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-   -   Pistons (http://legacygt.com/forums/showthread.php?t=201104)

islandborn 02-05-2013 08:19 AM

Pistons
 
Weak point in the engine... I know very little about. A quick search brings up a great selection from Rallysport! But.... I don't understand why the different sizes. If you ordered a set, which and why? Someone educate me! :spin: Here are the choices:

CP Piston Set 99.5mm Bore 8.2:1 CR Subaru Turbo 2.5L (inc. 2006-2013 WRX / 2004-2013 STI)

CP Piston Set 99.75mm Bore 8.2:1 CR Subaru Turbo 2.5L (inc. 2006-2013 WRX / 2004-2013 STI)

CP 99.5 Bore 9.0:1 Piston Set Subaru Turbo 2.5L (inc. 2006-2013 WRX / 2004-2013 STI)

Wiseco Piston Set 99.75mm Bore Subaru Turbo 2.5L (inc. 2006-2013 WRX / 2004-2013 STI)

Cosworth Forged Piston Set 99.5mm 9.2:1 CR Subaru Turbo 2.5L (inc. 2006-2013 WRX / 2004-2013 STI)

CP Piston Set 100mm Bore 9.0:1 CR Subaru Turbo 2.5L (inc. 2006-2013 WRX / 2004-2013 STI)

Wiseco Piston Set 99.5mm Bore Subaru Turbo 2.5L (inc. 2006-2013 WRX / 2004-2013 STI)

Wiseco Piston Set 100mm Bore Subaru Turbo 2.5L (inc. 2006-2013 WRX / 2004-2013 STI)

Cosworth Forged Piston Set 99.75mm 8.2:1 CR Subaru Turbo 2.5L (inc. 2006-2013 WRX / 2004-2013 STI)

CP Piston Set 100mm Bore 8.2:1 CR Subaru Turbo 2.5L (inc. 2006-2013 WRX / 2004-2013 STI)

CP Piston Set 100mm Bore 8.2:1 CR Subaru Turbo 2.5L (inc. 2006-2013 WRX / 2004-2013 STI)

Cosworth Forged Piston Set 99.5mm 8.2:1 CR Subaru Turbo 2.5L(inc. 2006-2013 WRX / 2004-2013 STI)

CP 99.75 Bore 9.0:1 Piston Set Subaru Turbo 2.5L (inc. 2006-2013 WRX / 2004-2013 STI)

Cosworth Forged Piston Set 100mm 8.2:1 CR Subaru Turbo 2.5L (inc. 2006-2013 WRX / 2004-2013 STI)

**Now to my credit, I do know that Cosworth is the top by most standards.

acumenhokie 02-05-2013 08:24 AM

Subscribed.

frank_ster 02-05-2013 08:37 AM

you can choose compression ratio : lower compression ratio if you plan to make more boost. higher compression ratio if you want better efficiency at cruise

and the sizes are for over bore . so if you were to just change out pistons you would choose the smallest one. the others are if you rebored the cylinders if there are scratches .

GTTuner 02-05-2013 08:49 AM

Don't forget the type of alloy used to manufacture.

2618- Low silicon content, high expansion rate requires a large piston to wall clearance. Generally a nitrous or drag piston since the alloy looses strength (becomes brittle) after repeated heating/cooling cycles. These are usually noisy due to large piston to wall clearance.

4032- Higher silicon content, low expansion which allows you to run "stock like" piston to wall clearances. Normally much quieter that 2618. This is considered a daily driver forged piston. Not as strong as the 2618 but still much stronger than cast. They will survive repeated heat cycles better.

FYI I have a set of 4032's. No noise, no oil consumption.

Rutchard 02-05-2013 09:35 AM

There's a good piston FAQ over at NASIOC http://forums.nasioc.com/forums/showthread.php?t=907570 You'll probably get a bit more info over there or IWSTI in the built motor sections than here in the 5th gen section.

The only built engines here are in our 5th gen hopes & dreams right now, lol

fahr_side 02-05-2013 09:58 AM

Quote:

Originally Posted by islandborn (Post 4278880)
Weak point in the engine... I know very little about.

In the previous iterations of EJ255/257 this is definitely the case... I've seen small hills made from broken OE pistons.
Fifth-gen? I haven't seen a single broken piston in this series, nor any other major failure for that matter. So far the worst failure I've seen is a faulty OCV.
If you've found complaints that the '10~ pistons are weak I'd like the link please.

Austin2334 02-05-2013 10:17 AM

^ I think it has sparked from Surly's GT. It isn't torn down yet but from the sound of things, pistons may not look new when they are pulled. Just assumptions.

islandborn 02-05-2013 12:12 PM

So a question might be - are our pistons shared with the WRX or the STi? Or are they unique to our engine? I know, might be wrong, that the STi pistons are made differently?? As a few of us are pushing our engines into the 300+ hp and having them as DD with bursts of "kiss my a$$" moments, having built internals just becomes a CMA issue.

fahr_side 02-05-2013 03:12 PM

Quote:

Originally Posted by islandborn (Post 4279351)
So a question might be - are our pistons shared with the WRX or the STi? Or are they unique to our engine? I know, might be wrong, that the STi pistons are made differently?? As a few of us are pushing our engines into the 300+ hp and having them as DD with bursts of "kiss my a$$" moments, having built internals just becomes a CMA issue.

No, the '10~ pistons are different. As I understand it the new heads have a chamber volume similar to previous models but with dual AVCS added. They're not the same heads as the GRB EJ257 ('08~ STi).

Therefore, the dish volume has to be smaller to come up with the higher compression ratio. I don't know if anything is different.

We make jokes all the time about the stock pistons being made from only the finest chocolate. Truth is they're not bad, just very intolerant of knock. I know loads of people running past 300whp on stock internals for a long time. I'm at 2 years already myself. A mate did 2 years at 360whp before a HG went. It takes careful tuning and logging and being able to tell your MAF and MAP sensors apart. ;)

islandborn 02-05-2013 04:08 PM

^thanks!!!!

SurlyOldManMN 02-05-2013 05:03 PM

Relevant to my interests, this thread is.

frank_ster 02-05-2013 08:43 PM

fyi i use these pistons
http://www.summitracing.com/parts/wi...iew/make/buick

fahr_side 02-05-2013 11:56 PM

I know there people going to sleeves and 102.xx bore in order to use pistons for the SBC. A little beyond what the OP envisions methinks. ;)

minuccims 02-06-2013 11:20 AM

2 Attachment(s)
Quote:

Originally Posted by fahr_side (Post 4279739)
No, the '10~ pistons are different. As I understand it the new heads have a chamber volume similar to previous models but with dual AVCS added. They're not the same heads as the GRB EJ257 ('08~ STi).

Therefore, the dish volume has to be smaller to come up with the higher compression ratio. I don't know if anything is different.

Didn't have an opportunity to post this sooner.

Yes, the pistons are different for the 2010-12 LGT. The crown shape of the piston head has been modified.

For the heads, slight changes. The combustion chamber has been modified. Cam timing changes for low range TQ; valve springs are different as well.

(Interesting that for the 2.5i non-turbo, the heads were changed to improve cooling and the pistons changed (shape and 26 gram weight reduction)

islandborn 02-06-2013 11:58 AM

Ok... sooooooo..... "IF" one was to replace pistons (when I realized that doing so would run into the $2k+, I decided to put it off for a loooooong time) would the aftermarket crown have to be like the stock crown? Or does it really matter?


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