat, the second is my Corvette. For my boat, I want to maximize the combustion. The Corvette is intended as primarily a street car that will operate below 5500 RPM bit might see some occasional track time at the drags and road course. Currently, the boat has rectangle port heads and I'm probably going to use peanut port heads on the Corvette. Peanut ports on the Vette because I need it to get responsibly good mileage yet look and sound impressive while being fun to drive.
My research indicates the following:
Intake valve size should be 50% to 52% of cylinder bore diameter. For optimal performance, this is critical within 1-1.5%. Exhaust valve diameter is generally about 72% to 76% of intake valve diameter.
The Valve throat is the location in the port where the machined surfaces of the valve seat meets the cast surface of the port. The valve throat is the smallest fixed cross-section in the port and controls the flow through the port.
Valve throat diameter should be between 91.5% and 92.0% of valve diameter, however some OEM's use a smaller percentage to increase part-throttle velocity. To emphasize how critical this ratio is, I learned that, my first opportunity on a flow bench, I learned that I lost 40 CFM @ 28" by increasing this diameter too much. No wonder it is called the Maximum Critical Ratio.
Curtain Area is the area around the perimeter of the valve circumference multiplied by valve lift. Playing with some numbers, I calculated out some lift points were the curtain area equals the throat diameter. The attached chart takes valve stem diameter into consideration because the stem occupies space in the port.
It's important to note that at valve lifts below a point where Curtain Area is equal to Valve Throat Area, the valve seat is the greatest restriction and the port has very little influence (if any) over low lift flow. This SHOULD emphasize the importance of a quality multi-angle valve job, regardless of which heads are selected.
The flow gods emphasize port velocity as pretty much the most critical element to port flow. General consensus is that 0.6 Mach is the optimal velocity. As velocities approach the speed of sound (1.0 Mach), quality of flow is lost in a buffetting effect. And, of course the actual speed of sound changes with temperature and pressure. A value recommend for the intake port is 678.5 Feet Per Second (FPS) or 0.6078 Mach.
Bottom line, I have not found calculations for port velocity. It is also challenging because few head manufacturers/porters supply average port cross-section areas. For now, I am using engine simulation software to calculate it for me. This indicates that my rectangle port iron 188 Chevy castings, with 299/318cc intake port volumes, are in the neighborhood of 0.4 Mach at 6000 RPM on a 467 CID BBC. I would have to spin around 8500 to get close to 0.6 Mach or increase displacement. These numbers only apply to big block Chevy.
Piston CFM Demand. I am a bit shaky on this topic. I am not sure if this is the average or peak requirement. Regardless, it is recommended that the smallest head, that supports the required flow, be used, so that port flow velocity remains as high as possible without going over 0.6 Mach.
Some other points to consider: The weight savings of aluminum. Detonation resistance is also a benefit. One disadvantage, everything being equal, an additional point of compression (or one pound of boost) is required to make the same power. Both of these points are due to the thermal conductivity of aluminum conducting heat away from the combustion chamber.
Cheers,
Joe
P.S. Here is a good read on peanut port building.
http://garage.grumpysperformance.com/index.php?threads/tips-on-building-a-peanut-port-big-block-combo.2900Edelbrock, at one time, offered a replacement part for peanut port heads, which have a smaller as-cast (not milled) combustion chamber. I can't put my hands on the part number right now though.
The posts by vortecpro on speedtalk have persuaded me to go the peanut port route. Well worth searching them out.
