Bumping this up from the grave (canx2k I'm going through all your old Off Topic threads). The information UK-SRi gives is actually half correct. The main part is incorrect though.
Traditional forged aluminum pistons have a low silicon content. They are very strong, and pretty ductile so they resist detonation very well. They are the choice for many racing engines both forced induction and naturally aspirated. This alloy is labeled 2618.
The problem with 2618 alloy pistons, is that they expand quite a bit when heated. This means that to have good piston-to-wall clearance when running at normal operating temps, the pistons will be undersized when cold or warming up. This produces a good bit of piston slap which adds a good bit to NVH levels. They basically sound like old diesel engines when warming up. The increase on piston and cylinder wall wear is also not to be ignored. Additionally, emissions suffer during the warm-up period because of the poor seal.
As you can imagine, those drawbacks are unacceptable in a new vehicle these days. Because of this, you have pistons made from aluminum alloy 4032. This alloy has a high silicon content. This raises the high temp hardness of the piston (less ductile so not as detonation resistant), and makes the volume much more consistent at cold and warm temps. These piston do not suffer from the expansion problem that the 2618 pistons do (as UK-SRi incorrectly mentioned). You can run tight tolerances (although not as tight as cast pistons) and avoid most of the NVH problems. These are the type of pistons found in the Lexus mentioned above. As well as other vehicles with forged pistons from the factory, such as the Honda S2000, and others. These pistons are not as strong or detonation resistant or as light as the 2618 alloy pistons, but they are still a good bit better than cast pistons in those regards. They cost more to make than cast pistons obviously.
Last edited by BenFenner
on 2012-02-09
at 19-43-11.
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Originally Posted by canx2k 
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