DIY Porting
Performing additional port work yourself on an out-of-the-box CNC head will be a mistake 98 out of 100 times. "Unless you have a tremendous amount of experience and a flowbench at your disposal, save yourself the aggravation," Tony opines. Any changes made to a highly optimized port will more likely have a negative effect than a positive one, which is why it's as good as it is right out of the box. If you need more flow, opt for the competition version of the product you are interested in or perhaps the next larger head in that lineup. "Don't think just making a port larger will guarantee it flows more. It's the classic mistake and assumption most people make." In fact, when it comes to making power, bigger isn't necessarily better even if it does flow a little more air. "The shape of the entire flow curve and the velocity of the charge are far more important than a few more peak cfm. I have personally flow-tested dozens of 230-plus-cc heads that don't flow as much as smaller, 195-205cc heads. When it comes to port design, shape is far more critical than size."
Angle of Attack
The vantage point the incoming air has to the back of the intake valve is sometimes referred to as the angle of attack. Due to its added height, a raised-runner head simply has a better angle of attack or vantage point for a straighter shot to the back of the valve. The added height also reduces the angle of the turn that the incoming charge must negotiate. "The geometry of a raised-runner port is superior to a similar nonraised runner design, since it allows additional airflow and a higher terminal velocity before it stalls or backs up," Tony explains. Another overlooked benefit to any raised-runner design starts before the cylinder head. "Due to the higher port inlet locations, manifold runner length is naturally increased, since the space between the left and right port banks is increased, which allows the manifold designer more room for a smoother turn radius from the plenum in an X-plane-style intake. In addition to having runner shape advantages, a raised-runner intake manifold has a much better approach angle from the manifold exit to the runner entrance of the cylinder head. In order to run our 23-degree, raised-runner design, our heads require a shaft-mount rocker system as well as a dedicated intake to properly align the raised intake ports."
Trial & Error
After establishing design goals, the next step in producing a new head is developing the most efficient, highest-flowing piece that meets the design parameters. This step requires all the intense flowbench time and constant trial and error, especially when trying to push the envelope of flow in relation to port volume. AFR does not use any computer port modeling. Designing ports is still done the old-fashioned way, by hand, which requires a lot of time, creativity, and dedication to the project. "Various valve jobs are experimented with, and every shape and contour of the port is addressed to try to achieve strong numbers at every lift point, not just at peak," Tony explains. "For example, our new 23-degree 195cc Eliminator street head (competition version) flows slightly more than 300 cfm at only 0.600 lift. Many aftermarket race heads that are 30 cc larger can barely muster that number by 0.700 lift, and some never get there at all, and that same 195cc port is already flowing 260 cfm as early as 0.400 lift. The entire curve is explosive, and that's one of the key factors in designing a cylinder head that's going to produce big power."