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.