Thin Piston Ring Technology Provides Large Benefits In Performance

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Advances in material technology have driven engine component development since the very first internal combustion gasoline engine was developed, and the materials used in the construction of IC engines have seen constant refinement through manufacturing techniques and consumer demand.

MAHLE Motorsports has been at the forefront of this development cycle over the last century; the company traces its roots to the Mahle brothers, led by Dr. Ernst Mahle, and his invention of light-alloy pistons in the 1920s, which replaced previously state-of-the-art gray cast-iron pistons for use in the developing automobile industry.

Today, the company has facilities all over the world and manufactures pistons and other products for both OEM and aftermarket use through a variety of manufacturing divisions. Recent developments in OEM requirements, coupled with their massive economy-of-scale advantages in manufacturing, have allowed them to bring thin piston rings – and all of the advantages they offer in engine operation – to the masses.

This side-by-side development drawing show how the 2.0mm oil ring stays away from the piston pin. In the process, the smaller dimensions also mean lighter rotating weight, and in a racing engine that can pay unseen benefits.

This side-by-side development drawing shows how the 2.0mm oil ring (left side) stays away from the piston pin, compared to how it intersects the pin with a 3.0mm oil ring (right).

OE Technology For Racing?

Those advancements over the years have MAHLE building a new product – their HV385 performance rings – that are designed to permit the sportsman racing and performance market to take advantage of new, thinner piston ring technology that’s now been adopted by the OE manufacturers, with dimensions of 1.0mm/1.0mm/2.0mm in contrast to older, thicker designs.

The reasons for using a thinner ring set are many. For a better understanding of the technology and its application, EngineLabs turned to MAHLE’s Trey McFarland to get the details.

This side-by-side cutaway shows the difference between a 1.5/1.5/3.0mm ring set on the left, and a 1.0/1.0/2.0 ring set on the right.

This side-by-side cutaway shows the difference between a 1.5/1.5/3.0mm ring set on the left, and a 1.0/1.0/2.0 ring set on the right. Look at the difference in the oil ring land thickness at the bottom of the piston.

“In the past, with thicker rings, piston manufacturers were forced to make the piston pin intersect the oil ring for many applications, which caused the final product to be heavier and add more cost, along with more effort in terms of installation of the piston rings,” says McFarland.

The OE adopting this technology has the thinner ring package coming into vogue in the aftermarket for a variety of reasons.

“A thinner package actually works better in nearly every situation. We’ve been using them in OE applications for some time, and before that in upper-level racing series – Formula 1, NASCAR, IndyCar, Pro Stock, they’re all running skinny rings. The only people who haven’t been are the sportsman and lower-level professional-level racer, and it’s largely due to cost,” he explains.

We control the production of the ring to a tight tolerance, and since we can count on that tolerance to be correct, now we can build our ring grooves in the pistons tighter to the ring. – Trey McFarland, MAHLE Motorsports

Up until now, thin ring packages have been very expensive, but through their work at the OE level, MAHLE has developed proprietary equipment built in-house to allow them to produce these in higher volumes at a much more reasonable cost. The application of new OE technologies such as direct injection create increased cylinder temperatures and higher cylinder pressures, and for EPA reasons the manufacturers keep moving the rings higher and higher on the piston, which has necessitated the development of the equipment at the supplier end to build piston rings that will withstand those operating parameters.

“We’ve also developed manufacturing processes that have allowed us to cut production costs, which makes up for the higher cost in the steel material,” says McFarland.

The HV385 rings offer lighter weight and lower drag as obvious features, but there are other, less-visible benefits to be found as well.

The use of the thin ring design also permits increased material dimensions between the ring groove and the valve pocket, improving piston strength.

The use of the thin ring design also permits increased material dimensions between the ring groove and the valve pocket, improving piston strength.

With the use of the thinner design, piston designers are freed up to be more flexible in the process of development. According to McFarland, the skinnier ring design is also much more conformable to bore irregularities, bore distortion, piston groove deformation, and will behave more consistently in tough situations.

Each part in the system has to work together. This improvement in conformability allows the ring to seal the cylinder and control oil more consistently with less ring tension – which results in the aforementioned low drag characteristics.

The advances in manufacturing have permitted MAHLE’s piston designers to be more precise when specifying tolerances during the design phase.

Material Details

The moly coating is applied to this stack of piston rings through the use of a jet-fuel-powered supersonic gun, which impregnates the cermet material into the face of the ring.

The cermet coating is applied to this stack of piston rings through the use of a jet-fuel-powered supersonic gun, which impregnates the HV385 cermet material into the face of the ring.

Piston rings can be constructed from any number of materials; cast iron, ductile iron, and PC479 ductile iron, along with rings that are constructed from any number of steel materials that have been gas-nitrided on their face. Gas nitriding is a surface hardening process where nitrogen is added to the surface of the steel parts using dissociated ammonia. The reason for gas nitriding is to increase the wear resistance of the ring face.

Stepping away from those types of materials, the HV385 ring package consists of 9254 steel on the top and second rings, which McFarland says is a much stronger, harder material that has a much higher fatigue strength than traditional cast or ductile rings.

The introduction of the new HV385 cermet material improves wear resistance through another method – the application of the moly through a supersonic, thermal spray process that promotes superior bonding to the 9254 steel underneath.

“HV385 is very different from what people are traditionally used to when they hear the term ‘moly’. We use a higher chrome content in the form of chromium carbides that is stronger, but the real difference is in the application. The material goes through a jet fuel, liquid-oxygen-powered gun, which accelerates the particles to a supersonic speed and allows the HV385 to actually impregnate itself into the base material. The bonding is much better than what most people know of moly,” says McFarland.

On The Pump

When installed on an engine, a vacuum pump assists greatly in promoting ring seal and oil control, but McFarland says for the vast majority of combinations it isn’t required when using this piston ring design.

“With most any ring package, a vacuum pump is going to be beneficial, and it’s just as beneficial with this ring package as it would be with any other – but it’s not a necessity. We have tested these rings to 7,800 RPM in a limited circle track application where they have to run a wet sump and aren’t allowed to make many oiling system modifications, and they’ve performed very well. The racers using these are reporting the same or ‘drier’ engines. We’ve used these in thousand-horsepower applications and they were even installed in a car that raced the Texas Mile with success,” explains McFarland.

The face of the HV385 rings are put through a double lapping process giving them a very smooth friction reducing finish that is still forgiving to the cylinder bores. McFarland says the rings work quite well in high-boost power-adder applications, although he stops short of suggesting that they’d be ideal in a Pro Mod level application just yet. The material wears better than traditional moly rings, but it has the same forgiving nature.

The second ring is constructed from the same 9254 material, with a Napier-style face to help scrape the oil from the cylinder walls and return it to the bottom end of the engine. Napier rings get their name from their hooked design, which allows only the minimum amount of oil through from the bottom end that is required to lubricate the top ring during its operation. The second ring has been described in the past as the second compression ring, when it’s been determined that the vast majority of its responsibility is oil control – as much as 80% in some cases.

Most of the oil control is handled by MAHLE’s proven CP-20 oil ring set, designed to promote ideal oil control. The CP-20 features chemical polishing for better oil drainback, while using a 20-degree ear angle to ensure side seal around the ring itself.

“Whatever an engine builder has found to work best with a traditional moly ring, he can use that same formula to machine the cylinder bores for use with the HV385 ring package,” says McFarland.

Other Considerations

Using these ring sets, MAHLE engineers can now develop piston sets that have historically required the piston pin intersect the oil ring groove, such as in the popular 347 stroker Ford engines, 4.000-inch stroke GM LS engines, and others. With the thin rings in place, now there is room for the pin bore to sit below the oil ring groove, reducing complexity and improving performance at the same time.

The removal of the oil ring support rail reduces the ring pack’s weight, and less weight in a rotating assembly is a good thing for performance. In addition, the thinner ring groove permits the piston designer to increase the cross-sectional thickness in key high stress areas like the valve pocket to top ring groove area, along with improving ring land heights.

Side view of the 1.0/1.0/2.0mm piston ring pack. Not intersecting the pin is a good thing, both from an oil control and rotating assembly weight perspective.

Side view of the 1.0/1.0/2.0mm piston ring pack. Not intersecting the pin is a good thing, both from an oil control and rotating assembly weight perspective.

In Conclusion

All of these benefits may seem like small advances, but when combined together they make for a more consistent, better performing engine. McFarland says overall performance has been excellent in all of the company’s testing along with the evidence provided by OE manufacturer use of these thin ring products – there are millions of passenger cars on the road using the technology – and now it’s affordable to the racing and performance enthusiast community.

About the author

Jason Reiss

Jason draws on over 15 years of experience in the automotive publishing industry, and collaborates with many of the industry's movers and shakers to create compelling technical articles and high-quality race coverage.
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