Pistons - Cast vs Forged vs Hypereutectic

[Lambolica]

This following a queation asked of me on MSN that whilst I know a little on the subject of eutectic phase changes I'm unsure of the resulting strength of a Hypereutectic alloy Piston.

I am asking for peoples opinions on the use of these different pistions in a street application as there appears to be a definate cost and strength benifit (from a little of the research I have done) from using them compared to using Forged pistons, however I'm pretty sure there is a question of brittleness. weather this falls in the realm of engine conditions or not I am unsure.

From what I gather:

Strength:
Cast pistons -----> Hypereutectic pistons ----> Forged pistons

Cost:
Cast pistons -----> Hypereutectic pistons ---->Daylight ------> Forged pistons

Hypereutectic pistons also have a lower expansion coeffcient than Cast pistons due to higher silicone content over Cast pistons thus closer tollerences can be achieved.

Thorghts Comments????

Research Update
From some searching on the net I have found further details. there is a general concensis that the strength of the pistons as I have shown above is correct however there are a few mentions of hypereutectic pistons having higher yield and fatiuge strength however the brittleness off sets this.

Looking at what I have found I will write this pros and cos style for each type of piston.

Cast:
Cast pistons origonally were made form mild steel however modern pistons use alloys with an approx silicone content of 4-6% (eutectic pistons) but are cast in a similar fastion

Molten material is poured into a mould and cooled slowly to get a fine crystalline structue as the molten mateial solitifies. forming a tight structure of the material.

Inherant properties of the casting means that the cast pistons don't thremally expand as much as Forged and piston/cylinder wall tolerance are tighter. there is also less noise during warm up and the benifits of less blow by etc.

Cast pistons are not good at handling severe detonation and hi temp engines (turbo, supercharged, Nitros) consensus says ~ 5-7 psi max

The process is inexpensive than forged thus suit mass production.

Hyper-Eutectic Pistons
Cast in the same fashion as Cast pistons however the silicone content is higher 12.5-16%
The casting must be done correctly in correct conditions (inert gas environment) to ensure supersaturation of silicone evenly through the structure to avoid voids.

Are harder (and thus more brittle) than cast pistons, and are generally regarded as being better than cast due to higher heat resistance (~30% higher)

Are suited to perfomance application where things don't excesivly heat the pistons (Turbo max boost 15psi, Nitros 100hp shot max, Compression 11:1 max) meaning a good upgrade for N/A applications of mild S/C, Turbo applications

Cost is only a little higher that cast

tighter tolerance advantages similar to cast.

Both Cast and Hyper-Eutectic pistons offer weight saving advantages over (most) forged pistons as forged pistons require a larger collar for structural rigidity (Conflicting information here more research required)

UPDATED RESEARCH
Hyper-Eutectic pistons can be treated further (T-6 Treatment) to lift the strength by approx 30%, this is done but heating then quenching the piston in flowing water and then artifically aging the alloy buy heating it to a definete temperature for several hours and cooling in air. This form of Tempering relieves the internal stress caused by the quenching in the first stage of treatment.

Forged
the process of forging is basically getting a block of matrial and bealting the crap out of it in a mould before machine finishing. this results in the origonal blocks crystalline structure being bent as the piston takes shape and the fine structure flows around corners adding structural strength.

Heat dissipation is increased due to the shape and size of the crystalline stucture. meaning pistons run cooler

Are generally more resilliant to detonation (however you can break them still)

Generally need greater wall top piston clearances. thus are more noisy during warm up periods. also less effecient in blow by.

Formed from one of a number of metal alloys, including 4032 and 2618

4032 grade usually results in tighter piston-to-wall clearances, while 2618 makes for greater piston-to-wall clearances because of its composition (higher copper content and less than one percent silicone

Pistons formed using 2618 grade are often recommended for applications which generate increased heat and pressure. Examples of such applications include those which utilize a supercharger, turbocharger, or nitrous oxide.


The process used to make these types of pistons is generally more expensive, and usually, forged pistons are the most costly of the piston types


So in conclusion

Cast OEM pistons are suitable for low temp (sub 11:1 compression ratios, N/A engines

Hyper-Eutectic Pistons are a suitable replacement for OEM Cast and offer an exellent bang for buck for N/A engines of Factory Turbo engines that run lower Boost levels <15 psi and Will handle a Nitros shot of < 100hp

Hyper-Eutectic pistons (T-6 treated) More research to be done however may make them more suitable for moderate boost levels. pending what happens to the brittleness.

Forged - Still for uber boost or where hi temps are place on the piston.


A eutectic or eutectic mixture is a mixture of two or more elements which has a lower melting point than any of its constituents. The proper ratios of components to obtain a eutectic alloy is identified by the eutectic point on a phase diagram.

[The Witzl]

good topic.... there was a VERY big discussion about all this on the old forums in threads regarding the 4a-gze pistons.

I'll try to dig up some links.... Oldcorollas will love it.

[JustCallMeOrlando]

*Sits down to listen*

[Lambolica]

Yes I must state that I was am looking for Stu's input here.

I am aware that cast pistons are eutectic, but from what I recall Hypereutectic is a different form of treatment resulting in a harder surface structure (however if I recall more brittle) It appears that a T-6 treatment is commonly used as annealing (if this is the correct terminology) to reduce the brittleness. (tempering if you will, once again my memory fails me)

[oldcorollas]

uuh, gimme a few days
i've only been thinking of rhenium and platinum elecroplating, and chromium and aluminium pack cementation for a while..
and it just started snowing 20cm in the last hour!! i'm so goin skiing this weekend....

will update post with relevant facts later

edit: good research do i even need to add anything here?

perhaps you could look at semi-solid formed pistons also..

and as far as forged and thermal expansion etc, iirc, with cast you can design consitent thermal expansion into the piston, but with forged, it is more dificult to control the overall shange in shape of piston due to the limitations of geometry of forging.

oh and eutectic is as you desrcibe, nothing to do with softening. it is just the point at which a single composition phase will form directly from a molten mixture, ie it freezes at a fixed single temperature, not over a range of temperatures

for hypereutectic piston since there is more than eutectic silicon, when cooling the first phase to form will be rich in silicon, and then as it cools, the silicon content of the solidifying material decreases until the last bit is eutectic...

[Lambolica]

Load of help you are.....

[myne]

Hyper-eutectic?
In soldering eutectic refers to a solder that has no plastic phase. It melts and cools directly from a solid to a liquid at a precise temperature.
Could this mean that the hypereutectic pistons dont soften at all? Would it be why they're supposed to have minimal expansion?

[Lambolica]

Intitial post revised now I have done some Googling....

[Joshstix]

Eutectic is about saturation of the alloying agent in the solution as far as I know.

i.e
Hypo-Eutectic <100% saturated
Eutectic =100% saturated
Hyper Eutectic >100% saturated

In this context it is discussing the amount of silicon in the alloy.

One other advantage of the Hyper Eutectic piston is that they are more resistant to wear as there will be pure silicon on the surface of the piston and ring lands etc which is a lot harder and resistant to wear than the rest of the piston.

[Lambolica]

Correct...

[1JZ.747]

great post, there will be a rep point coming your way, not that you need more.

only thing i would say is, i have factory pistons which would be cast items.


i am running consistantly 14 psi on the street with the big turbo on street fuel. with the twins i ran 17 psi all day every day.

on the track and dyno tuning i run 121 octane fuel and N20 40 shot of. now my boost recall has seen as much as 30 psi logged though it is set to boost spike to get it on boost faster it runs 22 psi now i am upto over 150 dyno runs and 80 passes on a race track and at times at extreme heat. ie intake temps 60 deg cel or more, my engine has done 187000 k's now.

so in my personal experience i have found some of the info not to be correlating to my personal exepriences.

its all very interesting, as i am at the point of deciding what to do next which a built engine is an option using forged pistons.

Shane

[Lambolica]

Thanks for that input Shane..... I think alot of what is written is in relation to detonation as opposed to heat. If you car is well tuned you really probably only have wear to worry about.

[Lambolica]

Updated at the top also.....

Hyper-Eutectic pistons can be treated further (T-6 Treatment) to lift the strength by approx 30%, this is done but heating then quenching the piston in flowing water and then artifically aging the alloy buy heating it to a definete temperature for several hours and cooling in air. This form of Tempering relieves the internal stress caused by the quenching in the first stage of treatment.

[adamaw11]

Cast OEM pistons are suitable for low temp (sub 11:1 compression ratios, N/A engines)

can anyone comment on the pistons in the 20v blacktop? They are cast OEM pistons aren't they? and the compression ratio IS 11:1 so are you saying its right on the limit with these pistons?

[Lambolica]

I think when they say that, they are talking about 11:1 when tuned to within an inch of it's life.

Toyotas generally run rich everywhere as a precation against detonation.

I believe the 20v pistons would be a Cast eutectic alloy not mild steel cast.

[The Witzl]

lets not forget about the completely excessive level of engineering design and specification that goes into a toyota engine.....

example no.1 -> factory 1UZ-FE

[psychofox]

Pistons are not made of mild steel in any modern engine. They are almost all made of alloys of aluminium.

The tolerance of pistons is almost always proportional to the tune - cast/hypereutectic/forged pistons will all suffer from failures (damaged ring lands, etc.) from prolonged detonation. Forged pistons will just take the punishment for a bit longer due to the extra tensile & compressive strength induced from the grain elongations of the forging process.

I think when they say that, they are talking about 11:1 when tuned to within an inch of it's life.

Toyotas generally run rich everywhere as a precation against detonation.

I believe the 20v pistons would be a Cast eutectic alloy not mild steel cast.

[kemicalx]

I'll be using hypereutectic pistons in my current 7m rebuild, so I'll get to find out first hand what they're like, I'm going to investigate into getting them T6 heat treated and also will look into getting the crowns coated as once it's all up and running again it will be getting more boost (10-12psi to begin, but I’d like to be able to venture up to the limits of the ct26 at 14-16psi yer?).
However it really is a case of cost, no point getting all this treatment done to hypereutectic pistons if it's going to put them only a couple of hundred dollars under forged ones

[Lambolica]

Tell em the price son.......

[kemicalx]

Well, i just rang the engine reco place to find out about heat treatment and coating. The bloke there wasn't a whole lot of help... Though he did say it can cost up to $400 to get the crowns coated, which really kills the value of the pistons which only cost a bit over $200 a set. Here are all the prices for my bottom end rebuild:

Bored/Honed - $160+GST
Decked - $60+GST
King (should I trust em?) Main and rod bearings - $130+GST
Pistons - $228+GST
Rings - $103+GST