We had a little discussion at work-
I though it was an aluminum alloy that had a high silicon content and had very tight tolerances. It was stronger than cast and expanded less under heat, but was much more prone to failure when exposed to detonation vs a cast piston making it a very poor choice for forced induction.

They guy at work said it was a new technolog devoloped by subaru that is better than cast but not quiet forged and used in the STi (300hp turbocharged H4).

I ignored the "new technology" part as it was probobly the typical "anything new in an import is techonologically advanced" but the part about them being an upgrade in the STi got me wondering. Why would Subaru use these pistons as an upgrade if they are less suited for forced induction than cast.

 

it sounds to me like you've got it right. hypers dont like detonation at all compared to forged or other pistons. but as for using them on something with forced induction... i can see the arguement for that less expansion = tighter fit to the bore. but in truth they are supposed to be stronger than a standard cast piston. i thought maybe wieght was another benefit too but i cant remember where silicon sits on the periodic table at the moment vs Al. as for it being new tech... i guess thats a relative term.

 

You are correct, they are an Al-Si alloy with 16% or greater Silicon content.
They are advertized as being better in all aspects to standard AL alloy pistons, including higher temps. and pressures.

Oh, and it is only relatively "new tech" and was not developed by Subaru.

 

A hypereutectic piston is typically still cast. Just wanted to clear that part up from your original question. The silicon gives some higher wear resistance in places like the top ring groove over a eutectic piston (approx. 13% silicon). That's one of the main reasons some of the OEs have switched to more silicon. Once you anodize the top ring groove though, most of that benefit (if not all) goes away.

It's certainly not new tech. I know they've been out for at least 10 years. Many of the transplants use a eutectic alloy with higher copper content for better high temperature fatigue strength than hypereutectic. That's not very common here in the US though.

Just saw the new LS7 piston today at a Mahle tech meeting. That was one nice looking piece. They weren't conservative with the design at all.