If size is the only thing that matters, then big-block Fords would dominate the street/strip world. They don't. At least not yet, but big changes are on the horizon thanks to Air Flow Research. Most commonly unleashed to the public in 429- and 460ci configurations, Ford's 385-series big-block has all the right stuff for maximizing displacement. With a 10.320-inch deck height and an extra-wide crankcase, it can easily swallow up a 4.500-inch crank. Likewise, the big Ford's 4.900-inch bore spacing yields cylinder walls thick enough for at least a 4.440-inch bore. Consequently, 557 ci can be yours using an OEM block that sells for peanuts at the local junkyard. On the flip side, easy access to cheap stroker kits and mega cubic inches means you need some serious cylinder heads to keep pace. While the big-block Ford crowd has been seething with jealousy over the endless cylinder-head options their big-block Chevy competition has to choose from, all of that's about to change if Air Flow Research has any say in the matter. The company's brand-new Cobra Jet–style heads are poised to revolutionize the Ford market.
Thanks to our incessant finagling, we were able to bribe AFR's security with some Q16 and VHT to check out these new cylinder heads in person. Although they're still a work in progress, the initial flow figures from AFR's prototype castings are very promising. The intake ports on AFR's entry-level 270cc heads check in at close to 400 cfm at just 0.650-inch lift. Considering that the factory Cobra Jet architecture is stricken with notoriously weak exhaust ports, the 280-plus-cfm AFR is even more impressive. AFR also plans on releasing heads with larger 285cc and 300cc intake-port configurations. Tony Mamo, AFR's R&D manager and chief designer behind the project, speculates the 300cc version could approach or clear 420 cfm on the intake side and more than 300 cfm on the exhaust.
Granted that AFR has earned an outstanding reputation within the small-block Chevy, big-block Chevy, GM Gen III/IV, and small-block Ford markets, the big-block Ford arena represents an entirely new frontier for the company. Nevertheless, AFR felt that the time to pounce is now.
"Over the years, lots of people have asked us to develop a big-block Ford cylinder head," Mamo explained. "We have many happy customers running our small-block Ford heads, so it made sense to round out our Ford lineup. There are certainly some good big-block Ford heads on the market, but we wanted to raise the bar considerably and felt that by ditching the large-and-inefficient, factory, egg-shaped intake-port design and coupling that with a vastly improved exhaust port, we could really give the BBF fans something to rejoice over. In fact, our new big-block Ford product is the most clean-sheet cylinder-head design we've ever undertaken. Despite the fact that it shares very little with most of the other heads in the marketplace, it's still a bolt-on upgrade that readily accepts OEM and aftermarket accessories."
Cobra Jet Shortcomings
Fully understanding where the new AFR castings stand on the big-block Ford evolutionary ladder requires examining the history of the 385-series cylinder heads. In the realm of high-end drag racing, options abound for big-block Ford heads capable of flowing well over 500 cfm. Much like a spread-port big-block Chevy casting, these 429/460 race heads trace their roots to NHRA Pro Stock. Likewise, they typically require extensive prep work, exotic valvetrain components, and intake valves too big to fit inside a bored-to-the-max factory block. Consequently, once factoring in the labor and supporting top-end hardware to make it all work, they can set you back more than $10,000. In other words, flow numbers aside, these aren't exactly street-friendly or budget-friendly options.
On the opposite end of the spectrum are aftermarket castings based on Ford's OEM Cobra Jet cylinder head architecture. Often referred to by the Ford faithful as "A429" castings, the factory Cobra Jet heads were plagued by a fundamental design flaw that positioned the valves too close to the exhaust-header side of the block. As such, airflow never quite lived up to potential offered by the big-block Ford's canted-valve layout and flat 15-degree valve angle. "With the stock Cobra Jet heads, the valves are crowded too close to the outer cylinder wall. You're not utilizing the entire circumference of the valve," Mamo said. "Air likes a continuous flow path, and you're essentially chopping off the curtain area if the valve is up against the cylinder wall. When that happens, you can end up with a 2.300-inch intake valve that performs like a smaller valve because it's so shrouded."
With a 3⁄4-inch-thick deck and beefy reinforcements throughout, AFR's big-block Ford cylinder head can handle power-adders with ease. It's drilled for 7⁄16-inch rocker studs and is fully compatible with the standard 429/460 valvetrain components.
Overlaying a stock intake manifold gasket on the AFR heads clearly illustrates how dramatically the intake-port geometry has been revised. AFR's flow testing revealed that there is little to no air movement around the edges of the inefficient OEM-style, egg-shaped port.
Since AFR will offer standard-height exhaust ports, as well as ports raised 0.250 and 0.600 inch, the head casting boasts plenty of material around the exhaust ports. Many aftermarket 429/460 cylinder heads utilize a big-block Chevy header flange pattern, but AFR stuck with the big-block Ford layout to maintain compatibility with existing headers.
AFR determined early on that designing the intake ports around existing off-the-shelf intake manifolds would severely limit its ability to optimize port architecture. Instead, AFR designed a new intake manifold around the cylinder heads, which allowed completely abandoning the inefficient egg-shaped factory port design. To eliminate sharp edges that restrict airflow, the new ports feature smooth, rounded corners. While they can technically be considered oval-port heads, they are much more rectangular than the stock egg-shaped ports (aka Mamo's "roval" designation).