Broadside Of A Piston: Discussing Piston Features With MAHLE Motorsport

Have you ever taken a close look at the side of a piston and asked yourself why they’re designed the way they are? Why are rings placed in the positions they’re in? In this article, we’ll take a close look at this often-overlooked area. Including how you can gain performance, improve emissions, plus the trade-offs and compromises that go with it.

To help further explain piston design, we sat down with Eric Grilliot, Product Sales Manager at MAHLE Motorsport. MAHLE is best known as a global OEM engine parts manufacturer and supplier across multiple industries, including the aftermarket and its motorsports division. Grilliot has been with the company for over 20 years, starting as an application engineer on the design side. “Having worked here long enough to start at the bottom and continue my way up through, I’ve seen a lot from the aspect of just understanding what’s happening with the piston, analysis, failures, and everything that goes hand-in-hand with that,” mentions Grilliot as he introduces himself.

Piston Side Design

Let’s begin by taking a close look at the side of a typical piston and understanding the terminology and its purpose. From the top down, we have the:

Now focus a little closer on the top compression ring and the distance to the crown. “I’ve heard this area called several other things, but internally we use the term ‘fire land’ to describe the distance of the top ring land from the edge of the crown,” explains Grilliot. The fire land is an area of hot contention, pun intended.

Designing the fire land starts with where the top compression ring is placed on the piston. When you go high, it reduces crevice volume. Crevice volume is the dead space in the combustion chamber that doesn’t achieve optimum flame propagation on the ignition stroke. When you reduce crevice volume, it improves fuel burn efficiency and effectively reduces emissions and increases performance.

“The higher you go, essentially, the more that you’re reducing crevice volume,” explains Grilliot. “And this was used for a long time in the automotive OEM world. If you pulled out a cast piston from the last, probably, 10-15 years, you’ll see the ring is placed very, very high on the piston. Their primary driver was emissions. Performance engine builders have been doing it for a while as well, particularly in naturally aspirated applications. The higher you push that ring, the more potential for power there is, and usually there’s some physical limitation that gets in the way before you stop seeing that benefit.”

Higher Isn’t Always Better

Like many aspects of engine performance, there are trade-offs. Planning and balancing expectations are key engine builder soft skills for achieving durability. With a smaller fire land, you’re increasing the temperature of the components, and ring damage can occur.

“Heat’s a big factor. That’s one of the trade-offs you’ll see. There’s a gradient from the top to the bottom of the piston that it operates. That could be a 200-degree Fahrenheit difference. The closer you are to the top, obviously, the hotter it’s going to be. Number one, that ring is a primary path of heat out of the piston. So it has to be able to transfer out, but it also affects the heat that the ring groove sees. There’s an interaction between the groove and the piston. The closer you are to that top temperature, is where they start to micro-weld. Micro welding can become a real issue. You have to find that balance for the application the engine is intended to run,” Grilliot eplains.

Micro welding occurs when excessive heat causes the ring to momentarily fuse to the piston groove surface and deform the piston ring geometry. This can cause loss of compression sealing, cylinder wall scuffing, and ring breakage.

The question arises: can the rings themselves be built more robustly? Such as a taller height and/or a wider radial dimension. Radial dimension is the horizontal measurement across the ring from the edge facing the inside cylinder wall to the edge facing the outside piston groove. “A beefier ring will be stronger, but it’s usually related to the radial dimension of the ring. The inside diameter of the piston groove has a wider radial, so that’s where you’re running out of physical space. The wider the ring is in the radial, you tend to run into other [piston] features before you would if you had a very narrow ring,” explains Grilliot.

When we step into high-horsepower motorsports, we see less emphasis on top ring placement and radial dimension in favor of surviving down the track. “You get into the big two- and three-thousand-plus horsepower, I like to say you’re not building a piston anymore. You’re trying to build an anvil. There are a lot of circumstances where an anvil is the right tool for the job,” smiles Grilliot. “The trade-offs for performance and some of the things that you would do in other applications don’t apply, because making it survive is more important than anything that you’re going to gain from ring placement.”

The Art Of Compromise In Piston Design

Grilliot and team have a daunting responsibility, considering the wide range of piston designs for various applications that MAHLE offers. From a design standpoint, it comes back to compromise, understanding the customers and market applications, then offering quality, dependable catalog products. “If you look at off-the-shelf offerings, usually those have to be a compromise for a lot of different usages. Even just a traditional heavy small-block piston, we can’t push the ring up as high as you would want to make power, because you don’t know that somebody’s not going to stick 200 horsepower of nitrous on it. You have to compromise.”

In the same vein, MAHLE offers dedicated pistons built for a specific motorsports class, and that’s the intended use. “We’ll have that fire land pushed up as much as we think is reasonably possible for that. The NHRA Super Stock class is great at taking advantage of applications like that. Those are very specific builds where you can really fine-tune the design into how it’s going to be used.”

Holistically, it comes back to planning and understanding the intended engine application. An engine builder needs to know the combination of the piston crown, cylinder heads, stroke, and deck height to define where the top ring placement should be. Basically, a thorough understanding of what physically creates the compression ratio and the heat combustion generates. “You could have two identical engines, but how you would approach the piston and all that ring placement and compromise depends on which application you’re building for,” Grilliot concludes.

Experience Over Simulation

That’s plenty to consider, and MAHLE has the advantage of deep research and development connections. “Our company is an automotive OEM supplier. There’s a ton of simulation work and research done at the OEM level that we get to peek in on and see what’s happening,” says Grilliot. Motorsports move much faster and with greater variety. “MAHLE Motorsport’s performance division is actually a very small portion of the company,” Grilliot explains. “It’s a lot of experience. It becomes this working knowledge that you acquire based on applications.” The track experience and relationships built with race teams over the years is still the ultimate proving ground, as it validates modeling.

For example, high top-ring placement and a narrow fire land are a benefit for large-displacement naturally aspirated engines to help build max power. Experience has proven that piston ring material starts to come into play significantly more. “The old cast iron, ductile rings can be too brittle and snap if they’re too thin. Materials come into play, and newer steel varieties can affect what you can do with radial dimensions of the ring, and that impacts tension and the conformability of the ring to the cylinder.”

There are even limitations discovered there. “Smaller isn’t always better, because you’ve got that heat aspect, not only the heat the component itself can handle, but the ring we talked about being the pathway of heat out of the piston. It’s not just sealing, but it’s also helping to keep your piston alive. If you’re not transferring that heat out of the piston, you end up with situations where you’re just overworking the material capabilities of an aluminum piston. The ability to transfer heat and hold its shape under the high heat and high loads, that’s where sometimes you get that trade-off of smaller is not necessarily the right path for everything. And sometimes, again, you’re back to building the anvil that’ll have the durability to take what you’re throwing at it,” emphasizes Grilliot from decades of experience.

Ultimately, the piston and rings transform chemical energy into mechanical movement. Their design is about managing balance, performance, and durability. How the crown and side are designed, including the materials used, determines how efficiently the engine runs. Understanding these relationships helps engine builders make smarter decisions, especially when adapting a naturally aspirated design for boost or higher cylinder pressures.