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Pistons. What's hyperutectic mean?


datzenmike

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Had to-day off so I decided to learn, and in some cases, re-learn a few things.

 

What makes a turbo piston, a turbo piston?

Do I need hyperutectec pistons (sounds good) or not?

Why is a forged piston stronger than a cast one?

Well read on...

 

Almost all stock pistons are hyperutectic with the exception of special applications such as a stock turbo motor. The eutectic limit, (the point at which two elements will mix thoroughly together) of an aluminum and silicon mix is about 12% but commonly, pistons are made in the 15% to 19% range and are said to be hyper-eutectic. This excess silicon won't fully melt and mix with the aluminum and is held suspended like pebbles in a concrete mix. They are almost always cast in molds and it is difficult to keep this mixture homogeneous.

The high silicon content allows for pistons that expand very little on warm up, and so are a tighter cold fit in the bore to reduce emissions and allow instant cold start use. They are cheap to make, and are perfect for normal passenger car applications where excessive heat is not a problem. The high silicon content makes these pistons more brittle and less crack resistant, than aluminum alone, and will not stand up to the detonation prone turbo motors or excessive heat.

 

Performance pistons with 11% silicon (4032) are said to be fully alloyed and expand less than aluminum alone, are less brittle and can survive mild detonation better than the hyperutectic 'smog' piston.

 

Performance pistons with 2% silicone (2618) are said to be hypo-utectic and can survive the most abuse but expand the most from heat. These pistons are given the most cold fit clearances of all and should never be over revved when cold.

 

Where hypereutectic pistons are cast, which is cheaper, and good enough for their intended use, performance pistons are usually a thicker casting and placed while hot into a die and forged under extreme pressure to remove and empty pockets trapped inside. This makes a stronger piston than that of casting alone.

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Well that's something you don't hear everyday...

 

Mike, any idea on the uses of the fully alloyed and hypo-utectic pistons? Is one set more commonly used as a turbo piston, or a race piston, etc? Do you have any idea on the relative cost between the three choices above? And finally (most importantly) are they available for the engines we use? :D

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IMHO :D The hypo-utectic would be good for heavy turbo and nitrous applications and very good for extreme performance non turbo engines. The middle (utectic?) 12% silicone would be good for hot street and lower boosted motors and DDs where cold start up noise isn't a factor.

 

I didn't mention it but even the hyperutectic pistons can be forged and or heat treated to T-3 specs which can increase strength up to 30%. There are ceramic coatings and other things but eventually it all costs the $ame.

 

As for costs no idea, would expect them to be pricey, very pricey. I'm sure there are custom L turbo pistons.

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