Preferences, Pros, and Con's?

 

Hypereutectic (cast) pistons are what most OE use for their non-high performance engines. Without going into much details, casting is the process of injecting hot liquid metal into a mold where it will take the size and shape. A cast aluminum piston has the advantages of light weight and saving cost. But because of the way casting is done, the alloy grain usually are not as tight as oppose to a forged piston. In a nut-shell, a forged piston is made by placing the semi-solid metal slug in a die and pressed into shape via a high pressure hydraulic press. A forged metal is a much stronger piece than a cast piece.

 

Depends on what you plan on using them for.



Hypereutectic pistons are more durable than plain cast aluminum pistons. Hypereutectic aluminum refers to a unique aluminum alloy that contains dissolved and free silicon. The material can be T6 heat treated to high strength and stiffness. Non-heat treated hypereutectic aluminum has slightly less strength than conventionally cast aluminum. So if it's not heat treated, there's no benefit to using Hyper pistons over plain cast aluminum pistons.



Forged parts are much DENSER that cast parts (Hyper, or plain). So they are heavier. In some cases that extra weight can be a bad thing(high rpm) and engine balance. Because forged parts are more dense, they also expand more when heated than cast parts made out of the same/similar metals. So in pistons, you need a greater piston/wall clearance with forged pistons than with Cast/Hyper. So this means poor ring sealing at start up, until the forged piston heats up and expands in size to allow the piston rings to properly seal the cylinder bore. It can also mean, if your forged pistons aren't sized properly, you can tear up the cylinder, break rings or break ring lands if they pistons are too big. If the pistons are too small, you'll have poor combustion chamber sealing, and decreased compression, along with combustion by-products(acids) in your oil, doing damage to bearings and other parts. Anyplace you buy a forged piston from should know your specs and be able to build it properly when talking about stock replacements. But when you're talking about overbore replacements, if your overbore company didn't remove quite as much material as you wanted/or too much, and if the piston is made just a tad too big/small, you can have problems.



Because of the increase in density, Forged parts are much stronger than any cast part. They can take more shock load (detonation/knock) and also take repeated load over more cycles without cracking like cast/hyper parts do/can. Forged pistons are best used for high power motors and/or motors that see very high RPM/High Combustion Chamber temps.



The only way I would go with Forged pistons if you were planning on getting more than 150-175HP per Liter out of the motor. At anything below that value, Hyper pistons will work just fine.



If you want to read more.

http://en.wikipedia.org/wiki/Hypereutectic_piston