How piston rings work
An engine is a simple mechanical device, an air pump really, that uses pistons and valves to pull air in and push air out. The pistons in an engine block must be sealed inside each cylinder in order for the process to be efficient. Because the pistons travel through the stroke millions of times throughout the life of the engine, the seals are critical. This is done with a series of rings that go around the top half of the piston, sealing it to the cylinder wall. When the piston rings wear out, the engine’s efficiency suffers.
How they work
Piston rings are made of metal, typical stock engines use cast iron rings, while high-performance engines may use ductile iron rings with a chromoly or chrome face. Cast iron is good for stock rebuilds, but that is about it.
Ductile iron rings are economical and good for performance engine builds. When left as is (no additional material facing), they are generally good to about two horsepower per cubic inch, so a naturally-aspirated 600-hp 350 cubic inch engine could safely run plain ductile iron rings. Stepping up to more power or adding boost or nitrous, the rings would need to be upgraded.
Many performance piston rings use the ductile iron ring and add a facing, such as plasma moly, which adds hardness and protection for added RPM and boost. Many top-fuel dragsters run plasma moly ductile rings for their durability, but they also get changed every run, so take that for what it is worth.
Modern engines have increasingly narrower piston rings, this is in part because of better manufacturing processes and material refinements. The narrower the ring, the more fragile they become. For this reason, a performance build using narrow rings means that you need the strongest material. Steel-based rings are 20-percent (or more) stronger than ductile iron rings, making them the ideal choice for narrow rings.
Horsepower is not the only benefit of a narrow steel ring. Because the ring is so narrow, it reduces friction and can conform to the cylinder wall better at the same time, so the engine is more efficient and seals better. This means less oil blowby and less emissions. Steel rings last longer, but they do take longer to break in. Steel rings may be plain carbon steel or stainless steel, however stainless rings cannot be used in a standard engine bore without titanium or chromium nitriding.
Ring coatings, such as plasma moly, titanium, chromium, and ceramic are good options for performance engines. These coatings are often used for specific engine builds, such as dirt track (chromium for dirt resistance). Chrome nitriding does not flake off like a typical chrome plating.
Moly rings break in faster than chrome, and they have better scuff heat resistance. The drawback for moly-faced rings is that under detonation, the moly material can be damaged significantly. Moly rings are not good for alcohol fuel either.
The design of the ring profile is another variable. Most stock rings use a square-face for both the top and second rings. This design is functional and seals the cylinder, but there are better alternatives. Barrel-shaped profiles, either center or offset, provide excellent sealing and long life. A square-faced top ring will eventually wear down to a natural barrel shape, having this shape from the beginning provides longer ring life.
The second ring is usually square, tapered or napier shaped. A tapered face is just that, a slightly ramped edge. This is for improved scraping of the cylinder walls to remove oil. The main job of the second ring is oil control, sealing the combustion chamber is a distant second. The Napier profile is a square-faced ring with a hook groove carved into the underside. This groove pulls oil away from the cylinder wall, reducing friction and improving oil control. Napier rings are susceptible to damage from heavy boost. This is due to the hook thinning out the ring at the edge.
The top ring is designed to control 90-percent of the combustion gasses, the second ring handles the last 10-20 percent. The last set of rings are called the oil rings, these rings scrape the majority of the oil off the cylinder walls. The oil rings are actually a series of three pieces- two thin chrome-faced or nitrided steel rings with a corrugated expander ring in the center. Performance and stock engines alike use the same design and for good reason- it works. The two thin rings seal the cylinder walls independently, unlike a single oil ring. The corrugated expander provides a drain relief to get the oil out of the way quickly.