Choosing a Ring

The lateral gas ports cut...        read full caption The lateral gas ports cut into this piston use the pressure created by combustion in the cylinder to provide sealing pressure for the rings against the cylinder wall.

In their efforts to make race...        read full caption In their efforts to make race engines as efficient as possible, designers have found ways to cut rings to the absolute minimum without sacrificing too much in the way of durability. This ring (above) from Race Engineering is pushing the envelope at just .0460 inch in thickness. The industry standard (below) is in the range of .0620 inch. The thinner ring reduces the surface area in contact with the cylinder wall, which decreases friction created as the piston moves along its path.

The radial thickness of rings...        read full caption The radial thickness of rings for racing also have been reduced. The Race Engineering ring (above) measures just .1480 inch across, while the more standard ring (below) is .1830 inch. The reduced radial thickness allows the ring to conform to the cylinder bore more closely than its thicker cousin, which provides a more efficient seal.

In today’s ultra-competitive racing environment, every component in an engine is expected to play its part in increasing power output. Gains can come from some unlikely sources—including the humble piston ring. In this article, we talk to four experts in ring technology—Bob Koch of Race Engineering; Jim Walther, former automotive sales manager of Wiseco Pistons; Dave Calvert of CP Pistons; and Tony Alteri of T/A Engines—about new developments in ring design and the latest products available to racers.

Engine builders have long sought to increase power by reducing friction, thus enabling an engine to accelerate faster. However, you can only make parts so light without getting into exotic materials, so thinner ring packages have become an area of interest. Borrowing from proven motorcycle technology, new smaller top and second rings, measuring 1.2mm or .047-inch thick, have been developed.

Normally, thinner rings do not provide long life, which is why street engines use wide 5/64-inch rings. To combat this problem, metric rings are now being made from aircraft steel with a chrome face to dramatically increase durability. These rings should not be confused with the older type of iron-based chrome ring. Those materials did not bond well, but Wiseco’s steel-alloy metric rings are compatible with the chrome, and there is no risk of flaking or cylinder damage. The second ring is manufactured in high-tensile ductile iron.

The benefits of these rings include reduced friction, lighter cylinder tension, and lower weight, which makes them resistant to high-rpm flutter. The metric rings also have a thinner radial thickness (the face-to-inside dimension of the ring), which improves the way the ring conforms to the cylinder wall for improved seal. These 1.2mm rings offer much greater life than .043-inch moly rings. A set costs about $135.

One drawback is the characteristic of the top chrome ring to require more seating time, which can be a problem for racers. However, Race Engineering offers an alternative metric set with a composite nitride top ring, which seats immediately with virtually no penalty in ring life. Expect to pay around $148 for a set.

Another great friction-saver is the three-millimeter oil ring. Almost 10 ft-lb less torque is required to spin the assembly by hand. You can check this figure with a torque wrench. While these rings offer terrific life (many stock production engines use them), their narrower radial dimension promotes improved cylinder conformity and oil control. Race Engineering has obtained excellent results with these rings, which the company uses on all of its two-ring oval-track pistons where there is no second ring to assist in oil control.

The company’s Pro Star pistons now exclusively use the 1.2mm ring package. These components feature lateral gas ports to improve top-ring function and seal. The ports are machined from the circumference of the piston; they cannot be seen from the top of the piston and do not clog with carbon. Additionally, they do not create a path for oil to access the combustion chamber (a potential detonation source). Rather, they gas-load the top ring to improve sealing during the combustion cycle. A piston equipped with lateral gas ports costs between $10-$15 more than a standard product.

Engine builders also have begun to pay attention to the ring grooves in the piston. Wiseco machines a slight degree of uptilt to the top ring groove. When combustion pressure hits the ring, it reverts to a level seating. If the groove was machined perpendicularly, pressure would force the ring to tilt downward. Uptilt helps to promote power and reduces the possibility of the ring micro-welding itself to the piston.

CP Pistons of Irvine, California, has been working on new machining methods. According to Dave Calvert of CP Pistons, the company used highly accurate inspection equipment to understand how machining methods affect the flatness of the ring groove and other piston characteristics. Their experiments enabled them to achieve ring-groove flatness measured to a millionth of an inch—a tolerance never before available. Calvert says he feels as piston technology has improved, ring manufacturers also have enhanced their production methods, and the racer has benefited.

“You can’t make power with a bumpy ring and a flat ring groove, or with a flat ring with a bumpy ring groove,” he says. “They both need to work together to make the best power.”

Tony Alteri uses Race Engineering’s two-ring pistons in engines he builds for the highly restrictive divisions of Late Model and Pro Stock for New England asphalt racers. His only warning is that a team needs to be careful not to overheat the engine.

“When you overheat these thinner rings, they lose their tension and ability to seal,” he says. “As long as you follow the manufacturer’s recommendation for end-gap and cylinder-wall finish, they will work great. Their reduced cylinder drag and lighter weight will produce more power in the applications they are designed for.”

As piston companies improve their products by achieving tighter tolerances, ring manufacturers have provided options to help engine builders, and recommendations to aid the assembly process. Power advantages can be found in products many engine builders overlook. Attention to detail, new technologies, and correct application makes some racers visit victory lane, while others drive back to the pits.