Two of the most common alloys used in pistons are 2618 and 4032 aluminum. JE manufactures pistons from both alloys, and Crawford says that the pros and cons of each determine the applications in which they should be used. "The main differences between the two are their material composition and thermal and fatigue characteristics. JE's 4032 performance piston alloy has a silicon content of approximately 12 percent, while 2618 has less than 0.2 percent," says Golya. This means that the 2618 alloy expands approximately 15 percent more than the 4032 alloy when exposed to elevated temperatures. Some people prefer 4032 alloy for their street-driven vehicles since they require less cold clearance and reduce startup noise. Mechanically, both are very similar, with 2618 having higher strength at all temperatures. "When selecting the proper material for a piston, we don't just consider the room temperature strength but also strength at operating temperatures. This is where 2618 outperforms 4032, as it's significantly stronger at temperatures of 500 degrees F and more. Since many racing engines operate above that temperature range, 2618 has the clear strength advantage for these applications. Consequently, 2618 is used extensively in Formula 1, NASCAR, and ALMS, while 4032 isn't. Currently we're testing new materials that could potentially yield the benefits of both alloys."
Piston skirts provide stability within the bore, but also create friction. When designing a piston, the trick is achieving a balance between maximizing stability and minimizing friction. In essence, the piston skirts allow the piston to perform its primary and secondary movements. The primary movement of a piston is when it traverses from TDC to BDC and back up to TDC again. Its secondary movement is the effect of the piston rocking in the bore. The rocking effect is caused by frictional and viscous drag, piston center of gravity location, constantly changing side loading, and changes in temperature. The key in controlling skirt wear, reducing parasitic losses, and improving ring seal is predicting the secondary motion. Frictional losses associated with the piston skirt are substantially influenced by its width and length, hence the surface area of contact between the skirt and cylinder bore. As the contact area is decreased, the viscosity will tend to fall and so will frictional force. However, as the bearing area and viscosity decrease, so does the oil film thickness. If the film thickness approaches the sum of the asperity heights on the two surfaces, this will result in boundary lubrication and an increase in friction. The contact area and the access of oil to that area therefore need to be optimized. There are two ways to reduce the contact area: reducing skirt length and changing the surface shape of the skirt. The first will reduce friction but increase the secondary motion effects influencing ring seal. The second is more successful, as the contact patch and secondary motion can be reduced. At JE, we are constantly developing new skirt shapes that focus on these factors.--Alan Stevenson
Engine builders have been debating second ring gap for quite some time. For most applications, JE says it's better to run a larger gap on the second ring than the top. "Many tests have proven that a larger second ring gap increases the top ring's ability to seal. The larger second ring gap helps release cylinder pressure that passes by the top ring," Stevenson explains. "Without this relief, the pressure can lift the top ring and impair ring seal. In some situations, the top ring seals well enough to eliminate the need for a larger second ring gap. This is typical in naturally aspirated engines that do not experience high cylinder pressures. Ring bore conformity, thickness, and tension play into the equation as well, but for most performance engines a larger second ring gap is preferred."