Thread: what's the difference

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When it comes to pistons the word Hypereutectic keeps coming up. I guess I'm getting old because we just ran cast or forged. What's the difference with the Hypereutectic? By the way, thanks for the advice on the rods.

hypereutectic

(hī'pər-yōō-těk'tĭk) Pronunciation Key

adj. Having the minor component present in a larger amount than in the eutectic composition of the same components.




eutectic   

/yuˈtɛktɪk/ Show Spelled Pronunciation [yoo-tek-tik] Show IPA Pronunciation
Physical Chemistry

–adjective 1. of greatest fusibility: said of an alloy or mixture whose melting point is lower than that of any other alloy or mixture of the same ingredients.

2. noting or pertaining to such a mixture or its properties: a eutectic melting point.

–noun 3. a eutectic substance.

Origin:
1880–85; < Gk eútēkt(os) easily melted, dissolved (eu- eu- + tēktós melted) + -ic

Or, in simple terms......better than cast, not as good as forged.

Great question. There's really a bit more:

As defined above, “hypereutectic” literally means “over” (hyper) eutectic. Eutectic is a condition in chemistry when two elements can be blended or combined at a molecular level, but only up to a very specific point, after which any additional secondary element will retain a distinct and separate form.

Most factory stock pistons are aluminum but typically contain small amounts (trace, i.e., less than 2%) of copper, manganese, and nickel. The major alloying element in pistons is silicon. When silicon is added to aluminum they only blend together evenly on a molecular level up to +/-12% (silicon content). Remember that silicone is just another name for sand that has been pulverized. The problem becomes any silicon above 12% content remains in its granular form instead of melting. At about 25% silicon there is a significant reduction of strength in the aluminum alloy (typically evidenced by failure at the wrist-pin bosses). Most hypereutectic pistons have between 16% and 19% silicone.

Modern engines typically use steel cylinders (or sleeves in an aluminum block) and aluminum pistons. Combustion temperatures of a “warm” engine typically run 600+/- degrees. Ideal coolant temperatures maintain the overall block temperature around 190 degrees. Because aluminum expands more than steel, the piston must have clearances when the engine is cold to allow for this expansion.
The hypereutectic technology of adding silicone actually came about as a result of the EPA’s requirements to reduce the fuel leakage. Studies at the “heaven-only-knows EPA skunk works” showed that, on cold engines especially, small amounts of fuel became trapped between the piston rings. As the engine warmed and the pistons expanded, the trapped fuel was released and added to the hydrocarbons in the exhaust. These are very minute amounts but multiplied by a gazillion pistons, the EPA felt there was a need for concern. By adding silicon to aluminum, piston expansion was reduced dramatically. Engineers to thus able specify a much tighter fit between the piston and the cylinder and reduce this tiny bit of trapped fuel to near zero.

The biggest drawback of adding silicon to aluminum is that the piston becomes more brittle (as noted above, re; wrist pin bosses) as the ratio of silicon is increased. This becomes a huge problem if pre-ignition or detonation occurs – resulting in cracked pistons.

When hot rodders want to increase power, we force induction (super/turbocharger). By compressing air and fuel, the power is increased. However, this also increases the heat and pressure in the cylinder. Forced induction increases the operating temperatures and if heat is added faster than engine can dissipate it, the increased temperatures will cause the air and fuel to prematurely ignite on the compression stroke. This is bad. I believe the technical term may be really complicated but the one I’ve heard usually contains “Oh s**t! This many times results in an immediate unplanned shut down and a taxation of the checkbook.

Several high performance piston companies advertise “4032 high silicone aluminum alloy”. The “4032” alloy has a silicon content of approximately 11%. This provides less expansion than a piston with no silicon and the silicon is “fully alloyed” on a molecular level (eutectic). This results in a stronger and more forgiving piston than than a stock hypereutectic piston with 16-19% silicone content. These pistons may survive mild detonation with less damage than stock pistons.

A piston advertised as “2618” aluminum alloy has less than 2% silicon. It is actually “hypo” (under) eutectic. This is by far a stronger alloy but needs greater clearances. As such, engines using pistons made of 2618 are susceptible to “piston slap” and should be allowed to warm up gently to avoid scuffing them with high revs.

Hypereutectic pistons can be forged but typically are cast as the additional expense associated with forging is not justified as cast pistons are considered strong enough for stock applications.

Aftermarket performance 2618 and 4032 alloy pistons from Keith Black, JE, etc., are typically forged.

Regards All,
Glenn

That was a great answer!! Ive always wondered the same thing, and now i understand.

Glenn. Nice piece. I knew the hypereutectic pstons were not as strong, but didn't know about the silicon. Thanks for the info.

I knew I came to the right place with my question, Thanks

and dont be concerned with using them in moderate performance engines .. they go in everything i build .. stock to 450 horse with 200 nos shot .. never a piston failure .. just bend a rod now and then ..

they are factory in turbo and supercharged applications, but can come apart if you run anything that is non factory..