A well-informed game plan is essential for any engine build, and when it comes to durability, reliability, and overall performance potential, it’s doubly important that bottom end components are properly selected for a particular application.
Pistons are certainly no exception to that rule, but deciding which ones to go with for your project can prove tricky. Like any modern engine component, piston design continues to evolve with technological advancements, and those advancements allow the aftermarket to offer more choices to builders in order to best suit the needs of specific combinations and use cases.
While that’s certainly good news for builders who’re looking to get the most bang for their buck, it also means there’s some new considerations to take into account when making those selections.
To help take some of the guesswork out of the equation, we’re getting the lowdown on the latest piston technology trends from the experts at MAHLE, JE Pistons, and CP-Carrillo to get a better sense of the options that are now available to builders, and how to determine what options should be taken into account for a particular project.
Machine time accounts for the largest part of a pistons cost, and the machine time required for billets are considerably longer forged pistons, which increases costs considerably. There is a common perception that a billet is not as strong as a forging, which is denser due to it being forged into shape instead of machined. Although technically the denser material is slightly stronger, in use the difference in strength is minimal. If there is no suitable forging available, billet pistons are often the only option.
When it comes to the debate of billet versus forged pistons, there are more factors in play than just strength and cost. “The number one reason why billet pistons exist is the advantage of virtually limitless customization,” says Mark Gearhart of JE Pistons. “Forged pistons are pressed into a forging blank. Different blanks can accommodate different bore diameters, compression heights, skirt thicknesses, and overall piston heights. If a forging isn’t available for the needed design, or the design itself is radical, billet pucks are cut from bar stock and become the architecture for the billet pistons.”
This sentiment of easy adaptability is echoed by Trey McFarland of MAHLE as well. “The benefit of machining pistons from a solid billet is complete freedom of design,” he explains. “When machining from a forging many aspects of the piston are already loosely preformed, limiting some areas of the design to certain parameters. Machining from a solid billet piece of material, every aspect has to be completely machined and, other than the total height and diameter, the design is unrestricted.”
On the left, the undercrown profile for a piston is being machined on a puck of billet aluminum. This is typically part of the forging process for a forged piston, which means this process creates additional production time for a billet piston versus a forged one. However, due to the tooling involved in the forging process, it's much easier, faster, and cost efficient to use machined billet for uncommon designs or specialty applications.
Billet pistons often see use in research and development purposes when narrowing in on a design before a forging or casting can be produced. “Machine time accounts for the largest part of a pistons cost, the machine time for billets are considerably longer increasing cost greatly,” McFarland adds.
Changes to a billet piston’s parameters can be implemented in subsequent versions in literally hours at minimal costs, compared to the time and costs associated with making even slight changes to existing forgings. They are ideal for the R&D of new engine platforms and designs where changes may be needed many times, especially in the initial stages of the project. -EdwardUrcis, CP-Carrillo
While engineers can look at a computer generated rendering or even a full scale 3D printed plastic model of a design, those can’t be put in an engine for real-world testing. “This development process can be completed much faster when employing billets than when using forgings,” says Edward Urcis of CP-Carrillo.
“Changes to a billet’s parameters can be implemented in subsequent versions in literally hours at minimal costs, compared to the time and costs associated with making even slight changes to existing forgings. They are ideal for the R&D of new engine platforms and designs where changes may be needed many times, especially in the initial stages of the project. Billet pistons are also ideal for limited runs of pistons where existing forgings do not exist that meet the dimensional requirements of the order. When a new material for piston use is considered a billet is often the quickest and most economical way to test it.”
Although billet pistons offer flexibility in design and production for both development purposes and unusual design requirements, forged pistons have distinct advantages of their own which should be taken into account as well.
“If available a forging is always preferable,” McFarland explained. Once a design has been narrowed in on a forging can greatly reduce cost and production time resulting in shorter lead times. Any reduction in machining reduces the possibility for errors.”
This in turn often results in lower costs to the end user. “The cost of forged pistons is a big advantage over billet,” says Gearhart. “The longer a piston spends in a CNC machine the more it costs to make. The rough under crown and skirt shape will be established in a forging, whereas a billet is a puck of aluminum that must be completely sculpted to shape.”
Forgings are typically produced in large quantities and require less design and machining time to produce an individual piston than a billet does. As a result, the end cost a forged piston is typically lower than a billet equivalent. But because of the resulting grain flow when the material is forged into the rough shape under heat and extreme pressure, a forging is actually stronger than an identical shaped billet piston, although the difference is easier seen in theory than in actual usage.
But beyond cost considerations, forged pistons generally offer greater strength and durability than their billet counterparts. “All things equal, forged pistons are always going to be stronger than a billet,” Gearhart adds. “During the forging process the grain structure is established and has more consistent material properties.”
While those strength advantages can sometimes be difficult to quantify in the real world, the science behind this production process is undeniable. “Because of the resulting grain flow when the material is forged into the rough shape under heat and extreme pressure, a forging is actually stronger than an identical shaped billet piston, although the difference is easier seen in theory than in actual usage,” Urcis tells us.
Although the strength differences are ultimately negligible, when durability is the highest priority in an application forged pistons are typically the go-to option, particularly when forced induction and/or nitrous are part of the performance equation.
“If a forging is available that can be machined into a piston that meets the requirements of the application, production time, and cost I would recommend it as a first choice over a billet.”
Asymmetrical Skirt Designs
When applied properly, asymmetrical skirts can reduce weight, drag, wear and chances for scuffing. “An asymmetrical skirt design reduces friction by keeping the proper amount of skirt where it needs to be,” Gearhart explains.
“By reducing the skirt panels’ width through an asymmetrical, forged side relief (FSR) design, the piston can also be considerably lighter than a full-round design. An asymmetrical piston will have a wider skirt panel on the major thrust side and smaller panel on the minor thrust.”
Asymmetrical skirt shapes like this one are becoming more commonplace in OEM applications, where the incremental benefits of such designs can be validated through the extensive testing that’s required. Due to the number of factors involved that can affect the potential benefits of an asymmetrical skirt, it is generally difficult to apply this feature across a range of applications, making this an application-specific design in most cases.
While overall opinions on asymmetrical skirts vary, all agree that the benefits are subtle, very application-specific and typically require extensive testing to validate.
By reducing the skirt panels’ width through an asymmetrical, forged side relief (FSR) design, the piston can also be considerably lighter than a full-round design. An asymmetrical piston will have a wider skirt panel on the major thrust side and smaller panel on the minor thrust. - Mark Gearhart, JE Pistons
“The thrust side of the piston is subjected to more load, which can create higher stress and more friction than what may be applied to the non-thrust side,” Urcis explains. “Because of this reduced load and friction on the non-thrust side, a piston blank can de designed (if it is a forging) and/or machined (if a billet) with less supporting structure on that side of the part. This can save some weight and reduce the friction on that side of the piston, but in actual practice the weight savings and reduction in friction is very insignificant. This design is offered by some aftermarket manufactures, but because the gains are relatively insignificant it can be viewed more as a marketing tool than an actual cost effective benefit to piston performance.”
Indeed there are a number of variables that affect the potential benefits of asymmetrical skirts, and that makes applying the advantages of this feature difficult across a range of designs.
“Asymmetrical skirts have become more common in OE applications, where numerous validation tests are performed,” McFarland points out. “In some cases the smaller skirt is on the thrust side and others it is on the anti-thrust side. This is a difficult feature to apply across a range of applications, generally an application specific, purpose designed forging or casting is necessary. Asymmetrical valve pockets on the other hand are a widely beneficial feature, using appropriately sized intake and exhaust pockets (typically smaller on the exhaust) instead of two intake sized pockets which is more affordable will reduce the total chamber volume increasing compression ratio. This is especially beneficial in class rule limited applications where additional compression is hard to come by.”
How Skirt Profiles Affect Performance
The profile of a piston’s skirt is one of the key elements to how well a piston performs. While it is virtually impossible to distinguish with the naked eye, a piston’s skirt is not simply a round, straight shape like a piece of tubing. “A piston’s skirt design is really defined in two shapes – barrel and ovality,” Gearhart explains.
“If you’re looking at a piston straight on from the wrist pin, barrel is the shape of the skirt as it protrudes away from the piston. While contrast of these shapes are minor, a piston’s skirt widens as it as you go down the piston then curves back in. Effectively the bottom portion of the piston’s skirt is what makes contact with the bore. The idea is to create the least amount of contact with the bore while maintaining the piston’s stability.”
Piston skirt profile might look perfectly round to the naked eye but they are not, and the shape features have a significant impact on the piston's performance as it expands.
“The second portion, ovality, is the roundness of the piston, as pistons aren’t perfectly round,” he continues. “Think of squeezing a stress ball, the outside portions of the ball squeeze out. This is effectively what’s going on with the shape of the piston. The top and bottom (major and minor thrust) is where the piston contact mainly occurs. This shape also centers the load on the trust bearing surface and allows for minor skirt deformation as required by loading.”
At its core, the profile is responsible for controlling the shape of the piston as it grows while warming to its full operating temperature – as the piston grows it will consume the majority of the clearance that is present between the piston and bore when cold.
Because of the complex structure and varying thicknesses in its design, different areas of the piston don’t expand by the same amount as the piston warms up during operation. A well-sorted skirt profile will reduce drag and wear, yielding more efficient operation.
A proper profile will reduce drag, wear, noise and load resulting in a stronger, quieter, more consistent and higher-performing piston. This operation is so critical that MAHLE manufactures their own machining equipment that only machines profiles. -Trey Mcfarland, MAHLE
“A proper profile will reduce drag, wear, noise and load resulting in a stronger, quieter, more consistent and higher-performing piston,” says McFarland. “This operation is so critical that MAHLE manufactures their own machining equipment that only machines profiles. Every application is designed with its own unique profile.”
Also, as the piston reaches operating temperature, some areas are warmer than others and expand by different amounts. ”This is the reason that a piston has more clearance in the bore when it is cold than when it is at operating temperature,” Urcis points out.
“Much experience and testing is required to develop the proper shapes and clearances for the many different forgings and shapes of billets, and to apply the correct values for each piston and application.”
While the folks creating the latest piston technologies and designs have a tendency to keep projects that are still in development under wraps until they are ready for the spotlight, we can still glean some insight into the direction things are headed simply by looking at how the design and production processes are evolving every day.
“CP-Carrillo is constantly working on new and emerging ideas and technologies,” Uris tells us. “We work with many high level race teams and select OEMs in the motorsport world (along with individuals and organizations from all forms of motorsports), and this provides us with opportunities to refine and develop new products for particular applications.”
Emerging technologies like 3D printing allows engineers to develop custom designs in rendering software and then validate clearances before producing a set of billet pistons.
“Our R&D department is always working on piston design characteristics behind the scenes,” Gearhart says. “Take our machining centers, for instance. Having the best CNC machines is like buying a new computer; the accuracy and speed will continue to increase. For example, our 9-axis Mazak Integrex CNC reduces the amount of handling required by a machine operator, and thus increases accuracy while decreasing machining time. The Integrex will literally hand off the part to the other side of the machine to perform operations between the dome and under crown of the piston.”
Still got some questions about choosing a set of slugs for your next project? The folks at JE Pistons, CP-Carrillo, and MAHLE can help point you in the right direction to ensure that the pistons for your next build meet the performance, durability, and cost requirements of your application. Give ‘em a buzz and find out what’s available for your build.