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

Tech | Piston And Ring Technologies
Whether you're considering new cars, or the high-performance aftermarket, engines are being built with more power than ever before. Items like power adders, multiple camshafts, and EFI tend to get all the glory, but it's true that virtually no stone has been left unturned in the quest for improved efficiency and power. Buried beneath all the flash, pistons and rings play an obviously critical, though somewhat overlooked role in the success of any engine. Influencing horsepower and durability, piston and ring technology has evolved through the years, and we felt it pertinent to review current thinking.

In reality, today's pistons serve the same purpose they have since the dawn of the internal combustion engine-converting the energy of combustion to mechanical energy. From there on however, it's a whole new world, with engineering and manufacturing capabilities at all time highs. Clean slate piston designs incorporate computer 3D modeling and Finite Element Analysis (FEA), helping identify areas of stress before a prototype is ever manufactured. CNC machining results in incredible precision, while space age coatings offer properties that were unheard of when the cars we love were first built.

In reality, the materials used for manufacturing pistons are much as they have been for years, with cast and forged aluminum being the real world choices. Castings are a perfect fit for many OEM applications due to low cost, and low thermal expansion, but their durability is not adequate for more demanding high-performance environs. Hypereutectic castings are a notable exception, and can be a sensible choice in a variety of performance endeavors (see our sidebar, Hyperwhat?). Billet aluminum is sometimes used in high-end racing pistons, but this aside; the most durable slugs for serious high performance are forgings. In fact, consider them mandatory in the world of high boost and big nitrous hits.

JE's Randy Gillis explained that the aluminum alloys used in today's forgings were developed before WWII by Rolls Royce, for use in aviation. These alloys are 2618 and 4032, which have different characteristics which are important to understand. The 2618 is the most durable in extreme environs, but with a higher thermal expansion rate than 4032, and requires greater piston to cylinder wall clearances when cold. The 4032 is made from material with higher silicon content, enabling tighter cold cylinder clearances that are particularly desirable to the OEMs due to emissions. However, the higher silicon content of 4032 (roughly 12 percent) makes it less ductile (more brittle) than 2618, thus it's ultimately not as friendly to punishing abuse.

The Science Of Sealing-Today's Rings
One of the biggest changes in piston design over the years is really all about rings. As a quick review, rings serve three functions in an engine: 1) they keep combustion in the chamber, 2) they help transfer heat from the piston to the cylinder wall, and 3) they keep oil where it belongs. In the heyday of the muscle car, common ring thicknesses were 5/64-inch for the top two rings, and 3/16-inch for the oil ring. If you haven't built an engine for a while, you may be surprised at the changes, because thin is in! As CP's Ed Urcis told us, 1/16-inch is the new 5/64-inch, while many applications are much thinner. Take for instance the GM LS and Ford Modular V-8s, which typically come from the factory with a metric sized 1.5mm, 1.5mm, 3.0mm package. Racing applications are often much thinner still, and make use of strategies such as gas porting, but the point is obvious-thinner rings develop less friction, and perhaps surprisingly, seal better as they conform more easily to the cylinder bore.

So how thin is thin? We asked several manufacturers about the thinnest ring set they would recommend for what we view as our broadest readership-those with hot street cars that see occasional track use. While there wasn't unanimous agreement, everyone was comfortable with the metric 1.5mm, 1.5mm, 3.0mm combo mentioned previously. Many were good with thinner 1.2mm or 0.043-inch top rings, including Total Seal's Kevin Studaker, who mentioned the edgy possibility of a 0.043mm, 0.043mm, 3.0mm package for normally aspirated and modest boost situations. Studaker also offered that when heading above 20 psi of boost, a bit thicker combination such as 1/16-inch, 0.043-inch, 3/16-inch is likely to result in better sealing and durability over the long haul. Of course, since pistons must have corresponding sized grooves, much of this will be dictated by whether you plan your next engine by considering your ring package first. As will become evident as you read this story, rings really are the prudent place to start.

Budgets will also dictate. Not only are thinner rings typically more expensive since they tend to use more costly materials, what you consider to be an ultimate ring package may not be available with an off the shelf piston. The necessary custom pistons can easily cost twice that of a modest forged catalog part, and often much more depending on options. Premium ring sets that include thin stainless steel top rings and Napier second rings are in the range of $300 per set, whereas a similar dimension ductile iron/non-Napier set for the same engine is in the $100 range. The financial equation was emphasized by Probe's Shawn Mendenhall when explaining why Probe's economical FPS forged piston line still uses a conventional 5/64-inch, 5/64-inch, 3/16-inch ring pack. Enthusiasts on a budget seem to appreciate the fact that such traditional size rings are noticeably less expensive than thinner sets, even when comparing like materials. Consistent with the philosophy of offering a wide range of products and prices, Probe's more expensive SRS forged line comes with a more modern 1/16-inch, 1/16-inch, 3/16-inch combination.