Cast or Forged Pistons?

[ANZAC_1915]

The intent of this thread is to collate information about the STi pistons.

If you want to whine and complain about wanting to start a class action suit because you were sure the car had forged pistons and you wanted to run it at 500HP then please go to SOA Issues forum.

I will leave this thread open and sticky under the rules that it strictly stay on the topic of the technical information about the pistons, and published descriptions.

* Registered users of the site do not see these ads.

[ANZAC_1915]

Subaru Australia Forester XT description: http://subaru.com.au/downloads/929305.pdf

The pistons feature improved high tensile strength aluminium alloy (AC8A->UAC12H) along with optimised piston crown shape for improved fatigue resistance.

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Subaru Media info for STi: http://vocuspr.vocus.com/VocusPR30/D...0-65d2b0f27407

In the section "300 Horsepower and ..." it says the STi has forged aluminum alloy pistons.

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SOA STi News Release on subaru.com: http://www.subaru.com/content/newscm...ry=2&article=8

The 2.5-liter horizontally opposed ("boxer") four-cylinder engine is based on a specially reinforced "semi-closed deck" engine block with forged aluminum alloy pistons, forged high-carbon steel connecting rods and sodium-filled exhaust valves.

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Car and Driver Forester XT review:

http://www.caranddriver.com/article....rticle_id=6854

There's a patented new trick to this engine, too. Ichikawa-san's engineers found that the aluminum rear main bearing cap was letting the crankshaft wobble a bit, opening up the bearing clearance over time and generating a rattling noise. So they invented a new kind of main bearing cap that has a shaped sintered-steel core and an aluminum wrapper that does the job with two conventional bolts. The 2.5-liter XT turbo engine also gets a new semi-closed-deck block, heavy-duty connecting rods, forged pistons, and a drive-by-wire throttle setup.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Most of these from Jon in CT.


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From Specul8r:

Here's my question to SOA:

YOUR ORIGINAL MAIL:

I purchased a 2004 STi last July, and have one quick question. I see
information on your website that states that the STi comes with forged
aluminum pistons. A well-know engine performance company disputes that
they are forged. My question to Subaru: Are the 2004 STi pistons
forged aluminum?

Here's their response:
Thank you for visiting the Subaru Web site and for your inquiry. The
2004 WRX STi is equipped with forged steel pistons and forged aluminum
conrods and crank.

Thanks for the opportunity to be of assistance. If you need any future
assistance, please feel free to contact us again.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Info posted by APS Australia: http://forums.nasioc.com/forums/show...hreadid=456606


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Other random stuff:

From subaru.com STi specs: http://www.subaru.com/shop/specifica...I_SEDAN#engine

(it is silent on pistons)

Horizontally opposed 4-cylinder intercooled high-boost turbo with die-cast aluminum-alloy block and aluminum-alloy cylinder heads. Intercooler has driver-controlled manual water spray cooler with low-fluid indicator.

News on subaru.com, STi specs (which is alongside the core technology doc, but is silent on pistons): http://vocuspr.vocus.com/VocusPR30/D...7-ef33041a1de7

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Part numbers for USDM STi:
12006AC390 (std grade A)
12006AC400 (std grade B)
12006AC410 (OS 0.25mm)
12006AC420 (OS 0.5mm)

[Dave_RalliSpec]

Since RalliSpec was the original source of information that led to the mayhem in the original thread (which I just now read for the first time since I rarely have the opportunity to visti NASIOC), I felt compelled to say a few things.

First off, the information posted on the website regarding the Ver.8 piston (which was uploaded around a year or so) was based on the marketing brochure for the 2002 STi model in Japan. In this brochure they clearly described a change from the forged pistons used previously to a new type of cast piston which they claimed resulted in a 20% improvement in strength. This applied to all Japanese STi models. They did not specify what this new casting method was. In reviewing various technical resources on piston technology I took an educated guess and said that perhaps it was a hypereutectic design. But I have no information from Subaru one way or another on this.

Hypereutectic pistons are cast with increased silicon content. Basically so much silicon is added to the aluminum that not all of it will be absorbed by the aluminum and therefore silicon particles are suspended in the aluminum forming a matrix. This improves the temperature limit of the piston, helps control the expansion characteristics of the piston, and has other benefits. To be honest I do not know all of the details. There are some downsides to hypereutectic pistons as well.

Forged pistons also have their strengths and weaknesses. Forged pistons have a high temperature limit and improved toughness over cast pistons....and therefore will withstand detonation better. Forged pistons, however, expand at a higher rate then cast pistons. This requires increased piston to wall clearances in order that the pistons do not seize in the bore once up to operating temperature. Therefore forged pistons are noisier when not up to operating temperature and this can also lead to increased waar rates on road cars where the engine spends a good deal of time under idle and light cruise conditions. Some types of forged pistons are worse in this regard then others as it has a lot to do with the alloy used. I believe STi will have used a forged piston with a lower expansion rate.

I do not have any information on the EJ257 piston outside of what is publicly available. I can say with some certainty, however, that these pistons are not forged. It is easy to tell a cast piston from a forged one by looking at the backside of the piston. Forged pistons have a smooth appearance to the grain of the metal and usually you will see a wide parting line somewhere (usually one the pin bosses if it has not been ground away). Cast pistons have a rougher finish (where they have not been machined) and usually there will be evidence of casting flash.

As for cost....forged pistons are almost always more expensive simply because forging is a more expensive process. The forging dies are very expensive to manufacture and they require some serious equipment to provide the thousands of tons of pressure typically used. However, economy of scale also comes into play. Only STi models had forged pistons and previous to the release in Europe, North America, and elsewhere these models were produced in small quantities. Therefore the cost of buying replacement pistons from Subaru is quite high. The price of one single STi 2.0L piston is 23,000yen retail cost in Japan. With the release of the new cast 2.0L piston for the STi, this price still has not changed. Perhaps it will in the future as more and more STi models are produced. As for the EJ257 & EJ255 (which are identical in almost every way except turbo & intercooler), I do not know why SOA chose to price the pistons (or shortblocks for that matter) at such low cost. I can easily find a thousand part numbers that are rediculously expensive from SOA when compared to buying from Japan...and then I can find a thousand part numbers where the opposite is true.

As for STi 2.0L engines being hand built and 2.5L engines being mass produced. This is a fallacy....they are both essentially mass produced. I don't think you will find some old Japanese dude hunched over a cylinder head somewhere porting away to his heart's content. Some special models like the 22B, S201, S202 and perhaps even the Spec C's probably recieve a little bit of extra attention. But all in all, these motors are built with the same precision as the ones we are getting. This is production line manufacturing.

Well I hope I cleared up a little bit of confusion.

Dave
RalliSpec, Ltd.
www.rallispec.com

[Dave_RalliSpec]

Oops...sorry, the information came from the 2003 model STi brochure (Ver.8....aka revision "C").

--Dave
RalliSpec

[strangerq]

Thanks Dave.

[STiWanabee]

(edited by GW - info already contained in links on 1st post)

[cjm04WRX]

(deleted)

[Jon [in CT]]

Does SOA's 2004 New Model Update technical reference booklet shed any light on this issue? The description for that document says:

Literature # MSA5P0301B
Price: $7.64
Media Type: Paper

Also called Module 913, this Technical Reference Booklet contains an overview of such subjects as (but not limited to): Impreza WRX STi, ETC Components, the Variable Valve Timing System, WRX STi Emission Controls, Intercooler Water Spray, 6 Speed Manual Transmission, Reverse Lock Out, Front Differential, Rear Differential, Driver Control Central Difference (DCCD), Semi-Automatic Air Conditioning, High Intensity Discharge Headlights, Brakes, Impreza, Forester, Baja, and Direct Control 4-EAT.

Subaru of America, Inc. reserves the right at any time to make changes or modifications to systems, procedures, descriptions, and illustrations contained in this book without necessarily updating this document. Information contained herein is considered current as of April 2003.

[ANZAC_1915]

Originally posted by Jon [in CT]
Does SOA's 2004 New Model Update technical reference booklet shed any light on this issue? The description for that document says:

No, it has very few specifics on the engine beyond the ETC system, AVCS details. It has a quick blurb (p36) on the engine but only mentions new piston crown shape.

A good book to get, glad SOA is finally selling them direct. Pity the poor people who paid big bucks on ebay to get one...

Glenn

[8Complex]

From this post:

Originally posted by Axis Power Racing
It's a cast hypereutectic piston. I took it to CP and had them check it out and that's what they told me. They couldn't tell if it was hyper or just plain cast but it's def. not forged.

*edit out non-pertinent info*

Ron

Thats about as definitive as you get.

[Snax]

I'll just say that cast does not mean crap. My last car came with a 1.6L turbo motor using cast pistons that routinely handled 400 HP without problems. If we can expect anywhere near that kind of realiability, these motors should be able to put out over 600 HP with proper tuning.

[jaypine]

Hypereutectic pistons are cast with increased silicon content. Basically so much silicon is added to the aluminum that not all of it will be absorbed by the aluminum and therefore silicon particles are suspended in the aluminum forming a matrix. This improves the temperature limit of the piston, helps control the expansion characteristics of the piston, and has other benefits. To be honest I do not know all of the details. There are some downsides to hypereutectic pistons as well.

I've read that "Hypereutectic pistons will take a lot of pressure. You can stand a mountain on them and they won't break under the weight. However, the minute you ring them with the frequency that nitrous causes when it detonates, they shatter like glass. They don't melt; they break. Don't use them in a nitrous engine." Sports Compact Nitrous Injection by Joe Pettitt
I'm not sure how they work in a non-nitrous engine.

[specul8r]

Spoke with SOA this morning regarding the cast/forged debate. I asked the rep on the phone for clarification. After being on hold for about ten minutes while she checked, she responded that they were indeed forged aluminum. I told her that a well-known performance company disputes their claim. She stood by her statement, and said that she'd talked to a Subaru technician at her location, and had found it in an STi reference guide that she had access to. Either the pistons are forged, or every Subaru rep that I've been in contact with via phone and email has lied.

[8Complex]

Not lied, been misinformed. They just have access to the same marketing documents that we've all been fed since the beginning of time.

[Conduit]

Just to add more fire to this debate, SOA said they have been asked this question so many times that it has now gone back to SOJ for discussion. The conclusive response was that the STI pistons were INDEED forged. This is as of today.

We are going to have to do something about this. They are not selling what they are...selling. The sad thing is, they don't even know.

Is there actually any desire to do something about this? Or should we just be refered to the implied whineyness of the SOA issues forum.

[8Complex]

Do something about what? What do you care about the construction techniques, as long as it can put up to a stock 100k miles, it is perfectly fine for them to be selling.

And perhaps adding some actual information to your post up there to bump it out of the 'rumors' catagory would be a decent idea.

[MJU1983]

Yeah for sure, anyone can look at any new age STi piston and tell its cast. Version 8 JDM STi and USDM 2.5 STi both have similar casting marks. Version 7 were obviously forged.

Im sure they switched for a certain reason.

US STi piston:


Pictures:
1st pic is Ver 7 STi piston from APS website



2nd Picture is Ver 8 STi vs. ROSS forged piston.



3rd picture is of the underside of both... NOTE the similarities to the USDM STi piston with the JDM 2.0 ver 8 STi piston.



4th picture is of JDM ver 7 STi piston from APS.

[Conduit]

Originally posted by 8Complex
Do something about what? What do you care about the construction techniques, as long as it can put up to a stock 100k miles, it is perfectly fine for them to be selling.

And perhaps adding some actual information to your post up there to bump it out of the 'rumors' catagory would be a decent idea.

???

How about do something about the fact that they are not selling what they claim to be selling? If I purchase a vehicle that is advertised to have 18" wheels, and it comes with 17"s, I'm not just going to say "oh well, it's ok, they think it's just as good". If they are not forged, then they should not be advertising, telling people and "verifying" that they are forged when they obviously are not. How much clearer can this be? I am frankly surprised to see someone as experienced as yourself taking this position.

I apologize, I should have clarified that I called for myself to find out the answer. That is simply what I was told when they called me back with the answer. I called on 2/23/04 at about 2:00pm. I didn't record it, if that's what you are wanting. I can do so next time, if you like.

[8Complex]

I'm not arguing a point like 17 or 18" wheels, I am arguing a point like if the block was high pressure cast or just low pressure cast. It does not matter to any normal driver and it is not in their paperwork for the vehicle. Everything anyone has ever quoted was stated that it was "based on" a forged internal motor, not that it was one.

[Conduit]

You are correct, in fact. I just noticed that the original information regarding their being forged on the main website has changed.

I am still volunteering to have these analysed to put this to an end. Anyone care to donate a broken (or whole) ej257 piston? I have already contacted a consulting materials engineer in the area who is willing to provide an affadavit regarding their construction process for a relatively modest fee. www.materialsengineer.com

[specul8r]

I'm with you Conduit. When I purchased my car, the Subaru website stated that the pistons were forged. That said, I don't mind necessarily if they're not forged, as long as there is a good reason that they went to cast. All I'm really looking for is some factual information from Subaru about their decision to abandon their forged pistons. I'd like to see some specs, and technical data on the pistons that reside in my engine. Tell me they're not the same type of design that is in an engine that was designed with 165 horsepower in mind. Tell me that they're constructed in a way that leads me to believe that they'll live up to the reputation for quality, and durability that previous STis are known for. I'll tell you this: If I ever have any piston related problems, with my stock motor, I'll be visting my attorney--I don't care if the warranty is up, or not.

[GotHP?]

I think we're all in agreement now that these ARE in fact cast, but to put the last nail in this issue I asked an STi design engineer about this (automotive industry contact in Japan) and he also confirmed they are a new high strength cast alloy. To go much further, we are treading on Subaru's core technical secrets - do we really think they OWE it to us to offer all technical details which the Mistubishi engineers would just love to get their hands on?? Sure it'd be nice, but don't hold your breath for their forged vs. new cast dyno fatigue life data...Bottom line, as we upgrade just avoid detonation and we should be ok for fatigue life. Like specul8r said, if a stock setup blows a piston, there's a problem...

[8Complex]

Axis Power Racing has already taken one of the pistons to be analized at CP. See my post halfway up the page.

[MJU1983]

Originally posted by 8Complex
Axis Power Racing has already taken one of the pistons to be analized at CP. See my post halfway up the page.

w0rd. And I dont see how there is any beef with SOA. I do recall it saying the engine was based on a motor with forged internals... Similar to, I dunno a version 7 STi motor? Seeing as all STi's worldwide use cast pistons now, I dont even know why there is a debate/problem.

-Mike

[ANZAC_1915]

The pistons are cast - no need to do an analysis.

Any pointers to current advertising material that says they are forged? If not, back to our usual programming.