There are numerous choices that must be made during the process of selecting parts for a fresh engine build; from oil clearances to crankshaft material to valve spring dimensions, each decision must be made after taking the goal of the engine project into account, as many choices could be swayed in one direction or the other by a small adjustment in overall goals.
One of these choices that’s incredibly important, but varies widely with the engine’s intended use, is the selection of the proper piston material.
This article will cover some of the differences between the two main alloys used in performance engines—4032 aluminum and 2618 aluminum—and discuss what an enthusiast needs to take into consideration during the initial engine design process in order to select the correct parts for the application.
The Difference Is In The Numbers
In terms of material composition, 4032 and 2618 aluminum alloy are very similar across the board with the exception of one critical component — the silicon content within. A much higher silicon content exists in the 4032 alloy; 12-13 percent, where the 2618 alloy’s composition contains the trace amount of 0.2 percent or less of the silicon component.
There are a number of advantages attributed to the use of the high-silicon 4032 alloy, which is why Mahle Motorsports uses the alloy in most pistons designed for street/strip and other moderate power applications. Most importantly is the lack of material expansion, which has many benefits.
“Silicon reduces heat expansion, while being hard increases wear resistance. The decrease in expansion allows for tighter clearances, reduced wear on both the piston and bore, and results in quieter operation,” says Mahle’s Trey McFarland.
The high silicon content in the 4032 alloy helps the piston to survive more heat cycles before the ring grooves and skirts want to distort; in effect, the piston retains its shape better under stress due to the silicon’s resistance to material growth. This has good and bad effects, dependent upon usage, which is covered in the next section.
“Silicon is harder and does not retain heat at the same level as the aluminum it replaces, which accounts for the lower expansion rate and increased wear-resistance characteristics of the 4032 alloy. These characteristics give the alloy greater resistance to micro-welding in the ring grooves and scuffing and galling in the pin bores, while also allowing for tighter clearances which increase performance and durability while reducing noise,” chimes in Mahle’s Craig Lancaster.
Those clearances are typically set at .0025-.0030-inch right out of the box with the 4032 alloy pistons. Piston slap is not an issue with the 4032 material, as the piston doesn’t need to “warm up” before it reaches its intended clearance to the cylinder bore—it’s already there as a function of the manufacturing process.
Each alloy has its advantages with the preference being driven by the engine application and its intended use. – Craig Lancaster, Mahle Motorsports
The hard compound of the 4032 alloy does have its limitations, however, and is not ideal for all applications. The lack of ductility — which is its ability to deform under tensile stress — means that it’s less forgiving in applications where the tuning is aggressive and the power level excessive. Not because the material can’t handle the power, but because it’s not designed for those applications where ductility is paramount.
More Abuse Is Handled By The 2618 Alloy
On the flip side of the discussion is the 2618 alloy material used in some PowerPak and all PowerPak+ pistons, which is more forgiving in high-powered applications where extreme abuse is encountered on a regular basis. Malleability is a key feature of the 2618 alloy, as it will flex and move under extreme loads before reaching the point of fracture.
McFarland explains, “This gives the 2618 alloy a greater resistance to the shock loads of detonation. The compromise is that the alloy softens at a much faster rate, allowing the piston to distort more rapidly. This makes the 2618 alloy best-suited for extreme-duty race applications where the engine will be serviced on a regular schedule.”
Depending upon the intended application, some PowerPak+ 2618 pistons also feature lateral gas ports that work in conjunction with the precision-machined ring grooves to promote ring seal, in addition to hard anodized top ring grooves to help with detonation that often occurs in high-boost engines.
Custom Machinery For Performance
As an interesting aside to the material discussion, the engineering team at Mahle has constructed their very own machinery where it did not exist, since one of the keys to providing consistent performance is uniform ring seal around the piston in all conditions.
“A flat ring groove is essential in achieving and maintaining the best seal possible. The quality of the ring grooves is so critical to peak and lasting performance, that Mahle designed and built the machinery used to machine the ring grooves for its Motorsport piston assemblies,” says Lancaster.
On top of that, the company has also developed their own piston ring coating rig, seen above. 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.
An added benefit to the 2618’s increased ductility over and above the 4032 material is what makes it a favorite choice of engine builders, who cannot dictate operating conditions for any of the engines they build and let out into the wild. With today’s laser-like focus on power adder engines, the builder is often forced to provide a piston that acts almost like a fuse, dictating servicing intervals shorter than the enthusiast might like.
“If the engine will be used in an aggressive race application, or with large power adders, the application will benefit from the increased strength and detonation resistance of the 2618 alloy. Most engines that fall into this category are competition-only applications being maintained on a scheduled rebuild cycle, thereby reducing the concerns for longevity,” Lancaster says.
Both PowerPak and PowerPak+ feature the company’s Grafal coating on the skirts (more on this below) to help reduce friction. They are also coated in phosphate, a dry lubricant that provides a film in the pin bores and ring grooves, which aids in break-in and reducing wear during startup.
Coating The Skirts
Over the last decade, piston coatings of all types have made their way into the industry. Mahle, one of the innovators in this field, has used their Grafal skirt coating on pistons of all types. Grafal is a resin-based phenolic formula that’s graphite-impregnated to form an anti-friction, anti-scuff layer on the piston skirt. It eliminates the direct metal-to-metal contact between the piston skirt and cylinder wall, helping to reduce wear and noise.
“One of the advantages of Grafal is that it can be sacrificial under adverse conditions to protect the piston and cylinder wall, but it was designed to stay on the piston for the life of the part. The same coating we apply to our off-the-shelf PowerPak pistons is used throughout the product range: from OEM passenger cars to HD diesel engines with million-mile warranties, all the way up to NASCAR and Formula 1,” Lancaster explains.
Which Material To Choose?
As touched on above, each piston material has its advantages and disadvantages in practice. But the line for choosing one material over the other is blurrier than one might think. This debate has always been an interesting one, and informal polling of various enthusiast friends finds each type of material in use in both small- and big-block V8 engines.
“A good rule of thumb is if the application is configured primarily for a performance street application, the 4032 alloy will offer reduced noise at start-up, increased wear resistance, and durability. If the engine will be used in an aggressive race application or with large power adders, the application will benefit from the increased strength and detonation resistance of the 2618 alloy,” Lancaster says.
In fact, in its Project Pony Jet engine build last year, EngineLabs used the Mahle PowerPak pistons with the planned use of a small hit of nitrous oxide—showing that the 4032 alloy is acceptable even with the 200-shot of giggle gas, although 118 octane race fuel was used for that testing to provide a margin of error.
“Each alloy has its advantages with the preference being driven by the engine application and its intended use. There is no single alloy that best covers all applications,” sums up Lancaster.