Peer under the hood of the fast cars at the 'strip or the local burger joint and you'll see the AFR logo etched into more than a few cylinder heads. Racers demand provenperformance, so it's no coincidence that many of them turn to Airflow Research for their cylinder heads. Almost forgotten due to the company's overwhelming success in the street/'strip arena is its decorated history in the most competitive ranks of professional racing. Over the years, AFR has powered top drag racers such as Bill Jenkins, Warren Johnson, and Bill Glidden to the winner's circle. Likewise, NASCAR legends Richard Petty, Darrel Waltrip, and Cale Yarborough all racked up multiple championships thanks to AFR cylinder heads. In other words, these guys make some darn good stuff.
Unlike other pack leaders of the industry that prefer to stay mum, AFR has never been shy about sharing its expertise with the public. The company's head R&D man, Tony Mamo, visits popular online forums on a daily basis. In addition to battling online myth-spreading with decades of head-porting experience, he encourages consumers to independently flow-test the heads they purchase and challenge the veracity of manufacturers' claims. His candid and unorthodox approach has earned him a reputation as one straight-shootin' SOB, so we're giving him the space to do what he does best: tell it like it is.
"The first step in designing a new cylinder head is to clearly define the direction and goals of the product," Tony says. "For our Gen III head, we wanted to produce a more efficient cylinder head that would make significantly more power but retain the factory port locations and valvetrain geometry. However, keeping everything in the stock location poses a greater challenge in port design because you're not working with any inherent advantages such as a raised intake floor, taller exhaust port, or raised intake roof. In the case of our BBC product, we decided to raise the exhaust port height to help facilitate a port design capable of exhaling enough flow to cover the much improved intake port. This would have been impossible without raising the port, as the height of the factory exit simply makes for too sharp of a turn, and no matter who designs the port, the laws of fluid dynamics would simply never allow a substantial amount of airflow with that sharp a bend."
Gen III cylinder heads flow exceptionally well due in part to their extremely flat valve angles, but Tony feels that too much emphasis can be placed on valve angles. "I'm certainly not going to say a flatter valve angle isn't advantageous, because it is, but the chamber design itself, the port design, the valve job, and the profile of the valve will have a far greater impact on flow," Tony says. A flatter valve angle doesn't guarantee more flow or additional power. It's the package as a whole that must be taken into consideration. Port architecture and location are much more important than valve angle. Much of the airflow benefit of moving from a 23- to an 18-degree head is attributable to better port geometry and layout. "Instead of a 90-degree turn in the intake port, an 18-degree head has a near-straight shot to the intake valve in addition to a higher intake floor, a raised port roof, and much larger valves; it's just a completely superior port layout on top of the fact that it has a flatter valve angle and better chamber design," Tony explains. "Another factor to consider is the point of diminishing returns. The factory LS1 15-degree valve angle is already an extremely flat valve angle, and moving it a few degrees flatter doesn't have nearly the same impact as it would in an application that started at, say, 23 or 26 degrees like the old-school SBC and BBC, respectively."
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," Tony reveals. 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," Tony says. "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," Tony says. "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."