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Performance Piston Rings

There are a lot of choices available and a lot of questions to answer

By Larry Carley

Page 1 of 2

When you’re building a performance engine, what kind of piston rings should you use? Rings run the gamut from cast iron to ductile iron to steel, with various dimensions, edge profiles and wear coatings. There are a lot of choices available and a lot of questions to answer. Some piston sets come with rings while others do not, leaving the choice up to you.

The type of rings that are “best” for any given engine will depend on how much power the engine will hopefully make, the kind of fuel it will use, and the kind of racing or performance environment in which it will operate.

An engine that is being built for a street performance application will not experience the same kind of loads and conditions as an engine that’s being built to run on a dirt track, a drag strip or in a boat. Whatever type of rings you choose, they should provide the best combination of sealing control, durability, heat resistance and oil control.

Ring Materials
Generally speaking, steel piston rings are more durable than ductile iron rings, and ductile iron rings are more durable than cast iron rings. For the past decade most stock production engines have been factory equipped with steel top compression rings to handle the higher operating temperatures and loads of today’s higher output engines.

Yet many Top Fuel dragster engines still run ductile iron plasma moly faced rings. Why? Because the moly rings provide the sealing and lubrication required in this extremely demanding application. Of course, the rings are changed after every run – which is something that would not be a practical option for most other types of engine applications.

Ductile iron moly faced rings are still the best choice for many traditional types of street and strip performance engines. They are the most economical choice for a typical naturally aspirated SB/BB Chevy or Ford engine, and can safely handle compression loads of up to about two horsepower per cubic inch.

Power adders such as nitrous oxide, a turbocharger or supercharger increase the heat load in the combustion chamber significantly, which also increases the risk of ring-damaging detonation. For higher output performance engines, moving up to a stronger steel ring is highly recommended.

Grey cast iron is a fragile material with very little tensile strength compared to either ductile iron or steel. Cast iron rings are less expensive than ductile iron or steel, and are perfectly adequate for relatively stock to moderate performance engines. But as an engine’s power output goes up, cast iron rings soon reach their limit.

Ductile iron is a strong ring material and has been utilized in both performance applications and in diesel engines for many years. Ductile iron rings are therefore a good upgrade over grey cast iron rings in higher output engines. Steel rings, however, can provide a 20 percent or more improvement both in tensile strength and fatigue strength compared to ductile cast iron.

One of the reasons why more and more rings are now made of steel is because of the narrower face widths that are now being used in so many ring sets (both original equipment and aftermarket). Thinner, low tension rings reduce friction and are more conformable to the cylinder wall. Consequently, they provide better sealing and less blowby.

A traditional 5/16˝ compression ring may generate 7 to 8 pounds of tension compared to maybe half a pound of tension in a 0.7 mm compression ring in a NASCAR engine. That’s a significant reduction in friction and drag. But it also requires a stronger, more durable ring material.

According to one ring supplier we interviewed, there are two basic types of steel rings being manufactured for the performance aftermarket. One is carbon steel, which is used in ring sets for both performance engines and late model OEM replacement sets. It is 35 percent stronger than grey cast iron, can be coated with moly, chrome plated or run unplated in the bore.

The other type of steel is stainless, which is the material of choice for NASCAR, NHRA Pro Stock and other high end applications running rings as narrow as 0.7 mm. Stainless steel rings cannot be run in a cast iron bore without some type of surface treatment such as gas nitriding or PVD (Physical Vapor Deposition) with an alloy of titanium or chromium.