Performance Piston Rings - Engine Builder Magazine

Performance Piston Rings

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.

Ring Coatings

Rings are available with no coating (plain faced), moly faced or with some type of PVD surface treatment (titanium, chromium or ceramic). Titanium nitride is a very popular surface treatment for high performance steel rings in high end racing engines because it reduces friction and improves wear resistance. Chromium nitride ring treatments offer many of the same advantages and are typically more popular for street performance engines and even many stock production engines.

Chromium nitride is also a popular coating for rings that go into dirt track engines because dirt won’t stick to chrome and scour the cylinders. But as another ring supplier pointed out, moly rings work just as well in dirt track engines provided the engine has good air filtration that keeps the dirt out.

Chrome nitrided PVD rings are not the same as traditional chrome-plated rings. The surface treatment on PVD chrome nitrided rings adheres to the ring very well and won’t crack and flake off like electroplated chrome rings can under adverse operating conditions. Chrome rings have been around since World War II and are made by electroplating chrome onto the surface of the ring.

Chrome rings became a popular choice for off-road and dirt track engines because of their ability to resist abrasion caused by airborne contaminants. However, chrome rings have about 1,000 degrees F less resistance to scuffing than moly coated rings. Since heat is a prime consideration in almost every performance application, moly has been the ring face coating of choice for many years.

 

Moly faced rings typically bed-in faster than chrome, reaching full sealing capacity more quickly, while providing significantly higher scuff resistance. Some ring suppliers improve the abrasion resistance of their moly coatings by combining it with a nickel chrome alloy (which increases face life by 65 percent compared to conventional moly faced rings).

The only disadvantages with moly is that the facing material can be damaged by severe engine detonation, and it may be incompatible with some bore coatings such as nickel/carbide or aluminum based alloys. For such applications, a PVD coated steel ring would be the right choice. Some ring suppliers also do not recommend using moly rings in engines where alcohol based fuels are used for extended periods of time.

Ring Profiles

Piston rings are available with all kinds of edge profiles including square face, taper face, center barrel, offset barrel and napier. The top compression ring will usually be some type of barrel face, while the second ring will often be a taper face or napier face ring. The reason for the different profiles is to optimize the performance of the ring for the job it has to perform.

The top ring is first and foremost a compression sealing ring. Its sole job is to seal the combustion chamber and prevent blowby. The top ring receives the brunt of the heat and compression loading so it has to be strong and durable.

A barrel faced top compression ring usually provides the best combination of sealing ability and wear resistance. A square faced ring seals well but eventually develops a barrel-like profile from ring flex as the piston moves up and down. Giving the ring a barrel profile to begin with reduces ring wear for longer ring life.

 

The second ring’s job is usually about 80 to 90 percent oil control and 10 to 20 percent sealing. The second compression ring backs up the top compression ring from a sealing standpoint but primarily functions as an oil scraper. A slight taper is applied to the face of the second ring (2 to 4 degrees) so it will scrape oil off the cylinder wall when the piston moves down.

Undercutting the bottom edge of the 2nd ring face to create a groove (hook groove or napier profile) improves the oil scraping ability of the ring even more, especially in naturally aspirated engines. The groove also provides a relief area under the ring face for blow-by gas evacuation. Because of this, a napier style second ring is usually the best choice for oil control.

An important point to note is that the second ring is obviously a directional ring and must be installed with the correct side up, otherwise it will pump oil in the wrong direction and increase oil consumption.

 

For engines running volatile fuels like nitromethane and engines running large amounts of boost like blown alcohol and diesel pulling trucks and tractors, napier style second rings are NOT the best choice. These type of engines run such high cylinder pressures that the second ring is forced to deal with substantially more gas pressure blowby.

Consequently, many blown alcohol engines run a ductile iron reverse twist style second ring or use a top ring in the second piston groove. Some also use a Dykes style ring with a barrel faced ductile iron plasma moly ring in the second groove.

The third ring is the oil ring, typically a 3-piece design with a pair of thin chrome faced or nitrided steel oil rails supported by an expander. Advantages of the three piece design are: low cost, good conformability to the cylinder bore, ease of installation and the ability for the ring manufacturer to modify the oil ring tension as demand requires.

The 3-piece design is widely used in both OE and performance ring applications. The general consensus is that the traditional 3-piece oil ring design works great so why mess with it? The 3-piece oil ring provides good oil drain back, good side sealing of the oil groove and allows the two rails to seal against the cylinder bore independently of each other, something a one-piece oil ring cannot do. A 3-piece oil ring also has less mass which reduces inertia forces for better ring stability especially at higher engine speeds.

One piece oil rings are used in many low rpm diesel engines and in some European gasoline engines, but are relatively uncommon here in performance applications. However, they do exist and are successfully used in a variety of F-1, NASCAR and NHRA Pro Stock engines.

The one-piece oil rings are available on special order, are very expensive and are quite different than a traditional 3-piece oil ring. But they do have their advantages. According to one supplier of these rings, their one-piece oil ring in a 4.180? cylinder bore with a 1.5 mm oil ring groove in the piston exerts only about 10 Newtons of load against the cylinder bore.

Of course, conventional 3-piece oil rings have also gotten thinner to reduce tension and friction. A traditional 3-piece, 3/16? thick oil ring that used to create 20 lbs. of tension has now been replaced by smaller, thinner 3-piece oil rings that generate only 3 to 4 pounds of tension. Less tension means less friction and more horsepower.

Ring Gaps

Regardless of what type of rings you choose for an engine, the ring end gaps have to be right for the application. The more power an engine makes, the more heat it generates in the combustion chamber and the more the top ring expands in response to all that heat.

As the ring expands, the distance between the ends of the ring in the ring gap narrows. Hopefully, there’s enough clearance so the ends of the ring don’t overextend and butt up against each other, causing the ring to scuff.

Engines with power adders such as nitrous oxide, a turbocharger or blower obviously create more heat the combustion chamber than most naturally aspirated engines, so requires more end gap clearance when the rings are installed.

A rule of thumb for the top compression ring is .0045? of end gap per inch of bore diameter for a naturally aspirated engine, and .006? for a power adder application. What’s more, many ring suppliers recommend opening up the second ring .005? to .010? more than the top ring gap in naturally aspirated engines to prevent gas build up between the top and second rings.

This can cause the top ring to flutter or bounce and lose its seal. But in a boosted engine, the second ring is exposed to more blowby and should be gapped nearly the same as the top ring.

Piston ring manufacturers publish end gap recommendations in their catalogs and on their websites. Always refer to these recommendations as they will vary from one manufacturer to another depending on the type of rings used, what the rings are made of (cast iron has a higher rate of thermal expansion than steel), and the application.

Final Thoughts On Ring Selection

Though books have been written on this subject, we can boil it down to the following points:

1) What is your total engine budget, and what percentage of that budget can you spend for rings? For example, if you buy a set of exotic one-piece oil rings that cost almost $600 for a V8, the rings alone might cost more than the pistons! Such an expensive outlay for a set of rings wouldn’t make much sense for a typical street/strip engine, but it might be justified for a high dollar Pro Stock engine.

2) Are you building a naturally aspirated engine or one that will be jacked to the max with nitrous oxide, a turbocharger or blower? The higher the power output, the stronger the ring material that will be needed to handle the heat and pressure.

3) How will the engine be used? Drag racing, truck pulling, circle track, marine, street driven, etc. If the application is a street performance engine, how much time realistically will it actually spend at full throttle? Choose a ring facing that provides the kind of durability that matches the application.

4) How often will the engine be refreshed? Will the rings have to last tens of thousands of miles (as they do on the street) or will they be replaced every season?

5) What type of rings are others using in similar applications? If a certain type of ring or facing material is working great for everybody else, trying to be a pioneer may prove to be expensive!

6) What kind of rings are available for the engine’s bore size and piston configuration? For some applications (like engines with unusually large bore diameters or extremely short lightweight pistons), this may limit your options quite a bit.

We thank the following ring suppliers for their input to this article: Federal Mogul/Sealed Power, Hastings, MAHLE, NPR and Total Seal.The type of rings that are 
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