Cylinder Head Design & Misconceptions - CHP Insider

CNC Accuracy
Improved software and machinery are two factors that have enabled AFR to replicate the design of a port on CNC machines much more precisely compared to just a few years ago. Once the digitizing of surfaces, programming, and tool paths are finalized, repeatability becomes the primary concern. This requires maintaining proper tool life and monitoring CNC tool and fixture coordinates, valve job diameter and heights, and other small details that can affect the flow of the heads. The rest of accurately replicating ports is attributable to good old teamwork. "Handling most of the design chores here, I will work directly with our engineering staff during the last phase of developing the ports and CNC programs to achieve a part that's as good as-and occasionally better than-the prototype it was copied from," Tony explains. "Once the hand-ported prototype is digitized, it allows us to be creative with that optimized shape in ways that would be virtually impossible by hand. This is how it becomes feasible for us to occasionally produce a CNC part that's actually better than the original, but trial and error is still the order of the day. This last stage can still require a great deal of time, but the end result is the ability to mass-produce an extremely efficient port design."

Quality Control
Since tiny variations in cylinder heads can make a huge difference in performance, maintaining tight quality control is essential. Tool life in the CNC machines is of paramount importance at AFR and is constantly monitored. An alarm will sound when it's time to change the tools, after a predetermined number of cutting hours are accrued. The carbides used on valve-job machines are also replaced at regular intervals. Valveguides are checked at the CNC machines quickly, using simple but very effective "go" and "no-go" pilots and are checked once again by AFR's QC department with a much more accurate electronic tool that records data in microns. "There are QC checks at all points through the shop, and our operators play a large role in the QC of our product," Tony explains. Anything questionable is immediately pulled off the line. Spot flow checks are also done routinely to verify that they meet or exceed AFR's advertised flow figures. The concentricity of valve jobs is also checked to make sure all the products seat the valves in their respective seats 100 percent of the time. "The bottom line is we have a lot of procedures in place to help ensure we ship a product that meets a long list of criteria," says Tony. "It's our opinion that QC really starts with the employees and the passion for the product they produce."

Design Refinements
"You have to always be looking for ways to improve your product," Tony says. "If you sit back thinking you have the best mousetrap, someone is going to catch you by surprise and produce a better one. We try to stay ahead of the curve by producing a truly exceptional piece that sets the bar two rungs higher, but due to the fact that most of us here are racers, we are never satisfied and are always looking for creative ways to improve upon what we already have. The redesign of our 23-degree SBC head, called the Eliminator, was a huge undertaking. Nothing from our former 23-degree line crossed over, as this was truly a clean-sheet design. The castings were redesigned both externally for aesthetics and internally for function with features like stronger rocker stud bosses and improved oil flow. We also took the recent advancements learned from our Gen III product and incorporated them into the new Eliminator line, specifically lightweight 8mm valves coupled with smaller and lighter springs, retainers, and locks. All of this helps to significantly improve valve control and performance, and extend the engine's operating range. Of course, the real meat and potatoes of any cylinder head is related to airflow, and the new Eliminator castings were shaped around all-new port and combustion chamber designs, ultimately allowing us to bring to market the most efficient 23-degree head we've ever produced. Instead of forgetting about the 23-degree segment of the market and focusing more on the newer Gen III/IV products, we embraced it and decided to swing for the fences by making it significantly better than before."

Manufacturing
"All AFR cylinder heads start as raw ingots of A356 aluminum poured at approximately 1,400 degrees," Tony reveals. "The mold it's poured into is maintained at a specific temperature as well. The casting process is a bit of a black art, and modern foundries are very specialized in what they manufacture. Once the casting is created, the next step is to drill pilot holes that allow us to accurately affix the castings to our CNC fixtures. Every machining operation except the actual CNC porting-such as drilling bolt holes, machining rocker-stud pads, and milling deck and flange surfaces-is done on one of our very large, five-axis, dual-pallet CNC machines. The twin-pallet machines work on a dozen castings at once. Before making their way to the smaller five-axis CNC machines for final porting, hardened seats and bronze guides are installed. Next, the dedicated porting machines handle the combustion chambers and intake and exhaust ports as well as some of the cylinder head engraving. At this point, the heads are pretty much finished but still need to still get a proper valve job, which is a very critical component of flow. The valve-job shape, diameter, and finish depth are all important. From there, it's off to the wash room, where the heads are finally deburred and thoroughly cleaned. Once cleaned, they are transported to our assembly room, where our operators carefully assemble the head with the correct springs, valves, and hardware. It's here that our final QC operator checks every head before it's placed in a box, confirming that the proper parts and machining operations have all been addressed."

Port Volume
Although some consumers equate larger ports with larger flow, AFR strives to keep port volume and cross section to a minimum to promote air speed and cylinder fill inertia. "If you think of air as a liquid, it's easier to understand how a faster column of air will have more energy and inertia to better fill cylinders, with far less reversion issues to boot," Tony explains. "This approach to cylinder head design is especially true if we are focusing on a product aimed more toward a street/'strip application than for race use. A reasonably small port that has a strong airflow curve just makes for an explosive package that you need to experience to really appreciate. If you've driven lazy big-port heads and cam stuff all your life in your daily driver and step into a package I just described, you'd never go back." According to Tony, with the proper traction and gearing, that type of combination can yield quicker e.t.'s, since so much of drag racing is about what happens in the first 60 feet.

Porosity
Porosity in aluminum castings was more common years ago, and much of it was attributed to pour technique and speed, as well as how the mold was vented. "While the material itself can play a role, it was the actual pouring process that was a larger issue, as the mold must be heated prior to the pour," Tony says. "How the mold is designed also plays a role, and additional risers in the casting help provide a solid, nonporous surface. Technology and technique march forward, and while porosity may always occasionally rear its ugly head, most modern foundries have it figured out by now. Of course, in the event we machine a head that's porous, it's immediately flagged and pulled off the line so no additional machine time is wasted on that particular casting."