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Editor’s Note: Seasoned Outlaw 8.5 and 275 Drag Radial racer DJ Reid is documenting the build of a new LSX engine for his ’68 supercharged Camaro. DJ will be sharing not only from his vast experience in engine development but also talking to numerous experts as he selects and matches the components. Following is a story on the piston ring selection.
As in any project, the heads, camshaft profile, piston design and block choices were all evaluated, then chosen based on our objectives. A critical component often overlooked in this cocktail of high-performance parts is the piston ring, which has a simple but demanding function: hold the combustion where it belongs — in the combustion chamber. After all, what good is the countless hours spent on R&D and technology that goes into producing a 450 cfm small-block head if the power leaks past the piston?
The engine’s intended purpose dictates the shape, size, thickness and materials used in a given ring package. In addition to suggesting the proper ring package for our LSX engine, Keith Jones of Total Seal also answered long-standing questions about piston rings to help us understand the rationale and engineering behind those choices, which at the same time, allows us to bring you this in-depth look at piston rings, from shapes to coatings and much more.
Shape Matters
Rings are more than an iron or steel band wrapped around a piston to pick up cylinder wall clearance. The first concern when evaluating Total Seal’s designs is ring shape. This is where engine boost comes into play. As we discussed, conventional ring designs versus gas (or combustion) assisted ring designs, it was clear that the intended purpose of the engine could make or break cylinder fill and ultimately power if we chose the wrong ring package.
Left - The piston rings sit in grooves that are machined into the side of the piston called the ring lands. Gaps above and below the ring can be a source of combustion leakage. A properly shaped ring will shift and seal these gaps during the four-stroke process. Right - The piston ring must be sized to fit in the ring land and seal against the cylinder wall but with minimal friction. A properly sized ring will minimize this potential link but not eliminate it.
“Ordinarily, we want good ring seal on the intake stroke because we’re relying on ring seal to create a strong vacuum for adequate cylinder fill,” explains Jones. “In a boosted application, we’re relying less on vacuum to fill the cylinders and can sacrifice intake stroke ring seal for designs that will increase ring seal on the combustion stroke.”
We want a ring design that provides the most ring seal on the combustion stroke, even if that means sacrificing ring performance at other times — such as low rpm and during the intake stroke. Boosted engines, unlike naturally-aspirated and nitrous-fed engines, achieve cylinder fill with the positive pressure created by a turbo or supercharger. For our project we’re looking at a combustion-assisted ring sealing package, and the most common types are gas-ported pistons and Dykes-style rings.
The main purpose of a piston ring is to provide a complete seal in the cylinder and keep the air/fuel mixture in the combustion chamber for full burn. There are several leak paths in a standard piston ring setup. The ring gap (left) is one such path. Total Seal offers a gapless style ring (right) to prevent combustion leakage through the standard ring gap.
Gas-ported pistons are constructed with small holes drilled along the outer diameter of the crown. The holes lead directly into the rear of the top ring land, allowing combustion gasses to push the ring outward from the inside and assist in combustion sealing without trade offs found with other combustion-assisted ring designs. The downside is the possibility of the port clogging with combustion residue.
We chose a traditional supercharged-style Dykes ring for our setup. Introduced in the ’50s, Dykes rings aim to accomplish the same goal as gas ported pistons, which is, get gas behind the ring to promote sealing.
Ring manufacturers use many different ring profiles to promote better cylinder sealing. In our application, we chose the “Dykes” style profile.
They accomplish this task differently by increasing the gap between the piston ring land and the top face of the ring. The ring is then shaped into an “L” at the front facing edge. The “L” comes into play during the power stroke. As the air and fuel mixture ignites, the gasses attempt to escape the cylinder and are forced downward against the top ring. As this happens, the gasses push against the outer “L”, pinning the ring against the lower portion of the ring land and the cylinder wall. The result is a ring that provides very low tension during the intake and exhaust strokes but creates a ring seal that is proportionally increased as cylinder pressure rises.
Beyond selecting a boost-assisted Dykes style ring, nothing else was changed in ring design to accommodate our boosted application. Jones says he doesn’t change the ring recommendation based on the presence of boost but instead focuses on the harshness of the operating environment.
“As you go up in the materials, they become tougher – you have to build the engine from a clearance point of view and ring material point of view for the worst case scenario,” he explains.
In that respect, Jones likes to look at the increased cylinder pressure from boosted engines to select more robust ring materials and coatings that are matched to the most abusive environment the engine will see. He stressed that the most common environment was not the same as the most abusive. This meant that a “street” engine that “may” see nitrous, should have a ring package designed for the same abusive conditions that you would build into a nitrous engine. However, as a max effort 275 radial engine, we didn’t have to worry about any ambiguity in our build.
Diamond finish rings maintain axial tolerances of +/- .000050-inch and provide improved sealing between the piston and piston ring. The diamond finish ring is on the left and an uncoated ring is on the right.
Ring Materials And Coatings: The What, Where And Why
A Quick Note on Block Finish
Jones says, “I don’t hone blocks based on the face of the ring. Bottom line, my two questions on honing – how hard is the block and what are you doing with it? What we are determining is the depth of the cut or the volume of oil the cylinder wall has to hold. As cylinder pressure goes up, we want the block to hold more oil.”
Generally, he is looking to see a similar finish in most applications but based on tooling and block hardness, consistently achieving the proper finish can be very easy to get wrong. Traditionally builders would read a spec that specified a stone and get to work. Historically this method worked when there was one tooling manufacturer and an assumed block hardness. Today, with varying cutting machines, stones and block manufactures, there is no one size fits all direction for block prep. Because there are so many manufacturers out there, Keith strongly suggested that builders contact Total Seal with their intended application, block manufacturer and machine specifications. From there the guys at Total Seal are able to give the builder a custom recipe for proper honing procedures given the hardware they are working with.
Steel rings are more commonly used today in racing applications. Contrary to their iron predecessors, steel rings are made from coiled steel wire and can be made in small quantities to accommodate varying piston designs and smaller custom orders. In addition to being more versatile, the steel wire can be produced in carbon, stainless or tool variations.
Jones explained, “Piston rings are constructed from cast iron, ductile iron, carbon steel, hardened ductile iron, stainless steel, and tool steel just to name a few materials. From first to last they handle more heat and abuse and in turn are more durable. It is important for us to work with the builder to determine the application and what is correct for his or her needs.”
We were immediately guided in the direction of an Advanced Profile (AP) stainless steel ring due to its stiff and consistent finish properties.
“It’s a nice material, very easy on the bore and seats up quick. Ring material is easy, it’s all about durability. The harder the material, the more durable, and harder materials will withstand harsher environments than the softer materials,” says Jones.
This made our selection of a ring material relatively easy as we would need a tougher than average material for our high compression, blown small block.
Our C33 stainless steel top ring in a .017-inch Dykes configuration. Total Seal takes a standard 1/16-inch ring (.0625-inch thick) and then machines a .017-inch step from the inside of it to form a sideways “L” shape. They also reduce the radial thickness to .175-inch.
After we determined an adequate ring material, we moved on to the topic of ring facing material (also known as ring coatings) and how our build would benefit from them. We first discussed the more familiar but less used coating such as moly and hard chrome.
Total Seal has moved away from these coatings as they tend to have problems with adhesion in high performance applications. Jones explained, “In a race application where you’re getting into high cylinder pressures, detonation can become a problem, boost can be a problem, nitrous can be a problem and it will blow that coating off the ring.”
Total Seal uses several machines that have been modified and sometimes built in-house to meet tight specifications. Above is a ring coiler (left) and a spool of stainless ring material (right).
As we discussed the coatings we would consider in our build, he noted, “What we get into now is called Chrome Nitride and Titanium Nitride. These are coatings that are applied in a vacuum using a process called particle vapor deposition and it literally sticks to the ring at a molecular level. They’re not coming off, they’re not going to chip and they’re not going to flake.”
In our application, we arrived at Total Seal’s “C33” coating on a stainless ring. The C33 coating is a Chrome Nitride (CrN) coated ring that acts much like a chrome faced ring but without the adhesion problems and difficulty in seating that most commonly accompanies the older hard chrome process.
Ring Selection
After our call with Total Seal, we ended up with a solid design for our ring package and a set of instructions for Ross to use during piston construction. We were clear in running with a race only setup but as Jones explained, this distinction is not always easy. “Guys often tell me they are running a street/strip setup. In these cases, the rings will need to meet mostly racing engine specifications”.
It is important for us to work with the builder to determine the application and what is correct for his or her needs. – Keith Jones, Total Seal
“The standard radial thickness is called “D wall” and ring manufacturers calculate it with this formula: (Bore Diameter)/22 = (“D Wall” radial thickness). So in our case a standard D wall radial thickness would be 4.165 / 22 = .189-inch. Reducing this to .175” reduces drag in the engine,” says engine builder Jason Pettis.
Our choice for the second ring was a ductile iron Napier-style .043-inch with a .140-inch radial thickness. Both Jones and Pettis explained that ductile iron is a bit tougher than a regular cast iron ring. This in turn makes the ring hold up better to heat and pressure. Jones further explained, “A taper face is a scraper ring profile that is used on the second ring. The Napier combines that taper face with a small notch in the lower leading edge that aids in returning oil down the bore for improved oil control.”
Finally we went with a fairly standard 22 pound, 3/16-inch conventional oil ring. As Pettis explained, “Total Seal can put together an oil ring combo in just about any tension, in pounds, that you like. For power adder engines like this we prefer something in the 20 to 25 pound range.”
Our final choice was a C33 stainless top. A ductile iron Napier .043-inch ring with a .140-inch radial thickness made up the second ring. Last, included was a 22 pound 3/16-inch conventional oil ring.
Block preparation is very important when it comes to ring seal. Relying on recommended stones, feeds and speeds may not achieve the desired results. The only way to be sure is to measure the surface using a profilometer. The most common surface roughness measurement used is Ra.
Appropriately matching the ring design to the intended purpose of the engine is critical in both performance and ring longevity. Luckily enthusiasts and professional engine builders alike have access to great minds like the people at Total Seal and Pettis Performance. From material selection, ring design and block prep, there are industry experts like Keith Jones available for the guidance needed to accomplish the goal. In our case, the coated AP Diamond Dykes rings have brought us one step closer to 275 radial success.
