Hypereutectic Pistons & High Zinc Oils

[garageman402]

I have heard recently that using high zinc oil in an engine with hypereutectic pistons will "remove the hypereutectic coating & ruin the piston".

My understanding is the piston is an alloy of silicon & aluminum to limit expansion & allow tighter cold clearances. How can zinc degrade that?

 

[CheezWhiz]

sounds bullshittish.

 

[Jetronic]

A lot of pistons have coated sideskirts, but zinc or phosphor won't remove that. Wear will remove it and at that point your oil needs to put a new sacrificial layer on, like phosphate glass, or MoS2 etc...

 

[Slick3914]

More horse feathers.

 

[OVERKILL]

I have heard recently that using high zinc oil in an engine with hypereutectic pistons will "remove the hypereutectic coating & ruin the piston".

My understanding is the piston is an alloy of silicon & aluminum to limit expansion & allow tighter cold clearances. How can zinc degrade that?

Whoever told you that has no idea what a hypereutectic piston is, so you can safely completely disregard their opinion.

 

[dkryan]

I have heard recently that using high zinc oil in an engine with hypereutectic pistons will "remove the hypereutectic coating & ruin the piston".

My understanding is the piston is an alloy of silicon & aluminum to limit expansion & allow tighter cold clearances. How can zinc degrade that?

I couldn’t even pronounce “hypereutectic” let alone figure out what kind of piston that would be. To save other BITOGers a little head scratching and research time, here’s a couple of excerpts from Wikipedia:

“A hypereutectic piston is an internal combustion engine piston cast using a hypereutectic aluminum alloy with silicon content greater than the eutectic point of 12 weight percent silicon. Most aluminum-silicon casting alloys are hypoeutectic (Si content lower than the eutectic point) and contain relatively fine elemental silicon crystals formed through the eutectic reaction during solidification. In addition to fine silicon crystals, hypereutectic alloys also contain large primary silicon crystals that form before the eutectic reaction, and as a result contain a much higher phase fraction of silicon. Consequently hypereutectic aluminum has a lower coefficient of thermal expansion, which allows engine designers to specify much tighter clearances. The silicon content of these alloys is typically 16-19 weight percent, and above this content the mechanical properties and castability degrade substantially. Special moulds, casting, and cooling techniques are required to obtain uniformly dispersed primary silicon particles throughout the piston material.”

“Compared to both 4032 and 2618 alloy forged pistons, hypereutectic pistons have less strength. Therefore, for performance applications using boost, nitrous oxide, and/or high RPMs, forged pistons (made from either alloy) are preferred. However, hypereutectic pistons experience less thermal expansion than forged pistons. For this reason, hypereutectic pistons can run a tighter piston to cylinder clearance than forged pistons. This makes hypereutectic pistons a better choice for stock engines, where longevity is more important than ultimate performance. Some vehicles do use forged pistons from the factory.”

 

[wag123]

What destroys hypereutectic pistons is pre-ignition or detonation, not zinc.

 

[OVERKILL]

What destroys hypereutectic pistons is pre-ignition or detonation, not zinc.

Bingo! Don't want to rattle those slugs, they shatter.

 

[JeffKeryk]

Just stab a hydraulic roller and run M1. Just my 2 cents...

 

[oil pan 4]

I have heard recently that using high zinc oil in an engine with hypereutectic pistons will "remove the hypereutectic coating & ruin the piston".

My understanding is the piston is an alloy of silicon & aluminum to limit expansion & allow tighter cold clearances. How can zinc degrade that?

"Hypereutectic" isn't a coating.

 

[ripcord]

Hypereutectic just refers to the composition of the aluminum alloy. Just like adding a sufficient amount of chromium to steel makes it stainless, adding a high enough silicone content to an aluminum alloy makes it hypereutectic. As previously mentioned, this has zero to do with any coatings applied to the piston and their properties.

 

[RDY4WAR]

A hypereutectic piston is just a cast piston with a high silicon content. It has a lower expansion rate, allowing for tighter cylinder to wall clearance and ring gaps, making it a great choice for daily commuter engines striving for better fuel economy and less blow-by across a wide temperature range. The problem is the high silicon content makes the piston brittle. Therefore, any sudden shock (ie: detonation) can crack the piston. ZDDP is irrelevant, including coatings.

 

[Vercingetorix]

The Chevrolet Vega used a hypereutectic aluminum block and high zinc oil. I don’t think any of them survived past the 80s.

 

[OVERKILL]

A hypereutectic piston is just a cast piston with a high silicon content. It has a lower expansion rate, allowing for tighter cylinder to wall clearance and ring gaps, making it a great choice for daily commuter engines striving for better fuel economy and less blow-by across a wide temperature range. The problem is the high silicon content makes the piston brittle. Therefore, any sudden shock (ie: detonation) can crack the piston. ZDDP is irrelevant, including coatings.

Yup, Ford migrated the 302HO from TRW forged slugs to hypereutectic ones in like 91 or 92? I can't remember the exact date, but the idea was to tighten up piston-to-wall clearance, reduce blowby and positively impact emissions performance. These engines were more fragile than their predecessors when you added boost.

 

[Trav]

The Chevrolet Vega used a hypereutectic aluminum block and high zinc oil. I don’t think any of them survived past the 80s.

I don't think many survived past 30K without torn up cylinder walls. Later versions of the 2.3 had a better cooling system but it was too late for GM to pursue further development of that design. GM had to call it the Dura-Built engine LOL.

 

[Fabulous50s]

Using oil in an engine increases the sliption, also reducing the sticktion. I prefer to run an airgap engine. Using oil just fouls up the airgap.

Therefore, I only run air in my engine.

 

[LongTime_Lurker]

Using oil in an engine increases the sliption, also reducing the sticktion. I prefer to run an airgap engine. Using oil just fouls up the airgap.

Therefore, I only run air in my engine.

Haha…maybe if your engine runs fast enough! This reminds me of those plastic Air Hogs engines that I played with as a kid.

 

[SubieRubyRoo]

A hypereutectic piston is just a cast piston with a high silicon content. It has a lower expansion rate, allowing for tighter cylinder to wall clearance and ring gaps, making it a great choice for daily commuter engines striving for better fuel economy and less blow-by across a wide temperature range. The problem is the high silicon content makes the piston brittle. Therefore, any sudden shock (ie: detonation) can crack the piston. ZDDP is irrelevant, including coatings.

Nitrous tuneups on hypereutectic pistons are always on the “fat” side with retarded ignition timing to soften the shockwave when the combustion pressure spikes. I’ve run as high as 175 shot on a stock ‘95 5.0 Ford short block with no issues proving that hypereutectics can live like this, so it can be done, but you always have to creep up on the tune. If you go for the gusto without verifying spark, fuel, and nitrous levels things will be disastrous very quickly!

If you keep it safe like this, IMO the limiting factor is only when the piston top and top ring gets so hot that the ring gap closes completely; imagine popping dandelion heads off, but with hot aluminum and disastrous results

 

[chris719]

As previously noted, eutecticity is a property of the alloy and eutectic alloys melt and solidify at single lower temperatures than any of their individual constituents. This is very helpful in certain applications like soldering; you get a better joint.

BMW S55 and later M engines use Mahle 2618 pistons with Grafal or other Mahle coatings and specify a full SAPS oil in the US. I have never heard of any coating issue with the ZDDP content in normal FS Euro oils.

 

[Eddie]

Please give us the source of this information garageman. Ed