Diversified Automotive Media
automotive event coverage, special automotive media assignments and more!
The Lowdown on Hypereutectic Pistons
Written by Phil White - Posted October 19, 2019
Hypereutectic (hy·per·eu·tec·tic) pistons are cast pistons made from aluminum with over 16% silicon content for strength and durability. The term
“hypereutectic” comes from eutectic.
Special melting processes are necessary to “super-saturate” the aluminum with additional silicon content. Special molds, casting and cooling
techniques are required to obtain finely and uniformly dispersed silicon particles throughout the material.
These newer pistons are very hard, thus brittle. They have proven to be unforgiving with engine knocking. For this reason they are great in naturally
aspirated engines, but should never be used when building a nitrous, supercharged or turbocharged engine. Generally speaking, forged pistons are
a much better choice for high boost engines.
Hypereutectic pistons are used in some original equipment engines. They are favored because of reduced scuffing, improved power, fuel economy
and emissions. The reduced thermal expansion rate allows the piston to be run with reduced clearance.
Advantages of Hypereutectic Pistons
In general I would say that most automotive engines use aluminum pistons in an iron engine block. The combustion chamber temperatures can
reach as high as 3800+ degrees Fahrenheit, but normal operating temperatures are approximately 1300-1800+ degrees Fahrenheit, and the coolant
is usually around 185-195 degrees Fahrenheit. As we all know aluminum expands more than iron at equal temperature ranges, so for the piston to
properly fit in the cylinder at a normal operating temperature, the piston must have a fairly loose fit when the engine is cold.
With ever increasing vehicle exhaust emissions in the mid-seventies the U.S. Government decided to form the Environmental Protection Agency
(EPA), which began creating and enforcing various rules and regulations that would require automobile manufacturers to undergo drastic changes
to make their engines operate much cleaner in regards to our environment. As time went by, vehicle exhaust emissions had noticeably improved,
but more stringent regulations would force vehicle manufacturers to adopt the use of electronic controlled fuel injection systems, OBD, and
hypereutectic pistons. It was discovered that during cold engine start-up, a small amount of fuel became trapped between the piston rings and the
piston lands. During engine warm-up, the piston would expand and allow a small amount of unburned hydrocarbons into the atmosphere.
With the addition of silicon to the piston's alloy composition, the piston expansion was greatly reduced. This allowed for much tighter engine
design tolerances between the piston and the cylinder wall. Silicon expands at a much lesser rate than aluminum does, but it also acts as an
excellent insulator, thus preventing the aluminum piston from absorbing as much of the operational heat as it otherwise would. Another useful
benefit of adding silicon to the piston alloy is that the piston becomes much harder and is less susceptible to scuffing and premature wear as
compared to a soft aluminum piston. In conclusion, a hypereutectic piston is very durable during high rpm, cold engine operation, and extreme