Cast pistons are made by pouring molten aluminum alloy into a mold; forged pistons are pressed from a solid billet under great force, which produces a denser, stronger part. Cast and hypereutectic pistons are standard in most factory engines: they are quieter, cheaper and run tighter clearances. Forged pistons are stronger and tolerate detonation, boost and high rpm better, which is why they are used in built and boosted engines, but they expand more and need larger clearances, which can mean noise when cold.
Forged vs cast vs hypereutectic
| Cast | Hypereutectic | Forged | |
|---|---|---|---|
| Made by | Casting | Casting with high silicon content | Forging |
| Strength | Adequate for stock power | Better than standard cast, more brittle than forged | Highest |
| Expansion and clearance | Low, tight clearance | Low | Higher, larger clearance |
| Cold noise | Quiet | Quiet | Can make piston slap when cold |
| Cost | Lowest | Low | Highest |
When you need forged pistons
- Adding significant boost from a turbo or supercharger
- Nitrous or high-rpm racing use
- Engines prone to detonation, where cast pistons can crack or lose ringlands; see ringland failures and EJ257 problems
Hypereutectic pistons
Hypereutectic pistons are cast with a high silicon content, which reduces expansion and improves wear resistance. They are common in modern factory engines and in rebuild kits, and they handle mild power increases well, but they are more brittle than forged pistons when detonation occurs.
When cast pistons are fine
For a stock or mildly modified street engine, factory cast or hypereutectic pistons last a long time and run quieter. Many replacement engines and short blocks use them.
Forged pistons and cold noise
Because forged alloys expand more, they are fitted with more clearance. Until the engine warms, they can rock in the bore and make a light knock, often called piston slap; see LS1 problems for a factory example of piston slap and engine knocking.
Rings and ringlands
The ringlands are the thin ridges between the ring grooves on the piston. Under detonation, they are the first part of a cast piston to crack, which causes low compression, smoke and blow-by. Forged pistons resist this better, but no piston survives sustained detonation, so fuel quality and safe boost matter whatever pistons you use; see low compression in one cylinder.
Pistons are only part of a build
Strong pistons need matching rods, rings, bearings and, often, a stronger block deck; see closed deck vs open deck, piston rings and short block vs long block. Built short blocks for Subaru engines are listed on Subaru EJ25 engines; check each listing for its internal parts.
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