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Old 07-22-2003, 06:43 PM   #1
eclip5e
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Default GC8 2.5L vs 2.5L in USDM 04 STi

I have a GC8 2.5L (EJ25 Phase II SOHC)
and would like to see how feasable it would be to mod it into the same engine as the 04 STi (EJ25T-STi DOHC).

My current thinking (simple mindedly) is that i could throw a bunch of STi parts into it, and poof... $10,000 later an 04 STi comes out of the 2001 EJ25.

I've heard that the EJ25 pIISOHC is closed-deck where the EJ25TstiDOHC is open-decked. I don't know what that means.

Whats that mean?
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Old 07-22-2003, 07:14 PM   #2
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well lets start with
your motor ej251 open deck, cast pistons, cast rods ,10 to 1 compression
sti motor ej257 semi closed deck, hyper cast pistons (jury is still out on this), forged rods and 8 to 1 comrpession

this is not a cheep
my guess is that to go to change the pistons and rods/crank your looking at the same price as a short block from subaru

i believe cobb charges ~$6000 for the complete transformation
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Old 07-22-2003, 07:24 PM   #3
eclip5e
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Quote:
Originally posted by Jaxx
well lets start with
your motor ej251 open deck, cast pistons, cast rods ,10 to 1 compression
sti motor ej257 semi closed deck, hyper cast pistons (jury is still out on this), forged rods and 8 to 1 comrpession

this is not a cheep
my guess is that to go to change the pistons and rods/crank your looking at the same price as a short block from subaru

i believe cobb charges ~$6000 for the complete transformation
Short block like the EJ22t?

Rumor has it that mine will do 19psi stock. What if i run 40psi (for example) what would happen exactly (besides the apocalypse?)
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Old 07-22-2003, 07:33 PM   #4
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Default Re: GC8 2.5L vs 2.5L in USDM 04 STi

Quote:
Originally posted by eclip5e
........would like to see how feasable........
How about......not feasable at all! How does that rub you?

I'm pretty sure that 99% of the parts in that engine are different than a SOHC EJ25 (are there even 2.5 sohc's?)....though, if you have a 2001 RS, isn't that a DOHC?

Anyway, do a swap if you have the money. Anything you want, either a wrx motor, sti motor, jdm wrx/sti, complete drivetrain, etc. Much more bang for the buck.

Kevin
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Old 07-22-2003, 09:56 PM   #5
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For $10k, you could get an entire V3 STi drivetrain installed.

Yes, there were SOHC EJ25s. They have been around since the 1998 Foresters, the 1999 Legacies, and the 1999 Imprezas.

I have sincere doubts the internals would just "swap" over between blocks. However, we never know until somebody proves otherwise.

-Jon
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Old 07-23-2003, 05:38 AM   #6
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All the parts are different. The blocks might use the same piston diameters and same bolt holes and such, but one is open the other is semi-closed like has been mentioned. Everything else is totally different.
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Old 07-23-2003, 10:08 AM   #7
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Default Oops

Not be be a Jerk but the SOHC 2.5 wasn't put in the Forester untill MY99, Same year as the RS went to SOHC. Imprezas and Foresters come from the same factory. The 2.5 Legacys, GT, Outback didn't become SOHC untill 2000 when they introduced the new body style. Even the MY99's are DOHC. All Leagacy platform cars come fromteh same factory.
Joshua
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Old 07-23-2003, 12:00 PM   #8
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Sorry, I got my years one year too early on the Legacies and Foresters.

-Jon
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Old 07-23-2003, 12:29 PM   #9
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Quote:
Originally posted by eclip5e


Short block like the EJ22t?

Rumor has it that mine will do 19psi stock. What if i run 40psi (for example) what would happen exactly (besides the apocalypse?)
Hmmm, for some reason I think your EJ25 would not be happy running 19psi for very long. Oh, and no, your EJ25 is NOT closed deck. The new STi EJ257(?) is semi-closed deck. Closed deck really just means less efficient cooling until you get to running really high boost pressures (on a turbo designed to run in that boost range).Once again, boost pressure means VERY little until you decide what turbo you are using.

- Trent
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Old 07-23-2003, 12:47 PM   #10
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whoa who told you a stock 2.5na block would handle 19 psi .. no no no no .. well it might do it once, but definatly not twice

i say start with a ej255 (forester xt) or ej257 (sti) short block
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Old 07-23-2003, 05:33 PM   #11
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Quote:
Originally posted by Jaxx
whoa who told you a stock 2.5na block would handle 19 psi .. no no no no .. well it might do it once, but definatly not twice

i say start with a ej255 (forester xt) or ej257 (sti) short block
what does it handle stock?

and what would happen if i put 40psi through it... like, what would break exactly?
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Old 07-23-2003, 05:50 PM   #12
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You will either crush a rod or crack a piston around the ring land/valve relief. Not to mention the static compression is too high to even attempt that kind of boost without detonating.

-Jon
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Old 07-23-2003, 05:55 PM   #13
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Quote:
Originally posted by stimpy
You will either crush a rod or crack a piston around the ring land/valve relief. Not to mention the static compression is too high to even attempt that kind of boost without detonating.

-Jon
"detonating"?
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Old 07-24-2003, 05:49 AM   #14
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Quote:
Originally posted by eclip5e


what does it handle stock?

and what would happen if i put 40psi through it... like, what would break exactly?
About 8psi at the most without internal modifications, and WITH extreme engine management to keep it from exploding and running perfect.
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Old 07-24-2003, 09:12 AM   #15
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Quote:
Originally posted by eclip5e


"detonating"?
Yes, detonating is when your fuel/air mixture in the combustion chamber ignites before the piston is a top-dead-center (TDC) and without spark from the spark plug. Usually caused by excessive heat or pressure (or both) in the combustion chamber.

This is bad, usually results in broken or melted pistons.

What is this obsession with high boost pressure numbers on the board lately?

- Trent
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Old 07-24-2003, 09:43 AM   #16
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Quote:
Originally posted by Kostamojen

About 8psi at the most without internal modifications, and WITH extreme engine management to keep it from exploding and running perfect.
Friends RS-T (Ludespeed Stg 2), is daily driven with 8psi. (About 5 months now, no problems still)

Can't say the same for mine......BOOM at only set spring 6.6psi.

Goes to tell you, EVERY setups are different, with different thresholds.
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Old 07-24-2003, 11:40 AM   #17
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Quote:
Originally posted by Wedge


Friends RS-T (Ludespeed Stg 2), is daily driven with 8psi. (About 5 months now, no problems still)

Can't say the same for mine......BOOM at only set spring 6.6psi.

Goes to tell you, EVERY setups are different, with different thresholds.
Exactly!! I just had a shortblock EJ251 replaced due to a crack in it an I'm N/A with no mods!!

There's a lot more research that needs to be done on your behalf....wait five months until SOA has reached their quota on the STI EJ25 blocks and order a shortblock...build your heads while you're at it.
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Old 07-24-2003, 01:13 PM   #18
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Quote:
Originally posted by latinskllz


Exactly!! I just had a shortblock EJ251 replaced due to a crack in it an I'm N/A with no mods!!

There's a lot more research that needs to be done on your behalf....wait five months until SOA has reached their quota on the STI EJ25 blocks and order a shortblock...build your heads while you're at it.
"build your heads"?
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Old 07-24-2003, 02:17 PM   #19
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Your stock heads will quickly become a restriction with a high power turbo motor. Since the engine will already be apart, it wouldn't hurt to put in stiffer valve springs, better breathing cams, and have it ported to flow better. See here for details.

-Jon
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Old 07-25-2003, 04:54 AM   #20
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Quote:
Originally posted by Wedge


Friends RS-T (Ludespeed Stg 2), is daily driven with 8psi. (About 5 months now, no problems still)
Exactly, the Ludespeed stage 2 is very tuned to work with that setup. Its tuned just about to as good as you can get without requiring internal modifications. Its a pretty darn good kit, but i'd still put that money towards an engine swap to be safe.
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