Originally Posted by
Wiggsy72
Short answer is that this is a performance street engine build with many years of fun driving mileage. It is not a race engine that will be brken down and rebuilt every season.
Long answer:
"There are two common ways of making a piston.
You can cast it in a mold, or you can forge it under extreme applied pressure in a forging press.
*Note*, this says absolutely nothing about what material the piston is made from, it is only the METHOD used to shape the material into the shape of a piston.
Now quite independent of HOW the piston shape is shaped, you then get to chose the type of aluminium alloy, and maybe heat treatment. There are two popular materials, pretty much pure aluminium, and aluminium very rich in silicon (sand !).
O/k now the advantages and disadvantages.
Cast pistons can be made absolutely any intricate shape, because after the molten metal has cooled solid, the multi piece mold can then be extracted from inside the piston. Very complicated slots, reinforcing ribs, oil holes and other complex features can readily be built in, and a piston can be designed to be both very light, and very strong where it needs to be made strong.
Forged pistons are stamped out in one rapid and very violent operation, where a male punch is driven into a female die to form the entire piston shape. This always has the limitation that the punch needs to be subsequently withdrawn straight out of the die backwards, meaning the inside shape has to be designed both tapered and very simple and smooth in form.
So cast pistons are almost always lighter in weight, and more intricate inside, and can be more cleverly designed.
Forged pistons will almost always be heavier and always smooth inside, the extra metal usually adding more to weight than to strength. Some forgings are truly massive, and very very strong, but also quite heavy.
Both methods can produce a perfectly usable piston, but if you either want something very light or very strong, one method may have some particular advantages over the other.
Now the material choice boils down to two very different classes of material.
Raw natural aluminium is soft, and it expands hugely with heat, but it is tough and bends before it breaks. The raw aluminium pistons in your top fuel dragster will take a hell of a battering. They will bend like a drink can or melt before they crack or shatter from detonation, these are true racing pistons. And they can be either forged or cast, but are usually forged.
The bad news is the high thermal expansion, they must be fitted very loose and they rock and rattle in the bores when cold. They also wear very quickly because they are SOFT. These racing pistons are ideal for a very high output engine that is regularly stripped after only a few races and rebuilt, but very bad news in a long life street engine.
Now the very high silicon hypereutectic pistons are very hard and brittle, have minimal thermal expansion, and can be fitted very tightly into the bores for good cold compression silent running and good oil control. They seal very well, and last a long time, and they are perfect for a long life road engine. These pistons can also be bought either cast or forged, but they have one unfortunate disadvantage.
Being brittle, detonation will crack them quite easily. But if you can keep out of detonation, they would be the best choice for most of us here.
So the most common types of pistons are:
Cast hypereutectic, very light, very long wearing, seal very well, but prone to crack if detonation occurs. What most of the car manufacturers fit, they are excellent pistons.
Cast aluminium pistons are just not made anymore, they have all the disadvantages and no advantage.
Forged hypereutectic, both heavier and stronger, very long wearing, seal very well best choice for high performance on the street.
Forged aluminium, heavy, very very strong, the toughest pistons you can buy for a real competition engine. They will rattle and wear fast and typically start burning oil after a few thousand road miles. But if you are building a 9,000 rpm six second car, you don't plan on expecting 150,000 miles between rebuilds, huh ?
All the major piston manufacturers offer their forged pistons in BOTH types of material, and the clearance will tell you if they are the hard or soft material.
Or put simply, "rubber" aluminium pistons for a race motor or "glass" hypereutectic pistons for your streeter.
So there are two things, HOW the piston is made cast or forged.
And WHAT alloy the piston is made from." - The Dude