Forging Pistons and Thermal Expansion: can't find data to support it.

[SaabTuner]

So for a long time I've been vexxed by the explanation given by piston makers for the increased tolerances in forged pistons. I've spent many long hours scouring matweb for any evidence to suggest that the forging/pressing process significantly affects the thermal expansion of an alloy in any way.

Certainly, 2XXX AlCu wrought alloys are going to expand more than the 4XX.X hypereutectic AlSi alloys, but that is not the fault of the forging. As well, pistons in high-output engines will require more clearance due to higher piston temperatures, but that is also not the fault of forging.

In many cases, the reccomended clearance for a forged piston is more than 10X that of the stock cast piston. (even for stock pistons which are not hypereutectic) What gives?

Does anyone have any hard data to support the idea that forging increases the thermal expansion properties of an alloy by any significant amount?

Thanks in advance for any information.

 

[Majik]

I don't see where your getting that the clearance between a stock cast piston with expansion bands runs a 10x tighter clearance than a forged piston. Most 4xx.x series aluminum silicon alloy pistons run a .0035-.004" clearance which is very tight. the 2xxx series pistons run about a .005-.006" clearance. A stock cast piston runs between .0025-.004". Numbers are close enough for me. No cast piston to my knowledge runs a .0003" clearance. And no forged piston runs .040" of clearance.

 

[ivymike]

first of all, I take it you're worried about clearances, not tolerances?

Secondly, when you say that the clearance is 10x, which clearance are you referring to? Top land clearance? Second land clearance? Min clearance on the skirt?

My suspicion is that if you compare an aftermarket forged aluminum piston to an OEM cast aluminum piston, you will find differences in several areas:
* application:
- since you're upgrading the pistons at considerable expense, is it not reasonable to assume that you'll be using them at higher power settings, or at higher load factors than would typically be seen in the OEM application?
* tolerances:
- face it, KS, FMO and Mahle probably make much better pistons than your aftermarket supplier. If you can't hold +/- 0.015mm on the crown land diameters, and +/- 0.005mm on the skirt profile, you can't very well run the same clearances can you?
* design:
- since the OEM has less thermal expansion to deal with, and tighter tolerances from the machining process, they can probably run tighter clearances all over with little concern about scuff/siezure
- The OEM and/or piston supplier has undoubtedly spent quite a bit of money running PisDyn, Glide, and/or an in-house application(s) to refine the skirt profile and crown clearances. They've been supported by thousands of hours of test cell time, and have a database of previous design experience and guidelines. They know pretty well what the operating envelope of the engine is, and what the boundary conditions for the piston are. They've come up with a skirt profile and ovality that give good piston guidance with relatively little potential for scuffing or abnormal wear. They run the top land tight for HC emissions, and the second land tight for reduced blow-by (another emissions driver). The aftermarket supplier, on the other hand, is pretty much winging it w/re to the skirt profile. If they've got the money, they may run a few piston marking tests to check the contact distribution on the skirt, and a full-power pull or two to check for top land contact. They know they need gobs of top land clearance because of thermal expansion, and they don't give a darn about emissions. Too much clearance will mean a bit more blow-by (but they can sell you "gapless" rings to help that) and a little more noise, but it will mean a bit less friction and much less chance of siezure or scuffing. As long as the piston doesn't come apart too early or stop when it's supposed to go, everyone is happy.

 

[Fabrico]

The forging process can't be irrelevant, as it can certainly put more material in approximately the same space. Forged pistons also have unavoidable heavy cross sections in some areas compared to cast. Even when machined internally to lighten them, forged pistons are noticeably heavier. Forging also limits material choices and silicon content. Silicon is a major factor in aluminum hardness and expansion.

 

[SaabTuner]

I meant piston-to-wall clearance. Minimum, not on the thinner part of the camming.

The ten-fold increase in clearance was from a Subaru STi piston which has stock piston-to-wall clearances from .0000"-.0009" listed as stock. That is where I'm getting the "ten times" clearances. It is a hypereutectic piston, however, my old Saab 900 non-turbo has an identical .0000" (rounded from .00003") minimum piston-to-wall clearance. The turbo was minimum of .00055". So I guess "six times" clearance in that case.

"The forging process can't be irrelevant, as it can certainly put more material in approximately the same space."

Except that forged aluminum is not noticably denser than cast aluminum ... at least according to matweb. I guess if anyone has any hard data to prove that wrong, I'm more than game to listen.

"Forging also limits material choices and silicon content. Silicon is a major factor in aluminum hardness and expansion."

I'm not so sure about that. I'm not saying you're wrong, as I don't have enough information in front of me, but there are wrought melt-spun alloys with a 50% silicon content and plenty which are hypereutectic, beyond the ~11% eutectic point.

Big thanks to Ivymike. Those are the conclusions which I reached. It's why I am so vexxed that the aftermarket continues to blame this on the forging process itself. However, even Mahle forged units run more clearance than most cast pistons these days. But I think the problem is, as I assumed and you have said, more to do with R&D than forging.

If anyone has any hard data about the forging process and thermal expansion, I would still be VERY appreciative!!

-Adrian

 

[ivymike]

even Mahle forged units run more clearance than most cast pistons these days

In comparable applications? ...or does the forged piston go in the high heat rejection app?

 

[EddyC]

I have no direct knowledge, but I've read repeatedly that the additional clearances are due to the material grain pattern that results from the forging process as compared to the casting process. Apparently the material grain pattern of a cast piston is more random than the pattern of a forged piston (or so I've read). This pattern supposedly influences thermal expansion.

 

[Fabrico]

Back to the original question, higher expansion is not the fault of the forging process as you suggest. Forging is part of achieving the strength requirement. The two main forged piston materials are 2618 or 4032. The 2618, highest strength for the most serious race applications, has less than .25% silicon and a high expansion rate. The 4032, for milder applications, has 11 to 13.5% silicon and has an expansion rate similar to standard cast pistons. Hypereutectics, which typically have 16.5-18% silicon, are among the lowest rates of expansion. This amount of silicon is not used to make the aluminum stronger. It is used to reduce expansion by rejecting heat.

There undoubtedly is some confusion and lumping together of long standing beliefs. Aside from materials, clearance is also affected by piston design and all other dimensions.

 

[Majik]

I'm not saying it's true, however one should read his nasioc post for more information on what he is trying to get at

http://forums.nasioc.com/forums/showthread.php?t=1061799&page=1&pp=25

It's interesting to read so far because the STI's do run that kind of clearance after reading up on them. I'm sure there's an SAE paper on cast pistons with zero expansion and benifits from this. or maybe it has something to do with expanding cylinders under heat as well but that wouldn't explain the mahle forged box in box subie sti pistons running 3.5thou clearance. While few motors do run this setup it would be interesting to note why this happens and what is the manufacturers goal?

 

[ivymike]

although I would add one thing about the comment on scupper slots: These slots allow for the skirt to flex and cushion any piston slap. ...perhaps more importantly, they're WAY cheaper to make.

 

[kenre]

Correct me if im wrong, but many factory cast pistons have steel expansion plates cast into the piston, supposedly to hold the piston in an expanded shape?? these plates are meant to control the expansion to allow tighter clearances.

Ken

 

[ivymike]

I saw one of those once. Asked a piston manufacturer about it - he said they hadn't done that in about ten years. I don't know if anyone does currently. As it was explained to me, the steel was to help carry thrust loads from the skirt to the pin (Al was not strong enough at the time).

 

[NickE]

"The forging process can't be irrelevant, as it can certainly put more material in approximately the same space."

SaabTuner has pointed out that this is a completely untrue conclusion of what forging does to the metal.

(Hi Adrian, how are you.... LyveWRX on Nasioc)

 

[Fabrico]

Thank you NickE, how about also pointing out what the process does to piston construction and profile. As mentioned above, forged pistons are also not of the same alloys.

If one reads this thread or the one from the forum that Majik posted above, SaabTuner seems to be the one blaming the forging process itself for the increased clearances.

What's in a name? This group of pistons made from significantly different alloys and a radically different manufacturing process, and having significantly different properties and profiles, just happen to be called "forged pistons".

 

[NickE]

Forging is working of the metal to obtain a desired shape by impact or pressure in hammers, forging machines, presses, rolls, and related forming equipment.... Some metals can be forged at room temperature, but most are made more plastic for forging by heating. Often above the recrystalization temperature. (paraphrased from ASM Metals Reference Book, I added the part about recyrstallization temperature.)

Thats the definition of forging. Yes, there is some gas porosity in any cast alloy. But the density does not significantly increase with forging. For the same volume the mass is roughly the same. So it doesnt have more material in the same space.

Alloys that are cast have a specific elemental makeup used to provide various benefits during melting, pouring, solidification, machining, heat treating, and final use.

Alloys that are wrought also have a specific elemental makeup used for the same reasons.

Personally I dont know what forging does to the piston's construction and profile, thats not my field. However I do know about metals and metallurgy, esp casting and a bit about forging.

Forging allows the designer to take advantage of various properties that directly change wrt the grain flow. Crankshafts come to mind here. It also allows different alloys to be used. Ones that dont have the large amounts of elements added to provide fluidity and prevent mold shrinkage. (A356 vs 6061, etc....)

Forged parts generally have better fatigue and creep properties. However that is not always true. (single X-tal turbine blades are cast.)



Nick
I love materials science!