Consequently, hypereutectic aluminum has a lower coefficient of thermal expansion, which allows engine designers to specify much tighter tolerances.
Special molds, casting, and cooling techniques are required to obtain uniformly dispersed primary silicon particles throughout the piston material.
In 1970, increasing concern over exhaust pollution caused the U.S. government to form the Environmental Protection Agency (EPA), which began writing and enforcing rules that required automobile manufacturers to introduce changes that made their engines run cleaner.
More stringent regulations forced car manufacturers to adopt the use of electronically controlled fuel injection and hypereutectic pistons.
[citation needed] As the engine warmed up, the piston expanded and expelled this small amount of fuel which added to the number of unburnt hydrocarbons in the exhaust.
This is also approximately the melting point of most aluminum alloys, and it is only the constant influx of ambient air that prevents the piston from deforming and failing.
Forced induction increases the operating temperatures while "under boost", and if the excess heat is added faster than the engine can shed it, the elevated cylinder temperatures will cause the air and fuel mix to auto-ignite on the compression stroke before the spark event.
This is one type of engine knocking that causes a sudden shockwave and pressure spike, which can result in failure of the piston due to shock-induced surface fatigue.
After the alloy cools and solidifies, it is removed from the mold and the rough casting is machined into its final shape.
Therefore, performance applications using boost, nitrous oxide, and/or high RPMs, forged pistons (made from either alloy) are preferred.
This makes hypereutectic pistons a better choice for stock engines, where longevity is more important than ultimate performance.