We chose to make the most intricate and difficult mold, one of the complete ports attached to the combustion chamber. This is the hardest mold to make, since it encompasses every aspect of the airflow path and can be a bit tricky to remove without tearing.
Many choose to make individual molds and then try to attach them. The method we took allows the reader to see the entire flow journey in and out of the cylinder and can be likened to what development engineers call a flowbox. A flowbox is employed during cylinder head development; it can be thought of as the ports and combustion chamber without any other aspect of the cylinder head. In the old days, these would often be made from wood. In more modern times, they are usually generated by a procedure identified as LOM or laminated object manufacturing.
 The best method to determine the actual port dimension is to measure it with a caliper
If you are only modifying the intake and exhaust runner of the cylinder head, then it is beneficial to make separate molds of each and not be concerned with producing a pseudo flowbox. Individual runner molds are the easiest to make, and will allow you to measure and create your own game plan for improving the flow with the least amount of complication.
To produce any mold, you will need to simply fix the cylinder head in the appropriate position, apply the release agent in the ports, and block or plug any area where the molding compound can escape, such as the spark plug hole and valve guide. Then mix the product, pour it in, and let it cure. Next, extract the mold to complete the process.
Silicone rubber does not like to stick to things, so there will be little to no clean up of the port, but due to the intricate and often changing shape of a runner, it may be tricky to get out. Patience is a real virtue when it comes to extraction, along with a proper release agent applied to the surface before the rubber is poured.
Studying The Port
What you do and glean from a port mold depends on your ultimate goal. For example, if you are interested in porting the cylinder heads on your Pontiac engine, then you will study the mold for restrictions, sharp turns, pinch point, and so on. Once these flow obstacles are located, you can use a piece of mechanic’s wire or soft welding rod to measure on the mold to identify the area you want to modify in the port. Place the wire in the port. Once the region is identified, mark it in the runner with machinist’s blue dye or a similar product.
 Use a piece of wire or welding rod to measure the top and bottom of the runner.
If you are trying to choose between multiple cylinder heads you have available for your Pontiac, then you can make a mold of the different choices and carefully study and compare each port, since it will now be in plain view.
It is important to note that the customary method to determine runner size is a cc volume reading. Though this is a good indicator, it is not all-inclusive. Let me explain. You may find one cylinder head has a more efficient bowl area that is properly blended into the seat and a slightly smaller runner when measured with a caliper. This head may pour out in a cc test to a higher volume. Theoretically it will flow more air and have higher port velocity, since the volume is being increased in a region that promotes efficiency and speed. If one were to only reference a cc reading, then the head may be rejected as being too large for your engine combination.
Other aspects of the port can also be readily seen with a mold, such as the short-turn radius, the entry to the valve seat, intrusion of the pushrod into the flow area, and the impact of the valveguide.
 On the workbench, measure the two lengths (top and bottom of the runner), add the valu
The same logic can be applied to studying the runner of the intake manifold. An easy-to-make mold will reveal the true port shape and critical runner length that can be added to the intake runner length of the cylinder head. Total intake path length is of paramount concern when choosing a camshaft. The valve opening and closing events (“Project Pure Poncho,” June ’13) need to be able to capture any inertia that the charge has. (This is especially true with Third-Generation TPI engines, since the extremely long runner needs to open the valve during the second harmonic pulse to achieve the best performance.) When total runner length is known, the camshaft company can better select the proper grind for your Pontiac.
Making a mold of the cylinder head ports and, if applicable, the intake manifold is normal procedure for an advanced engine program, but for the most part it has been absent in the Pontiac community. That is, until now that HPP has let the cat—or should I say tiger?—out of the bag!
An easy-to-make mold will reveal the true port shape and critical runner length that can be added to the intake runner length of the cylinder head