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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?

minuccims 02-06-2013 12:08 PM

When looking into this a few weeks back, forged pistons change the compression, but just slightly. If remembering correctly, if there is no damage to the engine the pistons and rings can be swapped out from underneath. IE, not an engine tear down. Labor is "not that bad".

GTTuner 02-06-2013 12:16 PM

Quote:

Originally Posted by minuccims (Post 4281284)
When looking into this a few weeks back, forged pistons change the compression, but just slightly. If remembering correctly, if there is no damage to the engine the pistons and rings can be swapped out from underneath. IE, not an engine tear down. Labor is "not that bad".

I'd like to see that done! :lol:

You would still need to pull the tranny to get the wrist pin c clips off of cyl's 3 and 4

minuccims 02-06-2013 12:20 PM

Haahaa! yes, very poor choice of "underneath". Thanks for clarifying.

fahr_side 02-09-2013 03:04 AM

Damn Surly, you didn't have to break a piston to prove us wrong eh!

Really sorry you were the first. Very unsure as to what options are open to you by way of replacements. I don't know of anyone making '10~ specific pistons.

Rutchard 02-09-2013 05:58 AM

Quote:

Originally Posted by fahr_side (Post 4286083)
Damn Surly, you didn't have to break a piston to prove us wrong eh!

Really sorry you were the first. Very unsure as to what options are open to you by way of replacements. I don't know of anyone making '10~ specific pistons.

I don't know if Surly's engine was a piston yet or not. They haven't torn it down yet, right? It's all spread out over a few threads so maybe I missed something along the way...

SurlyOldManMN 02-09-2013 08:30 AM

Quote:

Originally Posted by Rutchard (Post 4286124)
I don't know if Surly's engine was a piston yet or not. They haven't torn it down yet, right? It's all spread out over a few threads so maybe I missed something along the way...

Correct, we don't know yet. I will be keeping documentation here:

http://legacygt.com/forums/showthrea...ad-201225.html

We won't be looking at lgt-specific pistons since a bore is highly recommended due to probable damage. JE 99.75, 8.5:1 are a likely candidate. According to NF Performance those should give me 9.2-9.3 final compression:

http://www.jepistons.com/Products/296348.aspx

I plan on chatting with them Sunday during our Drive n' Dine to make triple sure that's what they want ordered. I'm tempted to wait until it's pulled apart so we can check for any design and/or damage surprises first.

fahr_side 02-09-2013 06:06 PM

Agree. I'd want to cc the heads and check deck height first. I haven't seen this data published anywhere yet.

SurlyOldManMN 02-09-2013 06:23 PM

Indeed, nothing out there that I've been able to find. Hopefully nobody else gets the "opportunity" to find out before I get a chance to document measurements.

SurlyOldManMN 02-09-2013 06:34 PM

May as well throw it out there standard 99.5 x 79.0 bore and stroke that you would expect with an EJ25. Probably goes without saying.

SurlyOldManMN 02-14-2013 01:48 AM

Quote:

Originally Posted by frank_ster (Post 4278901)
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 .

Just for a quick sanity check... higher compression comes with a reliability hit, correct?

fahr_side 02-14-2013 06:58 AM

Quote:

Originally Posted by SurlyOldManMN (Post 4293745)
Just for a quick sanity check... higher compression comes with a reliability hit, correct?

Given the same combustion chamber design / efficiency, you will run into knock faster as compression ratio increases. In theory that limits the useful boost you can run.
The fact that this new engine runs a CR a full point higher than the previous generation engine, yet can tolerate similar boost and timing, makes me wonder about a few things.

Maybe the cam timing (more overlap) in the midrange reduces dynamic compression a lot.

Maybe the ports are smaller than before, compromising VE and both increasing boost and allowing more timing to be run.

Maybe the combustion chamber design is much better than before.

Maybe there's a combination of the above factors.

Lots of maybe.

frank_ster 02-14-2013 07:40 AM

Quote:

Originally Posted by SurlyOldManMN (Post 4293745)
Just for a quick sanity check... higher compression comes with a reliability hit, correct?

well if you put it that way lower compression ratio would probably be tougher...
less chance of knock.


but BUT . i have issues with being to low of compression, where the compression ratio is so low the that knock doesn't happen when over advanced in timing. instead of what happens is the pistons and rings get to hot and you can have the top ring shatter due to the gap closing. and top of piston rubbing on cylinder walls! so the method of advancing timing until knock doesn't apply !

frank_ster 02-14-2013 07:46 AM

Quote:

Originally Posted by fahr_side (Post 4293853)
Given the same combustion chamber design / efficiency, you will run into knock faster as compression ratio increases. In theory that limits the useful boost you can run.
The fact that this new engine runs a CR a full point higher than the previous generation engine, yet can tolerate similar boost and timing, makes me wonder about a few things.

Maybe the cam timing (more overlap) in the midrange reduces dynamic compression a lot.

Maybe the ports are smaller than before, compromising VE and both increasing boost and allowing more timing to be run.

Maybe the combustion chamber design is much better than before.

Maybe there's a combination of the above factors.

Lots of maybe.

cam overlap can reduce dynamic compression,

yes all of those factors work together. yo should see the hayabusa engines turbo charged. they are 12-13 to 1 compression ratio running 25 lbs boost on pump gas!

SurlyOldManMN 02-14-2013 08:41 AM

Quote:

Originally Posted by frank_ster (Post 4293886)
well if you put it that way lower compression ratio would probably be tougher...
less chance of knock.


but BUT . i have issues with being to low of compression, where the compression ratio is so low the that knock doesn't happen when over advanced in timing. instead of what happens is the pistons and rings get to hot and you can have the top ring shatter due to the gap closing. and top of piston rubbing on cylinder walls! so the method of advancing timing until knock doesn't apply !

Interesting stuff... The preschool version I have so far is higher compression gives you generally better all around performance, but if/when something goes a little wrong, the ability to gracefully handle the event and keep it from going a lot wrong is severely hampered. Otherwise it would be a no-brainer to go HC all the time.

There's also a lot of FUD out there on what constitutes "high boost" for this application. The vast majority of opinions and articles I can get my hand on are wishy washy on this point. Mostly the conversation around FI and HC pistons centers lol-charging NA motors and we're talking single digit boost targets.

Assuming a conservative power goal that isn't going to require squeezing every last drop either way, does it make more sense to go HC and pull back the turbo or LC and let the FI get you the rest of the way? Which goes boom first?

frank_ster 02-14-2013 10:23 AM

go stock settings( compression ratio) put forged .. keep same ignition table have the car tuned on a dyno .. go conservative on ignition timing make up the power with a lil more boost.

fahr_side 02-16-2013 03:33 AM

Agree. If you lower compression you may gain power potential at the top end. Retaining that beefy midrange however will likely require careful tweaks to cam phasing which will take quite some time on the dyno.
I'd have JE make you some custom pistons.

SurlyOldManMN 02-16-2013 08:38 AM

Quote:

Originally Posted by fahr_side (Post 4296657)
Agree. If you lower compression you may gain power potential at the top end. Retaining that beefy midrange however will likely require careful tweaks to cam phasing which will take quite some time on the dyno.
I'd have JE make you some custom pistons.

Oh? You're thinking less than 8.5:1?

frank_ster 02-16-2013 08:46 AM

i'm at like 7.2-1 :)

fahr_side 02-16-2013 08:53 AM

Quote:

Originally Posted by SurlyOldManMN (Post 4296779)
Oh? You're thinking less than 8.5:1?

No, in your position I'd probably order some forgies with the stock 9.5:1 and as frank_ster suggests, go a little gentler on timing.

SurlyOldManMN 02-16-2013 10:50 AM

Stock is 8.4:1. The common offerings are 9.5 vs 8.5.

RabidWombat 02-16-2013 11:29 AM

Its possible Subaru is messing with the phasing as an effort to push gas mileage up.

The Toyota hybrid engines run a 13:1 CR on 87 gas. The trick is that its not a real 13:1, since the compression stroke is actually shorter than the exhaust stroke due to the way the valves open.

I'm guessing Subaru is trying to eek a little more mileage out by trying the same tricks.

fahr_side 02-16-2013 09:26 PM

Quote:

Originally Posted by SurlyOldManMN (Post 4296916)
Stock is 8.4:1. The common offerings are 9.5 vs 8.5.

Oh boy, that's quite a shock to my long-held beliefs. :eek::redface: I'm sure I read somewhere when the '10 was launched that it had higher compression than before. Not sure now where I found that info, and I'll be looking for that source.
I'm working on EDM models, and it could be the early ones did indeed have higher compression, or the initial press releases or brochures were wrong. It certainly explains why I can run so much boost and quite a lot of timing on these engines without problems!

I'd still be prepared to shop custom pistons even if sticking to 8.4:1 CR, since Subaru were very likely to have changed the combustion chamber shape with the new heads. You want to be sure you get the squish area right to avoid problems.

Subaru has long used what we call a 'tree-hugger spike' in AVCS timing to dis-improve VE and allow more overlap in cruise areas, improving fuel consumption and emissions by retaining more of the spent exhaust gasses in the cylinders, as EGR did in older cars. Flattening that spike out improves driveability dramatically, but costs a little in fuel economy. I'm sure that be able to change the phasing on both cams allows Subaru to refine this strategy a great deal.

SurlyOldManMN 02-16-2013 09:32 PM

Awesome, thank you!

8.4:1 comes from the USDM owner's manual.

Are you thinking it's worth pulling the pistons and sending them in to JE?

I'll call them on Monday to see what they think. The more insight I can have going in to that conversation the better. Very interested in getting it right the first time. I seriously do not want to have to go through this process twice...

fahr_side 02-16-2013 10:08 PM

I'd say you should measure the deck height carefully, and.cc both the piston dish and combustion chambers. Send that info to JE and see what they think.

I'd very much appreciate it if you could also measure the base circle and lobe height on the cams, so we can get an idea how they differ from the GRB items.

tytek 02-16-2013 10:15 PM

How many miles do you drive a year Surly?

Forged pistons will require a little bit of care o make sure they run for a long time. Low expansion allows for tighter tolerance. Thus leads to less slap and slower ring and piston wear. Warming up the motor to operating temps is critical. So is running quality oil at lower OCI. It is normal to expect a much shorter life of the components, before you start eating oil, and will require new rings and possibly a hone.

I do 30k miles per year and would not want forged internals. I would have probably gone for a new stock block. But your goals may be different. Just my $0.02.


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