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Old 12-07-2003, 08:59 PM   #3
Dave_RalliSpec
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Join Date: Oct 2003
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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
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