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Old 12-29-2008, 02:30 PM   #1
Maxwell Power
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Default EJ257 vs EJ255 facts with pictures

There is a lot of debate about the similarities of the EJ255 and the EJ257.

Until Jeff Sponaugle pointed out that the pistons had different part numbers, I too thought they were the same.

I have a set of used pistons out of an EJ257 and a set out of an EJ255.

Today I will compare them for you.

Results:
EJ257: -22cc dish (which is interesting because I got 19cc a few months ago. However, I did this one three times to be sure)
EJ255: -24cc dish.

The shape/casting of the pistons are identical with a few exceptions.
Dish profile is different to accommodate the the different volumes. The STi has a dome in the center of the piston that is 1.7mm from the top of the crown. The EJ255 has a dome that is 3.3mm from the top of the crown. Other than dome height, the dish profile is identical. Floor depth is the same at 4.75mm and the valve reliefs are identical.

The markings are obviously different with the EJ257 having one dimple to indicate FWD while the EJ255 has two dimples to indicate FWD. The EJ257 has the standard A or B designation for size while the EJ255 has AD or BD. Also, the casting says "00 3R" on the EJ257 and "00 4L" on the EJ255. If you look closely at the pic of the EJ255 you'd see it has a broken ring land.

Compression height is identical.

Ring land thickness, spacing and ring sizes are identical.

Now pictures:
the orange goop is the sealant I used to seal the pistons to the plate. It wasn't in the dish during the measurement but as I attempted to wipe it off it migrated. Too damn sticky to get off without solvent.

side by side with a bit of my burrett in the background:


top side by side..


bottom side by side..
you can see where we balanced the piston on the right. That isn't a factory end mill.


EJ257 side


EJ255 side.. see the busted ring land?



Hope that helps clear up some questions.

Now lets do the compression ratio calculations:

Combustion chambers:
06wrx heads (D25 casting): 50cc
04-07 STi (B25/V25 casting): 55.5cc (used for calculations, but they're actually 56)
02-05wrx (S20 casting): 49 cc

Factory compressed HG thickness .0236"

EJ257 pistons on B/V25 STi head: 8.28:1
EJ257 pistons on D25 WRX head: 8.78:1
EJ257 pistons on S20 WRX head: 8.88:1

EJ255 pistons on B/V25 STi head: 8.11:1
EJ255 pistons on D25 WRX head: 8.55:1
EJ255 pistons on S20 WRX head: 8.68:1
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Last edited by Maxwell Power; 02-15-2013 at 02:10 PM.
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Old 12-29-2008, 03:00 PM   #2
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Cool, thanks for the info

I don't know that much about engine design, but do the differences mean anything in terms of power and reliability?
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Old 12-29-2008, 03:05 PM   #3
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Quote:
Originally Posted by Bad Noodle View Post
Cool, thanks for the info

I don't know that much about engine design, but do the differences mean anything in terms of power and reliability?

The important part is at the bottom where I calculated compression ratios.

Basically it's just good info for guys who are building hybrids. I would recommend the EJ255 block before the EJ257 block for guys who are going to put the 2.0L WRX heads on it.

It also helps people figure out what parts they can swap over, as well as making sure they are using the right pistons if they are just replacing a blown cyl 4.

Not to mention it answers the millions of questions about compression ratio with different combinations.
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Old 12-29-2008, 11:32 PM   #4
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Indeed this exact topic, and said measurements came up in this thread:
http://forums.nasioc.com/forums/show...t=EJ255&page=2

a few months ago.. In that thread I posted the following:

Quote:
Originally Posted by sponaugle View Post
As I suggested in my original post, it is not typical for Subaru to have different part numbers for the same part, especially when it forces a cascade of part changes.

I picked up a brand new 2006 WRX pistons, and well as 2006 STI piston. (Of course I am $70 lighter just for the sake of clearing up this discussion. I don't like having questions about the accuracy of data hanging around. )

See the picture below.


WRX on the left, STI on the right

If you look carefully you can see that the dish on the WRX piston is deeper then the STI. In person it is very obvious, but I took that picture at that angle to make it eaiser to see.

The pistons are indeed different.

The STI pistons has a dish volume of 22ccs, while the WRX one is 25.5ccs, and difference of 15.9%. Given that the WRX EJ255 has a higher overall compression ratio, this would suggest that the EJ255 heads (in the WRX) must have a smaller combusion chamber by quite a bit.

This discovery leads to a few other interesting conclusions as well. More to come on that.

Cheers,

Jeff Sponaugle


Quote:
Originally Posted by sponaugle View Post
If you are doing a hybrid EJ257 SB + EJ205 WRX Heads you should consider instead using the 06-08 WRX EJ255 SB, which will net you lower (closer to normal) compression.

Consider the following:



I highlighted the 50cc number for the 06+ WRX heads as that is a guess. I have not actually measured them myself. If anyone else has made an accurate measurement, let me know. You can clearly see that if you are doing an EJ205 WRX head + EJ25X SB Hybrid, you are better off starting with the 06+WRX EJ255 block instead of the 04-07 STI EJ257 SB.

Cheers,

Jeff Sponaugle
Surgeline Tuning

My numbers are slightly different then the ones you got. For my 06 WRX EJ255 piston, the dish measured slightly larger. Both pistons were new, and I used a similiar technique. I still have the pistons so I could measure them again.

As for the head volumn, I have measured several new and used EJ257 heads and they always meaure out at 56-57ccs. I believe a few other builders have measured the same range, but it is difficult to be precise given the lack of measureing standards and conditions.

My resulting compression ratio values are similar, but not the same.

The head gasket thickness I used is .60mm, which is .0236". This is based on measuring several different used gaskets, and using the caliper to compress the gasket in a way similiar to the conditions it is clamped in.

If I were to adjust the piston dish to be the same, the numbers would be almost identical.

It is important to correct one thing:

It is not proper to catagorize these pistons as EJ255 pistons. There are two variants of the EJ255 shortblock. The first EJ255 shortblock used in the 05/06 Legacy GT, 04/05 *CORRECTED* Forester, and the Turbo Baja is identical to the EJ257 shortblock.

Thus you want to avoid grouping all EJ255 motors together, as the statements saying such would be incorrect. The correct way would be to either indicate the SB number 10103xxyyy.

Jeff

Last edited by sponaugle; 12-30-2008 at 11:03 AM.
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Old 12-30-2008, 01:19 AM   #5
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Quote:
Originally Posted by sponaugle View Post
...It is not proper to catagorize these pistons as EJ255 pistons. There are two variants of the EJ255 shortblock. The first EJ255 shortblock used in the 05/06 Legacy GT, 05/06 Forester, and the Turbo Baja is identical to the EJ257 shortblock
th 04-05 forester XT had the lower compression 257 style motor, 06- has the higher compression motor like the 06- wrx..I'm guessing a typo here
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Old 12-30-2008, 08:36 AM   #6
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Wouldn't the 8.87:1 compression ratio (given an 03 wrx with STI block) be better for low-end torque compared to the stock 8.42:1? I'm looking to get as much power down low as possible out of my build, which is why i was considering the STI over the 2.5 WRX block.

0.45 difference in CR doesn't seem like a lot, but i could see how it might cause a little bit of a funky powerband on a car without variable timing...
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Old 12-30-2008, 11:02 AM   #7
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You don't want too high of a CR in a forced induction motor. Look at how low the STI's CR is. Part of that is because of the turbo they matched it up with. I will be doing a hybrid motor but am just in the researching phase right now. I personally am looking to keep the CR around 8.4-8.6 just for engine life. Of course you can run a higher CR but then you should run lower boost, unless you just sleeve the block and build the piss out of it and have more money to play with when it comes time to rebuild it.
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Old 12-30-2008, 11:04 AM   #8
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Quote:
Originally Posted by a6n6d6y View Post
th 04-05 forester XT had the lower compression 257 style motor, 06- has the higher compression motor like the 06- wrx..I'm guessing a typo here

Righto.. misstype on my part. Actually I'll post up the engine code chart with the Forester on it as well, as it makes things much eaiser to read. Good catch! Thanks.

Jeff
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Old 12-30-2008, 11:21 AM   #9
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Quote:
Originally Posted by 305mouse View Post
You don't want too high of a CR in a forced induction motor. Look at how low the STI's CR is. Part of that is because of the turbo they matched it up with. I will be doing a hybrid motor but am just in the researching phase right now. I personally am looking to keep the CR around 8.4-8.6 just for engine life. Of course you can run a higher CR but then you should run lower boost, unless you just sleeve the block and build the piss out of it and have more money to play with when it comes time to rebuild it.
Fuel quality plays a big part in this decision. I run E85, so I could run a really high static CR if I wanted. If I was back in Cali where I bought the car, I'd go with a much lower number.
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Old 12-30-2008, 12:52 PM   #10
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Quote:
Originally Posted by sponaugle View Post
Indeed this exact topic, and said measurements came up in this thread:
http://forums.nasioc.com/forums/show...t=EJ255&page=2

a few months ago.. In that thread I posted the following:








My numbers are slightly different then the ones you got. For my 06 WRX EJ255 piston, the dish measured slightly larger. Both pistons were new, and I used a similiar technique. I still have the pistons so I could measure them again.

As for the head volumn, I have measured several new and used EJ257 heads and they always meaure out at 56-57ccs. I believe a few other builders have measured the same range, but it is difficult to be precise given the lack of measureing standards and conditions.

My resulting compression ratio values are similar, but not the same.

The head gasket thickness I used is .60mm, which is .0236". This is based on measuring several different used gaskets, and using the caliper to compress the gasket in a way similiar to the conditions it is clamped in.

If I were to adjust the piston dish to be the same, the numbers would be almost identical.

It is important to correct one thing:

It is not proper to catagorize these pistons as EJ255 pistons. There are two variants of the EJ255 shortblock. The first EJ255 shortblock used in the 05/06 Legacy GT, 04/05 *CORRECTED* Forester, and the Turbo Baja is identical to the EJ257 shortblock.

Thus you want to avoid grouping all EJ255 motors together, as the statements saying such would be incorrect. The correct way would be to either indicate the SB number 10103xxyyy.

Jeff

Thanks Jeff.
Don't know how I missed your thread. Figures I'm a few steps behind you...

I got 56cc when I measured the STi combustion chambers as well. However, 3 people chimed in, including TWE, saying they got 54-55cc. It is easy at times to be off by 1cc. I split the difference in case I was wrong. However, if you say you got 56cc as well, I'm with you. I didn't think I goofed it up, but it isn't beyond the realm.

Thanks for clarifying the EJ255. I should add then that the piston I measured was out of an 07WRX. The engine code of the sticker is EJ255BE9KB-0DB. Does the "BE" tell us more?

-Dominic
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Old 09-27-2009, 02:26 PM   #11
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Default EJ257 Swap

Just to have an idea, what are my biggest problems if I want to swap an EJ205 with a 257 block? I have never swapped an engine.
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Old 09-27-2009, 05:55 PM   #12
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Quote:
Originally Posted by WRCSubey6 View Post
Just to have an idea, what are my biggest problems if I want to swap an EJ205 with a 257 block? I have never swapped an engine.
they pretty much spelled it out in the first post. If you use the wrx heads, with a 257 block, you will have a high compression situation which is more knock prone.
ways around this are, thicker head gaskets, lower compression pistons, dishing out the wrx heads to accomodate a 257 block, or using a 255 block.
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Old 09-28-2009, 02:19 PM   #13
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Okay so US STi Heads from an 04-07 STi I could use straight up in the wrx without any compression issues on a 257 block. I'm also going to be installing a TBE later this year, and that should free up any further exhaust issues I may have? Anything else that I should know that would be of major concern? Thanks.
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Old 09-28-2009, 06:36 PM   #14
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Default 257 Shortblock

Intake and manifold issues? Need to jump to a 2.5L oil pan. Pumps? How would two separate compression maps work with EcuTek/Opensource?
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Old 10-03-2009, 05:34 PM   #15
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so bajas have the same shortblock as an sti?
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Old 10-14-2009, 12:55 AM   #16
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so Maxwell or whoever wants to chime in, what do you think about the ej255 with the bv25 heads? is that too low of a CR?
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Old 10-14-2009, 09:45 PM   #17
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it's all about how much boost you want to run and your total engine setup.
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Old 10-14-2009, 11:04 PM   #18
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Great info here, bump.
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Old 10-15-2009, 12:01 AM   #19
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subscribed for teh info
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Old 11-04-2009, 04:30 AM   #20
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subscribed as well
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Old 11-11-2009, 01:47 AM   #21
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So just to clear things up, The EJ255 and EJ257 Cases are the same. The differances are just in the piston domes, which in turn contributes to the C/R.
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Old 11-11-2009, 01:16 PM   #22
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yes, that is correct
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Old 11-12-2009, 09:14 PM   #23
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What I am trying to figure out is with a
turbo 2.5 block (resurfaced .001-.002")
79mm stroke
D25 heads (50cc)(head will be resurfaced .001-.002'')
99.75 bore
22cc piston dish

What size head gasket to go with to yield a 8:7 CR? Stock? What is the uncompressed size of the HG?

I would like to keep the C/R close to this as I am planing on running E85.

From what I have researched I will be close to 8:77:1 with out the resurfacing of the heads and block on a stock head gasket.

Last edited by ozzy370; 11-12-2009 at 09:32 PM.
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Old 11-13-2009, 12:04 AM   #24
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Volume lost from resurfacing will be (pi*r^2) * height.

Subtract those volume changes, add in the gasket volume, calculate BDC and TDC volumes, calculate CR.

Do the calculations in excel and you can play around with the HG thickness to get the desired CR.

Don't forget to check the squish clearance as well.
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Old 11-13-2009, 11:37 AM   #25
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from my calculations,

8.8:1 with the factory headgasket.
8.5:1 with 0.040" headgasket
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