Cast Vs Forged Pistons. Piston Basics

[240dx]

I know it's a lot to read, but it's worth it.

"CAST AND FORGED PISTONSThe majority of original equipment and aftermarket pistons are manufactured through casting. The technical description is 'gravity die casting'. However for the sake of simplicity, a cast piston is manufactured by pouring molten aluminium/silicon alloy into a mold. Forged pistons differ fundamentally in manufacturing and inherent character. As opposed to casting, the forging process basically takes a lump of billet alloy and stamps the shape of the piston from a die. Of course, both manufacturing procedures are a lot more complex and intensive that this simple analysis, but you get the broad picture. Casting and forging results in two different types of piston. A die for forged piston must be designed so it can easily be removed and, as a result, the forged blank (or unfinished piston) has a relative simple shape. Casting can achieve a more complex blank and, therefore, facilitate lightweight construction. Also, due to relative manufacturing procedures, forged pistons tend to be more expensive than cast items and forged 4.9 and 5.0 litre Holden V8 pistons have been traditionally scarce, in terms of availability. Although 5.7 litre Holden Chev units ca be sourced as off-the-shelf product, you may need to have suitable 304/308 forged pistons custom made, with a tidy price to match. CAST PISTON MATERIALSNo matter which piston you use, and no matter which engine a piston goes into they are all made from a combination of both aluminium and silicon. It is the amount of silicon though, which determines the pistons overall strength verses wear resistance properties. Silicon also controls the rate of expansion of the piston as the material becomes hotter (the less expansion then better!). Silicon content also markedly affects actual material hardness. More Silicon makes the piston much easier to machine-in the manufacturing phase. There are traces of many other metals in cast pistons, including copper, nickel, manganese and magnesium, all of these adding somewhat to the overall behaviour and strength of the piston.

CAST VERSES FORGEDThis is an age-old problem for engine designers. At what sort of power level is it necessary to change from a conventional cast Eutectic/Hypereutectic piston to a forged item? According to ACL's chief piston engineer, the only real disadvantage of a cast piston (in high output situations) is in the case of a piston failure, a cast items is more likely to shatter and damage the engine, as a whole, more than a forged piston. A big advantage with forged pistons is they generally result in a more ductile material, with the effect being the piston can take a higher level of detonation before failing. As far as I am concerned, this is not such a huge bonus as you engine should be tuned not to detonate in the first place. In extremely high rpm/high horsepower applications, the great strength of the forged piston can add reliability, with ACL recommending they be used once power levels rise past about 80hp per litre of engine capacity. This gives us around 450hp in a 5-litre Commodore before even thinking of changing to a forged piston."

[Lonismos14]

You make some great write ups.

[seang]

Also, do the heat transfer properties of the aluminum decrease as the silicon content is increased?

I read that hypereutectic pistons (because of the heat transfer properties) place a larger thermal load on the rings than less-eutectic blends, and this is why the ring gap is so important. If the gap closes, the ring has nowhere to go, and it shatters the ringland.