Pistons

There are 2 different ways that pistons are made, they are Casting and Forging.

Cast Pistons - Casting means that molten liquid aluminum is poured into a mold that is in the shape of the piston. Cast pistons are weaker than forged pistons and prone to cracking with use of nitrous and boost. There 2 types of cast pistons based on their silicon content.

Standard Cast means 12.5% Silicon Content, they expand uniformly but more than any other type of piston when heated, they need the loosest intalled piston to wall clearances, since expansion is due to low silicon content when heated. They are noisiest on cold engine start ups and has the most blow-by of the 3 cast piston types in a cold engine because it has the least silicon content and therefore expands the most when the engine warms up from being cold causing it to require larger piston to wall clearances. In addition they require reinforcement plates or struts near wrist pin area, since alloy expands more as temperatures increase. These struts often crack under high loads like detonation. They also require ring groove insert to withstand ring land pound out and wear.

Eutectic Cast means 12-12.5% silicon content and Hypereutectic Cast are 16-22% Silicon. Hypereutectic is an adjective that refers to the silicon, (material found in sand) content in the piston. Hypereutectic means the piston has more silicon added to the mixing with aluminum alloy. A special melting process combines silicon to -saturate the aluminum alloy. Special molds and casting and cooling processes are needed to get fine dispersing of silicon evenly or uniformly throughout aluminum alloy.

Hypereutectic cast pistons are more resistant to wear and scuffing, expand less with heat, and are stronger and are more able to withstand higher cylinder temperatures,pressure,and detonation than standard cast.

When more silicon, if uniformly added to aluminum alloy, it hardens the aluminum alloy further and expands less with heat due to a higher themal expansion co-efficient.

Hypereutectic pistons are stronger and can run narrower ring lands and tighter piston-to-wall clearances, since there is less heat expansion than standard cast. No wrist pin area re-inforcement plate is needed due to less rapid expansion with heat and no iron ring groove insert is needed to protect against ring land pound out. Ring grooves are more heat and wear resistant with higher silicon.

But the extra hardness also makes the piston more brittle, meaning that it's easier to crack under stress and loading, like high rpms or under boost or detonation and therefore must be handled more carefully on installation.

Not all hypereutectic pistons are alike. The size of the silicon granules used and how well the silicon is mixed and distributed in the aluminum alloy during the casting process affects the quality. If unevenly added and mixed, the silicon clumps and forms hard spots in different parts of the piston which crack more easily under high loads. Most stock OEM pistons are a high silcon casting.

Forged Pistons - The process of making a forged piston involves heating a slug of aluminum alloy that is in the shape of a long cylinder and then pressing it into the die's piston shape under very high pressure, and then machining the surface. The mixture of ingredients in the metal is already in place and there is no adding and mixing like in casting where they added the silicon to the aluminum.

The forging process creates a denser and less brittle. They are 600% more ductile or have less porous of a grain structure, that is stronger than a cast piston's crystalline grain structure. Forged pistons can handle nitrous and high boost are temperature resistant and stronger and better than cast pistons.

Forged pistons have high silicon content already mixed in the alloy but have less silicon than OEM hypereutectic cast pistons. For example, an aftermarket forged Wiseco piston has 7% less silicon content than OEM Suzuki hypereutectic cast pistons.

The less porous forged piston conducts heat faster and runs 20% cooler than cast pistons, even hypereutectic cast,reducing higher detonation-inducing temperatures. There is better heat transfer away from the piston's crown with forging. The heat can be transfered to the cylinder head when the piston is at the top of the exhaust stroke.

Forged pistons have greater thermal expansion than hypereutectic cast pistons and have greater clearances and can be noisier and may have more blow-by in a cold engine compared to hypereutectic cast pistons but it's not as noisy as eutectic or hypoeutectic standard cast pistons. Once the engine is warmed up, the clearances will then tighten and there is less noise from piston rock as it travels up and down the bore. In fact, this is another advantage of forged pistons: They seal up and close up the clearances better than when heated. The trick of the racing engine builder is to find the lowest cold engine clearances that they can choose so that when the forged pistons expands at warmup, the clearances are ultra-tight to get better engine compression and less leaking.

Some Race Only pistons remove the piston skirt contact with the walls altogether and recess them back. In addition to providing more strength and rigidity to the piston itself, these types of pistons also place an emphasis on reducing friction-robbing power from skirt wear on the walls, as the piston travels along the swept volume of the cylinder.

This, however, also places a lot of stress on the rings the piston and piston ringlands because they will endure the brunt of the piston sideloading against the wall due to the fact Race Engines tend to run very tight piston to wall clearances. This tighter clearance is also needed to minimize piston rock with a Race Only piston. A Race Only piston is not meant for your daily street performance engine.