Eric English
August 23, 2010
Photos By: Courtesy of the Manufacturer, emap usa Archives

Did You Know?
In our conversation with JE's Randy Gillis, he mentioned several basics of piston design we thought worth passing along. When looking down on a piston, the shape is slightly oval rather than perfectly round, though you won't detect it with your eyes. Gillis emphasized that aluminum acts like a heat sink, and where there is more aluminum, more heat will be absorbed. Such areas will expand more when heated, and since the supporting structure for the wristpins is one of the beefiest areas of a piston, the piston is 0.004-0.008-inch smaller in this dimension than it is at locations 90-degrees from the pins. As well, the side profile of a piston is not perfectly straight. Again, since there is more aluminum toward the top of the piston than there is in the skirt, the top will be 0.030- to 0.035-inch smaller in diameter to allow for more expansion. Speaking of temps, Gillis mentioned that the top of a piston can easily be 500 degrees Fahrenheit, whereas the skirt will typically run around 200 degrees Fahrenheit. As for rings, did you know that rings rotate around the piston during operation? It's true, otherwise the ring end gap would groove the cylinder wall. Also, the face of the top ring is described as a barrel-face, or rounded, which has been found to provide a better seal than the more square shouldered top rings of days gone by. As for the second ring, they have a taper to the face-the bottom of the ring touching the cylinder wall while the top is slightly narrower, resulting in better oil scraping action.

Hyper what? You've no doubt seen the term "hypereutectic" when it comes to piston composition, but what does the term actually mean? Keith Black's Scott Sulprizio explained that a eutectic alloy results when silicon is mixed with aluminum to the point of maximum saturation or absorption, meaning any additional silicon would fall out of suspension during creation of the alloy. A 12 percent mix of silicon is the eutectic point in an aluminum alloy, with a higher silicon content resulting in a hypereutectic product, and a lower silicon content technically being hypoeutectic.

Of course aluminum must be melted to mix silicon and other additives which comprise an alloy, but heating it to four times the melting temp of aluminum will also melt the silicon into the solution when creating a hypereutectic. When cooled, these silicon particles reform throughout the piston body, but have much different characteristics than before they were melted. "They're extremely hard, almost ceramic-like," says Scott. The result is a much tougher piston that has excellent thermal properties (low expansion and low heat absorption), and resists wear to a much higher degree than a standard casting. Good as hypereutectic material is, Scott went on to offer that "there isn't a perfect alloy that does it all. Hypereutectics keep more heat in the combustion chamber and have nice tight cold tolerances, but the flip side to adding more silicon is that you lose ductility." In other words, the product is more brittle than a low silicon alloy.

Pistons manufactured from forged material, where tremendous pressure is applied to create a denser crystalline structure and alleviate porosity, are considered more durable than any casting regardless of material, so when does one decide that they've exceeded the limits of a hypereutectic? That's hard to say, though Scott feels it's all about engine management. "As horsepower goes up, the engine management becomes way more important, with spot-on air/fuel ratios and avoiding detonation becoming absolute keys to piston survival. Hypereutectics are well suited to 400- to 500-horsepower applications when the tune is sharp, and the OEM's big-dollar computers are able to do this very well." What about the shade tree mechanic with his box-stock carburetor? It's easy to predict that his engine management skills won't be as sharp.

Federal Mogul's Scott Gabrielson was a bit more specific when trying to recommend a point where his Speed Pro/Sealed Power customers should consider moving from hyper-eutectics to a forging. He agrees that hypereutectics are plenty strong for many high-performance engines, articulating that they're physically stronger than a 4032 forging under normal operating temperatures. The caveat is that when something goes awry, the failure is dramatic, and Scott offered that, "hypereutectics aren't friendly to detonation." For this reason, a couple of his trip points to going forged would be if boost exceeds 8 psi, or if nitrous use of more than 150 horses is in the works. For the sake of comparison, hypereutectic alloy has 16-18 percent silicon content, 4032 has 11 percent silicon, and 2618 has about 1 percent silicon.

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