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AFR's Big-Block Ford Cylinder Heads

Dropping The Blue Oval Hammer

By Stephen Kim, Photography by Stephen Kim

Revised Valve Positioning

Considering the problematic valve positioning of the factory Cobra Jet cylinder heads, one of AFR's top design priorities was relocating the valves closer to the cylinder-bore centerline. "Relocating the valves closer the bore centerline gives you a lot more real estate for unshrouding both the intake and the exhaust valve. This allows the ports to become more efficient, especially in the lower lift regions of the flow curve," Mamo said. "It also opens up opportunities to install a larger valve without the typical shrouding associated with less-than-ideal valve positioning. This change really helped our exhaust-port flow, which, as most big-block Ford engine builders know, is the Achilles' heel of the OEM 385-series cylinder heads. Even the newer Cobra Jet heads—which are an upgrade over stock—are hampered by exhaust flow numbers that wouldn't even get a small-block Ford enthusiast excited. It's the major bottleneck of this cylinder-head architecture and the reason why most big-block Fords don't make the power that their intake-port cfm might lead you to believe is possible. They just can't properly evacuate the cylinder to make room for the next fresh charge of air during the intake stroke."

Filling the Floors

In addition to repositioning the valves closer to the center of the bore, AFR substantially filled the floor of the exhaust port to remove the dog-leg shape that's inherent to the stock head's design. "The airflow is trying to negotiate a 120-degree turn, and it simply can't do that once you start moving a reasonable amount of high-speed air through the port. By filling the exhaust floor, we removed an area of dead space that causes back swirl and turbulence," Mamo said. "Not only do you pick up cfm from doing so, you greatly enhance the port's efficiency because the smaller, higher-flowing port has a lot more air speed. That creates a better siphoning effect for the next intake charge, which helps get it moving into the cylinder on the overlap cycle of the valve events. With the improved valve locations, the filled floor, and a ton of time invested in the overall port design, our standard-height exhaust port will flow over 275 cfm, a very stout number for a bolt-on-style 385-series cylinder head." AFR will also have two raised exhaust-port versions of this design with flow numbers exceeding 300 cfm.

Valve Angle

Of course, great exhaust flow doesn't mean squat without intake ports capable of filling up the cylinders in the first place. Although a super-flat valve angle would have definitely been worth some wow factor, AFR took a more practical approach to designing its intake ports instead. "We could have gone with a very flat valve angle to make our heads look more impressive on paper. However, if you don't have the right port geometry to go along with it, it's not going to work," Mamo explained. "Unless you can substantially raise the height of the port entrance, a flatter valve angle forces the air to make a much sharper turn at the short-turn radius. As air speed and density increase at the higher lifts, forcing the air to negotiate a sharper turn will ultimately limit how much you can get through the port, and the airflow is likely to stall and go turbulent. To avoid this, we opted for a 14-degree valve angle, which is certainly flat in the grand scheme of things, but not as flat as some other heads on the market. Similar to improving the shape of the exhaust port by filling the floor, we also improved the ‘angle of attack' of our intake port by taking advantage of the tall, egg-shaped OEM ports. By filling and raising the floor considerably with our rectangular-oval-port design—which we call ‘rovals'—it helps reduce the angle the airflow must turn to get into the cylinders, creating a slightly straighter shot from the port entrance to the backside of the intake valve. We took the available real estate we had in the excessively large factory port design and used it to our advantage by positioning our smaller, more-efficient roval design at the roof—not on the floor like the OEM port architecture"


The combustion chambers should be an extension of the valve job, and AFR has accomplished exactly that. The entry-level 270cc head will come with a rougher level of CNC port work to keep machining time and costs down, while the 285- and 300cc variants will receive a finer level of full CNC porting. Reducing the speed of the CNC machine helps achieve a smoother, more accurate copy of the highly developed prototype cylinder-head design. Regardless of runner volume, all heads will come with a competition, five-angle valve job.

AFR spent hours profiling the area directly beneath the valve seats to ensure that air could flow freely around the valve's entire circumference. With a standard 2.250-inch intake valve in place, there's plenty of room for air to flow around the valve face.

In addition to unshrouding both the intake and exhaust valves, relocating the valves closer to the bore centerline also creates two similarly sized quench pads and both sides of the combustion chamber. For increased fuel homogenization and detonation resistance, quench is a very good thing.

In a factory Cobra Jet cylinder head, the intake-port floor is positioned so close to the deck that the incoming air charge is forced to move "uphill," which reduces air speed and wastes energy. By raising the port floor, AFR was able to increase air velocity while creating a smoother transition at the short-turn radius. This also helps stabilize airflow at high lift.

Customers opting for AFR's standard-height exhaust port will still benefit from a huge increase in flow. By filling in the port floors, AFR was able to dramatically improve high-lift exhaust flow. Once development work is complete, the company is hoping that its 0.600-inch raised port head will flow more than 300 cfm. As any big-block Ford fan will attest, that's a very big deal.

By Stephen Kim
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