Pistons, forged or cast?
The choice between using a Cast or Forged piston is dictated by the application. Forged pistons may not be absolutely necessary for all applications. However, the minor additional cost of a forged piston can save your engine from disaster in a critical situation.
The starting grid recommends the following well known forged piston brands:
By utilizing state-of-the-art CNC machines, most manufacturers can maintain exact specifications and tolerances. This makes them structurally more durable. Forged pistons are well known in the racing and performance industry because of their ability to withstand more heat, higher RPMs, higher boost and higher temperatures. But forged pistons are not just for professional racers. Another advantage to a forged piston is that they can be individually made to any custom size or specification without the high cost of casting.
The primary motivation for the growing interest in "Billet Pistons" is simply time management. There is no expensive or long-lead-time tooling involved, that is the biggest advantage, there are no significant strength benefits using them. A forged piston still is stronger than a billet it has a better grain structure and therefore more consistent material properties.
There are disadvantages also, of course. The piston will be weaker in structure. It is often heavier since the manufacturer has to increase wall thickness in order to achieve sufficient strength. The heat expansion cannot be controlled and is therefore often not completely round since the piston pin requires some material on the inside. They are also produced in bulk by most manufacturers only producing sizes that are close to factory specifications. This also makes them more affordable.
Although internal combustion engine pistons commonly contain trace amounts (less than 2% each) of copper, manganese, and nickel, the major element in automotive pistons is aluminium due to its lightweight, low cost, and acceptable strength. The alloying element of concern in automotive pistons is silicon.
Silicon in this context can be thought of as "powdered sand". Any silicon that is added to aluminium above a 12% content will retain a distinct granular form instead of melt. At a blend of 25% silicon, there is a significant reduction of strength in the piston alloy so stock hypereutectic pistons commonly use a level of silicon between 16% and 19%. Special moulds, casting, and cooling techniques are required to obtain uniformly dispersed silicon particles throughout the piston material.
The biggest drawback of adding silicon to pistons is that the piston becomes more brittle as the ratio of silicon is added. This makes the piston more susceptible to cracking if the engine experiences pre-ignition or detonation as well as when power adders are installed. Performance replacement alloys.
When auto enthusiasts want to increase the power of the engine they may add some type of forced induction. By compressing more air and fuel into each intake cycle, the power of the engine can be dramatically increased. This also increases the heat and pressure in the cylinder.
The 4032 performance piston alloy has a silicon content of approximately 11%. This means that it expands less than a piston with no silicon, but since the silicon is fully alloyed on a molecular level (eutectic), the alloy is less brittle and more flexible than a stock hypereutectic "smog" piston. These pistons can survive mild detonation with less damage than stock pistons.
Aftermarket performance pistons are most commonly made from 4032 and 2618 alloys and are typically forged. The 2618 performance piston alloy has less than 2% silicon. This alloy is capable of experiencing the most detonation and abuse while suffering the least amount of damage.
That clearly is an open-ended question because the answer will single out the uber-sophisticated pistons designed for Formula 1 and World Endurance Challenge (WEC), yet to be completely literal the response must acknowledge hand-modified cast factory pistons used in Hobby Stock racing at the local dirt track.
For the sake of argument, let’s define a racing piston as one that works best for any builder assembling an engine for motorsports competition. From there the discussion can move toward more specific talking points.
Typically, a racing piston, is a highly developed design that has undergone many iterations to optimize its use under a very specific set of conditions.
A simple comparison of pistons from the muscle car era of the ‘60s to today’s more advanced products will certainly validate that position. Manufacturers have improved materials and designs through advanced computer modeling to improve power and increase fuel efficiency.
To start this bench racing session, consider the basics of aluminum piston construction. Unless there’s a specific rules restriction, a forged piston will always be preferred in racing over a cast. Forged is the standard, offering excellent design flexibility and the best cost-to-performance ratio. Cast has no flexibility and can therefore not be developed or optimized. Cast pistons are also heavy and brittle, but they’re cheap to produce.
Forged pistons can be made of either a forged blank (right), where the entire shape of the piston is machined out of a billet aluminum cylinder, or created from a net forging that is squished closer to its final shape via a forging die (left). Low-power engines can survive with cast pistons since they won’t see the abuse of blistering heat or high RPM. There’s not much you can do to improve performance with a cast piston, other than grind out a little metal in the crown to clear the valves if a more aggressive camshaft is installed.
In the ‘80s and ‘90s, hypereutectic became a big buzzword in the piston industry. A hypereutectic piston is one that has greater than 12.5 percent silicon in its metal composition, usually around 16 to 18 percent. A standard cast aluminum piston has around 8-to-10 percent silicone content, which improves the hardness and helps reduce wear around the ring grooves, skirt and pin boss. The hypereutectic piston is still manufactured through casting but the alloy is a little lighter, and due to the improved strength the casting can be machined a little thinner to cut down even more weight.
Still, a forged piston will be much stronger and that’s why it dominates the performance market. There are two aluminum alloys popular with forgings: SAE 4032 and SAE 2618. Here’s where another choice is made that often distinguishes, or at least makes a good argument, for a racing piston—although one alloy isn’t always “better” than the other.
The term ‘better’ is tricky. If only one alloy were better at achieving every goal, there wouldn’t be multiple offerings. 4032 expands less so it requires less cold clearance which tends to run quieter. This could be important for EFI engines with knock sensors. 4032 is less dense so a given design would be lighter than 2618, however 4032 doesn’t have the high-heat annealing resistance that 2618 does.”
Again, silicon content is the differentiating factor. A 4032 piston has about 11 to 13 percent silicon while 2618 has less than .25 percent. 2618 has higher ultimate strength, better annealing resistance at elevated temperatures and has better ductility. In a racing environment, the goal is often to minimize weight without sacrificing durability in high-heat environments. For these reasons 2618 often gets the nod.
The piston on the right uses a 5/64in ring pack. This is a very common size used during the muscle car ere. The piston on the left uses a much thinner 1.00mm top ring. This reduces friction, freeing up horsepower and allowing the engine to rev quicker. While a majority of racing pistons are made from 2618 aluminum, there are some exotic materials used in high-end racing or at least they were tried before being banned. Aluminum beryllium alloy, which is exceptionally light and strong with superior thermal properties, was developed by Mercedes/Ilmor for the McLaren Formula 1 team in the late ‘90s. However, the alloy was quickly banned because beryllium dust is extremely hazardous, and in a fire the element will turn into beryllium oxide, which is extremely toxic.
The latest advanced alloy to draw banishment in motorsports is aluminum metal matrix composite, or MMC. Another very stiff, lightweight alloy, MMC is also banned by Formula 1 but continues to draw interest in other areas of motorsports where the rules are more open.
For many years, top engine builders have preferred billet pistons over forged versions and probably not for the reason you might think. Billet is not just a simple option to a forging. Billets are approached as complete engineered solutions that are put through several architecture design iterations using FEA modeling to optimize design to a very specific set of environmental conditions. Most popular configurations are offered as a catalog item, but if a combination comes up that doesn’t match, that’s where a forged custom piston can be designed and manufactured at in just a couple of weeks. This is half our business.
In other words, billet pistons are primarily used in tight-timeline development projects where critical changes can be made quickly without worrying that a proper forging isn’t available. The debate over billet vs. forged in terms of strength will have protractors and detractors on both sides, but generally speaking, a properly executed forging will have inherent strengths in grain structure that a billet piston will not. “Probably 98 percent of racing formulae have a forged option available in the aftermarket,” confirms Stevenson.
Anti-detonation grooves (arrow) are designed to help reduce pressure spikes while contact reduction grooves (just below the crown) reduce how much of the piston’s crown surface make contact with the cylinder wall. In addition to improved materials and advanced construction methods, specific features have been developed to boost horsepower through either thermal dynamics, reduced weight or reduced friction. Following are some examples of design elements often unique to racing pistons:
Thinner ring packages Although some exotic engines run 2-ring packages, the majority of racing applications stick with the tried-and-true 3-ring setups. What has changed dramatically is the thickness of those rings, especially in Pro Stock and other non-endurance engines. “You reduce friction and mass, freeing up power and allowing the engine to accelerate faster. The secret to power in any N/A engine is to use the thinnest rings possible, lapped flat and matched with a super-flat ring groove, and ensure minimal axial and radial clearance.”
Accumulator grooves work on every piston, but work best on gas-ported pistons. They work according to Boyle’s law; pressure and volume are inversely proportional. Through normal secondary motion (piston rock), the top ring tends to become momentarily unsettled as it rapidly changes direction through TDC. As this occurs near peak firing, combustion pressure tends to make its way past the top ring until it re-settles. Combustion pressure also gets through at the end gap of the ring. Accumulator grooves nearly double the volume below the top ring, reducing pressure according to Boyle’s law and preventing the top ring from becoming pressurized from the underside, which promotes ring flutter.
Smaller, Lighter Wrist Pins Less weight is always desirable but using a smaller or thinner wrist pin may compromise strength and engine durability. This often comes at a cost of using better, more expensive materials but also reduces mass to help the engine accelerate faster.
Shortening the piston skirt is a good place to remove mass, but also can be mandatory in stroker applications. Racing pistons can also have different skirt profiles to help them stay stable in the bore at high rpm.
Shorter Skirts Again, more design changes to reduce weight and friction but these efforts are usually dictated and restricted by the engine architecture, such as length of the cylinder sleeves and the stroke. The gauge point of the piston skirt must remain captured in the bore at BDC.
Thermal Coatings Some engine builders want to reflect the heat away from the piston, preferring that the valves and cylinder head dissipate the heat to the coolant instead of the pistons and rings through the cylinder wall. Thermal coatings are designed to repel heat from the piston crown, and in some cases the combustion chamber as best as possible.
Skirt Coating By far the most friction in an engine is from the rings. A distant second are the bearings. Skirts have minimal friction since they ride on an oil film. On racing pistons, top teams use coatings as an insurance policy against overheating. In OEM and daily driver conditions, they are used to protect against dry starts and other situations where oil on the cylinder wall is limited.
Application Specific Modifications Some modifications are inappropriate for one type of racing while done routinely on pistons made for other types of racing. Some examples are spinning the bosses, plunging the bosses, drilling holes in the skirts or struts, and 3D under-crown milling. 3D profiling is a precise milling procedure where the piston crown maintains the same thickness regardless of the dome profile. This step ensures that the crown has the necessary strength and heat resistance for competition with the least amount of weight. This is especially important when trying to maximize compression ratio, and it can also be used around valve reliefs to promote smoother flame travel.
The actual crown design and valve relief dimensions will be dictated by the combustion chamber and valve geometry. Another racing piston cue is that the engine builder will send a mold of the combustion chamber to the piston manufacturer so that the dome design exactly follows the profile of the chamber. Finally, some engine builders call for very slight adjustments in the overall piston shape and dimensions to suit their needs.
Skirt cam/barrel shapes and ring-land diameters are part of the black art of optimizing a design during a development program.
As you can see, a racing piston may have one or many of the features and modifications mentioned. The key is designing a piston that meets the needs of the engine builder assembling an engine for a specific competition.
By Alan Stevenson, Senior technical account manager JE-Pistons.
What Does the 2nd Ring Really Do?Piston ring packages are as carefully engineered as any high-performance part, but the “middle child” might be the most misunderstood. Here’s a look at the science that goes into second ring design. The ring package has three primary goals: Keep pressure confined to the combustion chamber on both the compression and power strokes, transfer heat from the piston to the cylinder walls where it can be removed via air or liquid cooling, and control lubrication to limit oil consumption and unwanted emissions. While it’s easy to look at the top ring or the oil ring at the bottom and intuitively understand their contribution to meeting these objectives, the second ring is more of a mystery. What’s it supposed to be doing, and why is it necessary?
How do the materials used and physical properties of the second ring affect performance?
A conventional automotive ring pack must work in harmony to seal combustion pressure, control oil, and transfer heat into the engine block. For our first question, we asked Stevenson whether the second ring had a role in containing compression or combustion gasses. “There was a time when bores were so bad in terms of surface finish, roundness, and so on, and ring materials were a lot worse, so that pistons used to have four rings; two for compression sealing, one for scraping oil, and one for pumping oil,” he explains. “The terminology hasn’t kept up with the technology. Referring to a contemporary second ring as a compression ring is a misnomer.” So what’s the contribution of a modern second ring to combustion chamber sealing? Per Stevenson, “Negligible. There have been SAE papers published that prove how enlarged second ring gaps actually increase top ring sealing and power. Combustion sealing is 100% the top ring’s job.” Combined with other piston features, the second ring’s role in this respect is to keep the pressure in the crevice space between it and the top ring as low as possible, giving any blow-by that makes it past the top compression ring a way to quickly escape to the crankcase. Note the subtle hook shape at the edge of the ring. This is a Napier-style 2nd ring and that “hook” helps it pull oil off the cylinder wall as the piston slides down the cylinder bore. “An accumulator groove works in concert with larger 2nd ring gaps,” Stevenson explains. “In short, there will always be some combustion pressure leakage past the top ring due to secondary piston motion and cylinder cross hatch. Any pressure that makes it past the top ring tends to get trapped between the top and second ring, which then pressurizes the top ring from underneath which leads to ring flutter (especially at high RPM). The accumulator groove creates additional volume which decreases pressure. This is where Boyle’s law is applicable; volume and pressure have an inverse relationship, so increasing volume decreases pressure. Coupling this with larger second ring gaps provides a smoother transition of the trapped gas out of that space and reduces top ring flutter.” Because the second ring is specifically intended NOT to be a pressure seal, it’s often constructed quite differently from the top compression ring. Stevenson says, “Many top rings have inside diameter bevels that cause them to twist opposite of the forces acting upon it in order to help keep it flat in the groove for better sealing. Second rings have a bevel opposite to that, so they actually twist the wrong way to help sealing.” While the oil rings do the brunt of the work “pumping” oil away from the cylinder wall face, the second ring plays a vital roll scraping it off the face of the cylinder.
The Heat Is On
So, having established that the second ring is most definitely not there to provide compression or combustion sealing, what about the second main objective of the ring package – transferring heat out of the piston and out to the cylinder walls, where it can be managed by the cooling system? It might seem like the relatively tiny amount of contact the rings make between the piston and the bore couldn’t possibly be a significant route for heat conduction, but it turns out to be the major provider. Per Stevenson, “There are many variables here, but the rings transfer about 70% of combustion heat from the piston to the cooling system.” The remaining 30% escapes via other routes, like radiation and convection cooling of the underside of the piston to the air inside the crankcase, conduction cooling through contact between the piston skirt and the cylinder bore, and heat carried away via oil splash from crankshaft windage. Some engines even employ oil squirters at the bottom of each cylinder bore that direct a spray of lubricant at the underside of the pistons specifically to aid in cooling. As rings continue to shrink in dimension to reduce friction, material and manufacturing becomes far more critical. Carbon steel is the material of choice for most performance applications, especially late-model–and offers robustness and durability far exceeding earlier, thicker rings. Other sources of heat transfer notwithstanding, the ring package handles most of the load when it comes to keeping the piston at an acceptable operating temperature. Out of that previously mentioned 70% of total piston heat, “The top ring transfers 45%, the second ring 20%, and the oil ring 5%,” says Stevenson. While the second ring definitely plays its part in this critical task, it’s still not the ring’s primary reason for being there. Thinner piston rings are far more likely to be damaged during installation than older-thicker rings. Using a tapered ring compressor should be considered an absolutely necessity when building a late-model engine. Oil Control For The Win As it turns out, the second ring has a lot more to do with lubrication control than the oil ring beneath it. “The second ring is what scrapes the oil,” Stevenson explains. “The oil ring is what gathers it and pumps it away from the cylinder walls via oil return holes in the oil ring groove.” The second ring’s main function is to continuously remove excess oil from the bore – as the crank rotates, oil escaping from the pressure bearings on the rod big ends is constantly thrown up behind the piston, coating the walls of the bore. On the downstroke, the second ring and the oil ring work in concert to clear all but a tiny amount of oil and return it down the bore to the sump. Stevenson says, “The top rings will always receive latent lubrication by oil trapped in the cross-hatch of the cylinder walls.” It’s that microscopic texture on the bore that retains just enough oil to keep friction between the ring package and the cylinder wall to a minimum, while the second ring prevents too much oil from making it up past the top ring and into the combustion chamber. As the piston moves down the bore, the second ring “scrapes the oil from the cylinder wall, both providing lubrication and keeping oil from entering the combustion area of the engine.
Theory Into Practice
Now that we understand each ring’s purpose in the package, we can see why different specific materials and ring cross sections are often used for the top and second rings. “The overall best top ring material is steel,” Stevenson continues. “Now, granted, some steels are better than others, but as rings get smaller and specific output increases, the demands on the top ring (which sees the most abuse) are highest.” Move down a groove on the piston, and the different job being performed places lower demands on the material being used. Per Stevenson, “Many second rings in racing engines are still cast iron or ductile iron. The second ring is not under enough stress and temperature to necessitate steel.” The shape of the ring profile also has a significant effect on how efficiently it removes oil, as well as how much friction it introduces, and both the interior and exterior diameters have a role to play. “Bevels are on the inside diameter of the ring and dictate the direction the ring twists to aid in scraping,” Stevenson says. Viewed in cross section, a beveled ring has one edge of the inside diameter cut at an angle – as Stevenson points out, this encourages the ring to dynamically twist in the groove as it moves down the bore and focus additional pressure on the outside corner, in order to more efficiently sweep excess oil away. Gas ports are another way piston designers can manipulate how rings work. By allowing combustion pressure to access the backside of the top ring, they increase ring seal increasing power, while reducing friction on the other three strokes. “Taper, Napier and steps are all variations of the outside diameter shape,” he continues. The goal with all these profiles is to concentrate contact into a narrow band to increase the efficiency of the scraping action. As the name implies, a tapered outer profile is narrower at the top than at the bottom, while a stepped ring profile has what looks like a notch in the cross section, oriented toward the direction of travel on the downstroke. A Napier ring is actually undercut at an angle or even hook-shaped on the outside diameter, further decreasing the contact area and providing space for scavenged oil to escape, away from the cylinder bore. “In order, the most efficient scraper is Napier, followed by step, followed by taper. Run a Napier if it’s available in your bore size and suits the groove in the pistons,” Stevenson concludes. What kind of a combination you are running will also influence the optimum choice for your ring package, including the second ring. Stevenson advises, “Thinner second rings are more prevalent in dry sump engines pulling gobs of pan vacuum.” Because crankcase vacuum helps ring seal across the board, it’s possible to get the desired results without working the second ring quite as hard. “Naturally aspirated with no vacuum help should usually be 1.5mm or larger, while forced induction should err towards larger 1/16-inch rings,” he adds. Setting the proper ring gap is paramount to achieving the desired engine operating characteristics. In any performance application, the 2nd ring gap should be larger than that of the top ring to allow blow-by to escape and prevent ring flutter from upsetting the top ring’s seal. “Of course, this is all relative to bore size; you can almost think of it like a ratio of ring size to bore size,” Stevenson cautions. “A big boost, four-cylinder engine will control oil just fine with a 1.2mm ring, while a 4.600 ̋ bore big block would be happier with a 1/16-inch ring. There are also substantial variables in crankcase efficiency when it comes to oil control. Modern engines with deep-skirted blocks, segmented oil pans, windage trays, and crank scraping/scavenging all have an effect on how much oil is thrown up into the cylinders. The more oil present, the harder the second ring’s job is.”
As with any other engine component, there are tons of different styles and ring materials that are suited to unique applications. Consulting an expert is always the best way to match rings to your engine build. As you can see, second ring design and engineering is a complex subject, but experts at Wiseco have the collective experience to provide you with sound advice for any need. While we can’t cover everything in a single tech article, we hope that what you’ve learned here will help you to better understand the ‘why’ behind a ring package’s specifications.