Excerpt from On All Cylinders July 15, 2016, automotive blog post by Stephen Kim.
Running 6.80s at nearly 210 miles-per-hour, today's 275mm drag radial door-slammers incinerate the quarter-mile thanks to obscene levels of airflow and grip. Producing close to six horsepower per cubic-inch requires immense cylinder pressure that can split blocks, blow head gaskets and melt pistons in a jiffy. Fortunately, the aftermarket has methodically eliminated each of these weak links in recent years. While enhanced short-block durability is obviously a good thing, it places an even greater burden on the cylinder heads to keep all that pressure sealed inside the block. To meet the demands of today's boost-hungry race engines, casting technology has evolved tremendously to spawn a new crop of incredibly durable cylinder heads.
While Edelbrock is best known for its extensive line of cylinder heads and intake manifolds, over the last 25 years, the company has quietly established one of the most sophisticated foundry facilities in the country. Initially, the foundry's primary focus was elevating the quality of its street castings, but as racers continued pushing the limits of durability, Edelbrock developed many key technological advancements in an effort to create the strongest race castings in the business. In fact, when engineers realized that they had reached the limits of their first foundry, Edelbrock built a second foundry in 2007 better suited to the low-volume manufacturing requirements of its race cylinder heads.
The combination of old school know-how and modern technology allows Edelbrock to dynamically adapt its casting process as circumstances dictate.
"We're always experimenting with new heat treat processes and testing new alloys and additives," Dr. Rick Roberts of Edelbrock said. "In the last several years, we've built a new permanent mold foundry and have introduced hot isostatic pressing into our race cylinder head castings. All of these technologies are an evolution of established casting techniques that ultimately create a superior end product."
Although building a foundry from the ground up wasn't easy, the long-term benefits for hot rodders have proven to be well worth the investment.
"Running a foundry in Southern California isn't the cheapest way of doing business, but 25 years ago, we felt that the only way to maintain the quality control we were looking for was to control the entire casting process from start to finish," Roberts said. "We're fortunate that Vic Edelbrock Jr. had the foresight to build a foundry from the ground up in 1991."
Hot Isostatic Pressing (HIP)
Every aluminum casting has tiny air bubbles trapped in it. The question is, how much?
"Hot isostatic pressing is a process originally developed by the aerospace industry to eliminate these microscopic air bubbles. The HIP process heats a raw casting inside a pressurized oven to about 900 degrees to make the aluminum pliable," Roberts said. "The oven is filled with argon to eliminate oxidation. As the pressure in the oven increases to 10,000-15000 psi, it compresses the aluminum and squeezes out the air bubbles. The process actually shrinks the casting, so we have to make it a little bit bigger to compensate for the increase in density. HIP'ing dramatically enhances the overall strength of the casting. A HIP'd casting isn't quite as strong as a billet cylinder head, but it comes pretty darn close."
When Edelbrock began machining its first batch of HIP'd heads, the improvements in hardness were immediately apparent.
As horsepower levels continue skyrocketing, the molds used in the casting processes have evolved to improve both casting quality and manufacturing efficiency.
"Our green sand foundry uses a process very similar to what foundries in the '50s used to cast iron blocks and cylinder heads. Green sand, which is actually black in color, is similar to the sand at the very edge of the water at the beach," Roberts explains. "You can almost see the imprint of your hand and feet in it, but not quite. Green sand can be packed down very tightly, but it also breaks down into loose sand easily. It's used to create the envelope of a casting where the liquid aluminum will be poured into."
Unlike a green sand mold, a permanent mold uses a steel envelope that works like a waffle iron.
"As the liquid aluminum is poured, the chilling effect of the aluminum coming into contact with the steel mold creates a casting that's structurally stronger," Roberts said. "Since the mold is re-usable, you don't have to make a new mold out of sand after casting each cylinder head. The downside is that a permanent mold is three to four times more expensive than a green sand mold, so it only makes sense when casting in high volume. Most of our Performer RPM small block heads are cast in our permanent mold foundry."
Fresh-out-of-the-mold cylinder heads clearly illustrate the path the molten aluminum flows through during the casting process. Liquid metal flows downward from the sprue, through the runners, and into gates positioned at the deck surface of the head. The metal then flows upward from the cylinder head cavity into the risers positioned on top of the head. Excess metal is trimmed from the raw castings before final machining operations can begin.
Green vs. Dry Sand
Shaping liquid aluminum to exacting tolerances with sand can be challenging, especially when the slightest deviation in the contours of a port can dramatically affect performance. Depending on the profile of the shape that needs to be made, Edelbrock uses green sand, dry sand, or a combination of both.
"Green sand is a mixture of sand, clay, and various chemicals. Its clay and moisture content make it easier to shape, but burns off after each casting run," Roberts said. "While it can be tightly packed together, it doesn't bind or get hard. That enables green sand to hold together well during the casting process, but you can still pull it apart by hand, replenish the clay content, then re-use the sand. Humidity levels can affect the moisture content of green sand, so the mixture must be monitored and adjusted as necessary.
"On the other hand, dry sand uses an adhesive as a bonding agent and its smaller grain size creates a smoother finish on the casting. It gets hard as a rock after sitting for 30 minutes, and you have to hit it with a hammer to break it. Green sand is versatile enough to shape all the features under the valve covers, like the oil drains, but it has its limitations. Dry sand works much better with casting shapes that have a lot of delicate and intricate features. Even in our green sand foundry, the cores used to form the ports and water jackets of a cylinder head are made from dry sand.
"Fine, narrow pieces of green sand don't mold or form very well and are prone to breaking. However, making shapes like that aren't a problem with dry sand. The dry sand is used to form the outer envelope of a cylinder head is re-usable, but the sand used to form ports and water jackets is not re-usable. That increases costs significantly, but sometimes that's what it takes to cast a cylinder head strong enough to handle the most demanding race applications."
While a casting mold determines the outer shape of a cylinder head or intake manifold, cores must be inserted into these molds to form any internal passages such as the ports and water jackets. The old fashioned method of making cores involves mixing sand with glue, packing it into a mold by hand, then waiting half an hour for it to harden. It's slow and tedious, but it works fine for low-volume production runs in Edelbrock's dry sand production line. However, newer hollow-core and sulfur-dioxide core processes can handle high-volume production more efficiently.
The traditional shell (hollow) core process uses a cast-iron box that looks like the actual cylinder head.
"After heating up the box and connecting a hose to it, sand is blown into the mold. The sand is impregnated with glue that's activated by heat, so it solidifies when it comes in contact with the hot cast-iron mold," Roberts said. "The sand in the center of each core doesn't heat up as much, so it falls out after it's removed from the mold. You're left with a hollow-shell core that's very accurate and well suited for high-volume production. The downside is that since the core box is made of cast iron, it's very heavy and difficult to modify if we want to change the shape of the cores. The cores used to build most of our Performer RPM cylinder heads are made using the hollow-core process."
About 20 years ago, Edelbrock began transitioning over to a sulfur-dioxide core process. It uses a different mixture of sand that has binders instead of glue. These binders are activated by sulfur dioxide, which means that hot cast-iron molds are no longer necessary.
"Instead of iron, we can use a plastic mold, blow sand into it, then fill it with sulfur dioxide to harden the sand cores," Roberts said. "Not only does this enable us to work at room temperature, it also makes it much easier to repair or modify the molds since they're plastic, not iron. Even though the sand is abrasive, the plastic is very tough and wear-resistant. Over the years, we have gradually converted our production lines from the hollow-core to the sulfur-dioxide core process."
The U.S. Aluminum Association establishes specifications that outline the composition and mechanical properties of aluminum alloys. For example, an A356 alloy is mostly aluminum, but the percentage of silicon and magnesium it contains must fall within a specified range to qualify as A356. These metallurgical formulas also place limits on impurities like iron. Many of these elements represent just a small fraction of a percent in the overall composition of the alloy.
Without proper heat treating, raw castings are not strong enough to survive the stress endured in an internal combustion engine.
Having complete control of the casting process makes it possible to experiment with different metal compositions and heat tempering techniques depending on the needs of a particular application.
After a casting is poured, the solidification process of the molten aluminum must be controlled very closely to prevent air pockets from forming.
"Aluminum shrinks as it solidifies. If you have a pocket of liquid aluminum that's surrounded by metal that's already gone solid, that pocket will tear itself to pieces when it solidifies because it wants to shrink, but it can't," Roberts said. "To prevent this from happening, we pour the cylinder heads with the deck facing downward. Ideally, you want the head to solidify from the deck to valve cover rail in a uniform fashion. A head that’s solid in one spot and liquid in another is a disaster in the making."
Thanks to modern technology, it's now possible to virtually pour a casting on a computer to identify trouble spots before they surface.
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