Most pistons, whether full round or slipper skirted are symmetrical in skirt contour; that is, both skirts are identical in size and profile. When you examine one of these pistons it is difficult to observe any discernable difference between the minor thrust skirt and the major thrust skirt. Most pistons are made with symmetrical skirts, but a different type of piston skirt pioneered and popularized by JE Pistons offers significant performance gains by altering the skirt design on the minor thrust side of the piston. The major thrust side retains the traditional style skirt with ovality and barrel characteristics appropriate to the specific application. The minor thrust side however, has a notably smaller skirt just large enough to still provide the stabilizing contact patch, but not as broad and robust as the skirt on the major thrust side. Because it is not subject to the higher thrust loads it can be reduced in size to reduce piston weight and frictional losses. The minor thrust side may also have its own degree of ovality and barrel contour that differs from the major thrust side.
JE asymmetrical pistons use a forged-side-relief design (FSR) with major and minor thrust skirts, inboard pin bosses and a shorter and stiffer wrist pin. Over the years, the skirt designs were tweaked, sometimes significantly, as designers refined their function according to changing performance requirements. The primary focus of the asymmetrical piston design is weight and friction reduction via the FSR’s smaller, lighter skirt on the minor thrust side. A key benefit of the asymmetrical piston is the shorter and lighter piston pin. On a Chevy LS engine, the piston pins are only 2.250 inches long, reducing piston weight by up to 10 grams.
The asymmetrical profiles do not throw the piston out of balance relative to the pin position The combination of lighter pin and pin offset keeps the piston balanced. JE offsets the pin toward the major thrust side making the balance of the piston on the pin axis almost ideal. So, the smaller skirt has no effect on the balance unless you grind on the piston yourself during balancing. This should be unnecessary since the pistons are manufactured as balanced sets. Each piston is different and will have its own unique skirt profile based on application and anticipated or verified loading. Engineers have a good handle on this, but they still test multiple skirt profiles, often in the same test engine to gain a real-world picture of how the skirt performs under stress. The engine’s stroke, rod length and maximum cylinder pressure all contribute to any given skirt requirement. Supercharged and nitrous applications also play a factor and will normally have a more robust skirt on the major thrust side.
In addition to ovality, barrel and skirt symmetry considerations the forged-side-relief configuration lends considerable strength to the skirt profile as it concentrates the greater mass of the piston toward the center and behind the major thrust surface. Piston are one of the few engine parts that incorporate multiple shapes and sizes that merge into an overall profile best suited for any application. Maximum gains in friction reduction are typically realized via stringent engineering protocols that define skirt profiles to meet the wide variety of performance conditions encountered in motorsports. As ring packs have evolved to thinner rings, the skirt contact patch became the final frontier of frictional challenges. By combining some, or all the available skirt shapes, designers have been able to reduce friction while improving piston stability and ring function. That translates to greater performance and component durability for all racers.