What kind of replacement pistons are best?
Ordinary cast pistons are usually adequate for most passenger car
applications. When it comes to modified or high output engines,
racing engines, marine engines, and severe-duty applications,
ordinary cast pistons may not be adequate in terms of strength,
durability and longevity.
Higher operating speeds, horsepower output, torque loads, and
thermal stress create an environment calling for a "performance"
piston. For most applications, that means forged pistons made from
forged aluminum slugs. Ordinary cast pistons are made by pouring
molten aluminum into a mold. The forging process increases metal
density, significantly improving strength, ductility and thermal
characteristics.
Hypereutectic pistons (which are also cast) are a low-cost
alternative to forged pistons for certain original equipment engine
applications requiring something better than an ordinary cast
piston.
Hypereutectic alloys contain a higher level of silicon (16% to 22%
versus 8% to 11% in a typical cast piston). This increases hardness
to reduce ring groove, pin boss and skirt wear. Most performance
engine builders use the forged variety. Pistons in high output,
racing, marine, or severe service applications are subjected to
forces far exceeding those encountered in everyday passenger car
engines.
In a 350 Chevy passenger car engine with a compression ratio of 8:1,
combustion pressures generated at wide open throttle typically peak
out at around 700 psi. This yields a total force of about 8,800
pounds pushing down on the top of the piston.
By comparison, a high performance 350 engine with a compression
ratio of 12:1 can generate upwards of 1,200 psi of combustion
pressure at wide open throttle. This translates into a downward
force of 15,000 pounds on each piston (nearly twice that of the
passenger car engine). Although a cast piston might be able to
survive this kind of punishment for a while, a forged piston is
preferred for long-term reliability.
The difference in strength between forged and cast pistons is
considerable. Cast pistons made of F-132 alloy (commonly used for
many OE pistons) typically shatter when their maximum yield strength
of around 27,000 psi at room temperature is exceeded. Hypereutectic
alloys stand up a little better than standard cast alloys, but are
also brittle and shatter when their yield limits are exceeded.
Forged, on the other hand, can withstand loads anywhere from 40% to
almost 100% higher without failing. When they do fail, forged
pistons tend to deform rather than shatter because the forging
process makes the metal up to 600% more ductile.
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