Here is some more info I found out of a Hot Rod article about the piston types:
When buying pistons you will be presented with some material options that you need to know about. Firstly, regular cast pistons are the cheapest option. To make them harder, the aluminum used for regular replacement and earlier-style OEM cast pistons is alloyed with silicon (which is in essence melted sand). The silicon content makes the piston harder and more wear-resistant, but set against this is the increased brittleness. This makes it the least desirable of the type of pistons available to us for performance use. Next on the list is a hypereutectic piston. These started gaining popularity for lower-cost performance applications in Europe back in the mid '70s. In the early '80s, Sealed Power started experimenting with hypereutectic pistons and subsequently introduced a successful line of them. This was followed by Silovolite, who introduced the Keith Black (KB) signature series performance-oriented hypereutectic piston.
Although the term "hypereutectic" is commonly used, it is often not understood. For "hyper - eutectic," read "super - saturated." An alloy that is at the point where no more of the principle alloying element will dissolve in the parent metal is said to be at the eutectic or saturated point.
A regular cast piston can have up to about 10 percent to 11 percent silicon in it. This is about the eutectic point. Any more silicone will put the alloy into the hypereutectic range and as a result, form free crystals of silicon. Normally, these free silicon crystals will cause a reduction in strength, but with suitable heat treatment a hypereutectic alloy can be toughened up to a level above that of a regular cast piston.
In terms of outright strength and toughness, a hypereutectic piston still falls short of a forged piston, but not by as much as the difference in material strength might suggest. The reason for this is that the process of casting a piston, as opposed to forging it, allows the material to be put right where it is needed to support the stresses involved. With a forged piston, the underside has to be designed such that the forging punch can be extracted after the piston blank is formed. It is only by extensive (and costly) post forging machining that a forged piston can rival a cast piston in terms of lightness. By using up some of this low weight advantage by incorporating extra material at strategic points, the cast hypereutectic piston, in practice, comes out looking a lot stouter.
As far as materials for forged pistons are concerned, there are three of note. Probably the most widely used, because it is also used for OE applications, is the Federal Mogul (TRW/Sealed Power division) VMS75 alloy. This was developed probably as far back as the 1960s as a good all-around high-performance alloy for pistons. Most of the rest of the piston industry uses one or both of two alloys, these being 2618 and 4032. The 2618 is most commonly used for outright race pistons as toughness at elevated temperatures (up to about 575* F) is good. On the down side, it lacks the hardness of 4032, which is a high-silicon alloy. Though slightly less tough, the 4032 alloy is a better material for high performance street and race use and would, in most cases, be the alloy of choice for such. At the end of the day, consulting your piston manufacturers about the choice is the way to go, as they are the experts here.
Either way, guess I will never know...
