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 light weight, low cost, and acceptable strength. The alloying element of concern in automotive pistons is silicon. Gold and silver have no eutectic point, which means they can be alloyed together in any ratio, however, when silicon is added to aluminium they only blend together evenly on a molecular level up to approximately a 12% silicon content. For the purposes of this discussion, 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 melting. 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.
By adding silicon to the pistons' alloy, the amount the piston expanded could be dramatically reduced. This allowed engineers to specify a much tighter cold-fit between the piston and the cylinder liner. Silicon itself expands less than aluminium but it also acts as an insulator to prevent the aluminium from absorbing as much of the operational heat as it otherwise would. Another beneficial effect of adding silicon is that the piston becomes harder and is less susceptible to scuffing which can occur when a soft aluminium piston is cold-revved in a relatively dry cylinder on start-up.
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.
The “4032” performance piston alloy has an approximate silicon content of 11%. This means that it expands from heat less than a piston with no silicon, but since its eutectic level of silicon is fully alloyed on a molecular level, this 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.
The “2618” performance piston alloy has less than 2% silicon and could be described as hypo (under) eutectic. This alloy is capable of experiencing the most detonation and abuse while suffering the least amount of damage. Pistons made of this alloy are also typically made thicker and heavier because of their most common applications. Because of the higher than normal temperatures these pistons experience in their usual application and also the low-silicon content allowing the maximum possible aluminium heat-expansion, these pistons have their cylinders bored to a very loose cold-fit. This leads to a condition known as “piston slap” which is when the piston rocks in the cylinder and it causes an audible tapping noise that continues until the engine has warmed to operational temperatures. These engines should not be revved when cold, or excessive scuffing can occur.
In the forging process the rough casting is placed in a die set while it is still hot and semi-solid. A hydraulic press is used to place the rough slug under tremendous pressure. This removes any possible porosity and also pushes the alloy grains together tighter than can be achieved by simple casting alone. The result is a much stronger material. Hypereutectic pistons can be forged but typically are only cast because the extra expense of forging is not justified when cast pistons are considered strong enough for stock applications.
Aftermarket performance pistons made from the most common 4032 and 2618 alloys are typically forged.