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Old 08-06-2018, 02:31 AM   #1
tehnation
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Default Piston Dome Volume



Trying to figure out which piston to get, I was intending on getting a -22cc dome volume piston but then I noticed the other varying dome volume piston and it got me curious! Anyone know the applications for the -17cc and -10cc in the 79mm STD STROKE (2.5L) section?
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Old 08-06-2018, 11:55 AM   #2
pcampbell
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Default Piston Dome Volume

Edit for correction:

57cc are probably for B series heads and 51 are probably for D series heads. Check your heads to see which are which.

Last edited by pcampbell; 08-06-2018 at 12:31 PM.
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Old 08-06-2018, 12:02 PM   #3
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Depending on what compression ratio you want, these pistons give you options. I've never ran the numbers myself to find out what dish volume I needed but it all depends on rod length, deck height, head volume, and piston dome/dish volume to get to your target compression ratio. There are a lot of CR calculators online that you can play with and see what kind of changes each piston would make. Are you building yourself?
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Old 08-06-2018, 12:11 PM   #4
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Machine shop is building, I'm waiting for them to clean and inspect the block first before getting a piston. I was going with stock specs. I have an ej255 from an 05 legacy gt, same thing as an 05 sti.
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Old 08-06-2018, 12:27 PM   #5
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This is actually not insignificant.
You should look at your head and confirm it's a B25 not a D25. It should be a B because it's an 2005 255. It is marked on the head in multiple spots.

please watch this video also.


Then you are probably going to want the -17's, not the -22's or else you will end up with very low compression. Unless you are running real big boost and want the very low compression (like 8:1 instead of 8.5:1 or something like that). Please confirm what I am saying with builder, not an expert!!! I just watch lots of youtube videos ha! But basically the wrong pistons will end up with wrong compression ratio.
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Old 08-06-2018, 01:45 PM   #6
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I have b25s.
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Old 08-06-2018, 07:22 PM   #7
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there you have it, you probably want the -17 pistons. for 8.5CR otherwise you will end up at like 8.1 8.2 or something
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Old 08-06-2018, 08:23 PM   #8
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Stock CR is 8.2:1 ain't it?
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Old 08-06-2018, 10:23 PM   #9
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https://forums.nasioc.com/forums/sho....php?t=1667168

Even more confused..... I was pretty sure the 05 lgt and 05 sti have the same longblock, or am i wrong?
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Old 08-06-2018, 10:54 PM   #10
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IIRC, you should have the EJ255 version 1, which, as you say, was the motor used in the 05-06 LGT, 04-05 Forester, and 04-06 STi. It uses the same shortblock and heads, but, if I'm not mistaken, the pistons may be different from the STi??

EJ255 v2 was used in the 06-08 WRX, 2007+ LGT, 2006+ FXT. Compression was higher for these cars vs STi, because STi, from factory, was running more boost. WRX is 8.4 or 8.5:1 (I forget which), while STi is 8.2:1. Since that link you posted was from 2008, it's possible those are EJ255v2 measurements and comparisons.
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Old 08-06-2018, 11:03 PM   #11
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I need the piston for the 05 sti! Anyone know which of these is for an 05 sti, or what the cylinder head or dome volume is? ?
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Old 08-06-2018, 11:38 PM   #12
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Are the EJ255v1 pistons the same in STi as well? I've always assumed STi's across the board has slightly different pistons is all.
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Old 08-07-2018, 05:30 AM   #13
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God damn Subaru....... Do they have this info in the service manuals? If the 05 sti and lgt have different pistons, I still want the sti ones! I'm running a billet 20g and bc stage 3 cams, so the lower compression piston would be better imo, right? I'm shooting for 400-500awhp.

But I just found this, which says they are the same pistons..... maybe... lol. So ej255 v1 is same longblock/pistons as 04-06 sti.

Quote from wikipedia as well as link to the reference
"EJ255
DOHC 16-valve turbo with sodium-filled valves originally designed for North American market, now sees usage in some European Imprezas and Legacies destined for Australia and South Africa. Power 210-265 hp

EJ255 Version 1: Used in the 2005 and 2006 Legacy, as well as the 2004 and 2005 Forester. This engine uses the same shortblock and heads like EJ257 in US 04-06 STI.[8]

EJ255 Version 2: Used in the 2006-2008 WRX, the 2007+ Legacy, and the 2006+ Forester. This is the newer AIS equipped EJ255, which uses a slightly different AB630 shortblock, as well as the AB820 heads. The only difference in the shortblock of this EJ255 and the 04-06 EJ257 is the pistons. They are of nearly the same construction, but have a slightly larger dish volume. Subaru lists the CR at 8.4:1 vs 8.2:1 for the 04-06 STI engine.[citation needed]

Usage in North America:

Impreza (WRX): 2006-2014
Forester XT: 2006-2013
Legacy GT: 2007-2012
Outback XT: 2007-2009
Baja XT: 2004 - 2006
SAAB 9-2X: 2006 only
Usage in the rest of the world:

Legacy/Outback: 2007-present
Impreza: 2005-present
Forester: 2005-2010"

https://legacygt.com/forums/showthre...nd-264989.html

Last edited by tehnation; 08-07-2018 at 05:38 AM.
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Old 08-07-2018, 05:41 AM   #14
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Don't I want 8.2 CR? I want my lgt motor to be an sti motor! And it looks like the 04-06 sti has 8.2cr.....so I'm back to the -22cc ??? From the looks of it , the CR was 8.2 for ej255 v1 if its the same as an 04-06 sti and the Ej255 v2 has 8.4CR
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Old 08-07-2018, 05:49 AM   #15
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Quote:
Originally Posted by tehnation View Post
God damn Subaru....... Do they have this info in the service manuals? If the 05 sti and lgt have different pistons, I still want the sti ones! I'm running a billet 20g and bc stage 3 cams, so the lower compression piston would be better imo, right? I'm shooting for 400-500awhp.
Big cams lower your dynamic compression ratio allowing you to run a little higher static CR. I don't see 8.4 being to bad on pump.
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Old 08-07-2018, 06:34 AM   #16
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Is 8.2 bad? Why does everyone seem to go towards the 8.4/8.5? The stock ej255 v1 had 8.2CR. Then again if it were good, they wouldn't have gone to 8.4/8.5 in the later ej257/55. There must be a reason they made a slight adjustment like that..... I'm about to just flip a coin =)
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Old 08-07-2018, 06:34 AM   #17
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Is 8.2 bad? Why does everyone seem to go towards the 8.4/8.5? The stock ej255 v1 had 8.2CR. Then again if it were good, they wouldn't have gone to 8.4/8.5 in the later ej257/55. There must be a reason they made a slight adjustment like that..... I'm about to just flip a coin =)
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Old 08-07-2018, 06:41 AM   #18
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Is 8.2/8.1 bad? 8.2 is stock CR.
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Old 08-07-2018, 06:53 AM   #19
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I don't even know if the -17cc pistons are available.... I don't see anyone selling them wtf.....Only place I see them is on ebay.... somethings strange...
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Old 08-07-2018, 08:04 AM   #20
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Default Piston Dome Volume

Did you check wiseco offerings as a comparison?

So wiseco pistons say they are 9:1 on a 51cc head and -19cc dish.

https://www.wiseco.com/Catalogs/Spor...aru-Toyota.pdf

You can check piston part number on parts.subaru.com

There is also a spreadsheet out there called the twe compression spreadsheet that you can plug in different values and itíll give you the CR.

Last edited by pcampbell; 08-07-2018 at 08:14 AM.
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Old 08-07-2018, 10:11 AM   #21
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This is f**king stupid.... manley needs better technical writers... it says 2004+ ej257 meanwhile there are like 2 or 3 different heads, which yield completely different numbers, so how the hell do they just generalize and say this one piston will give you this result when you can get several different answers based on different heads..... 2004+ is waaaaaay to vague and generalized.... I want to just look and say I have X, if I get Y then I get Z.... is that so hard?
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Old 08-07-2018, 10:29 AM   #22
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Old 08-07-2018, 10:32 AM   #23
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Is the Sti longblock the same specs from 2004+???
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Old 08-07-2018, 10:51 AM   #24
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Quote:
Originally Posted by tehnation View Post
Is 8.2 bad? Why does everyone seem to go towards the 8.4/8.5? The stock ej255 v1 had 8.2CR. Then again if it were good, they wouldn't have gone to 8.4/8.5 in the later ej257/55. There must be a reason they made a slight adjustment like that..... I'm about to just flip a coin =)
The latter 257 I think you're talking about had 8.2. That's what it says in my 2008 Impreza manual.

To be sure, I have a 255v2 and my owners manual says 8.4, so I'm not sure why I remember 8.5....sorry if it added to any confusing.

So many changes...so little info.

Here's some Factory Service Manual goodness if you want to browse through them.
https://sl-i.net/FORUM/showthread.ph...-Model-USDM-EU
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Old 08-07-2018, 11:09 AM   #25
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I don't even think it matters anymore, I don't think the -17cc pistons are available! I don't see anyone selling them, just some random guy on ebay and some website I never heard of. I emailed manley and am about to call to find out if they are even available, if not its back to the -22cc pistons! I just got the call from my machine shop and my block is good and I need a std set. I would prefer a 8.5CR, but 8.2CR works for me!
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