EJ20 to EJ25 2.5L short block swap help - Subaru WRX Forum
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post #1 of 23 Old 11-16-2015, 01:13 AM Thread Starter
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EJ20 to EJ25 2.5L short block swap help

Brothers, I'm new here and somewhat new in owning a Subaru. I have an 02 that I have been upgrading since last year. when I got my wrx the previous owner told me the two heads are new it was replaced due to valve problems. the car has 170Kmiles on it already assuming the ej20 block is of the same millage.

Currently I have a 16G turbo, after market headers, catless 3" down pipe. Cobb accessport V3 and a Cobb boost controller. Now sad to say I think my engine is starting to knock. I'm starting to hear it at around 3k RPM. I'm planning to replace the timing belt next week hoping some say it might be the tensioner or it might even be the oil pump. but I have a bad feeling its not the tensioner nor the oil pump.

Now when I was looking for my car I came across this WRX.. the owner told me it has a 2.5L EJ25 short block with a EJ20 wrx cylinder heads.. my question is.. can this be done? EJ20 block swap with EJ25? my wrx heads and the rest of my EJ20 components will fit the 2.5L EJ25 block? has anyone here have done this before? I have been looking for some materials regarding this project I cant seem to find any

Thanks,
Mike
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post #2 of 23 Old 11-16-2015, 07:08 AM
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Welcome to the fourms!

this has been done hundreds of times. quite reliable when done properly.

its 2.0 heads with a 2.5 block. you need to machine your heads to get the right compression ratio and fit the pistons or get special head gaskets and pistons. (short version)

ALL Hybrid 2.5L Shortblock & 2.0L Cylinder Head INFO Thread *POST HERE* - NASIOC

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post #3 of 23 Old 11-16-2015, 03:11 PM Thread Starter
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Welcome to the fourms!

this has been done hundreds of times. quite reliable when done properly.

its 2.0 heads with a 2.5 block. you need to machine your heads to get the right compression ratio and fit the pistons or get special head gaskets and pistons. (short version)

ALL Hybrid 2.5L Shortblock & 2.0L Cylinder Head INFO Thread *POST HERE* - NASIOC

Thanks Titter! I really need to have the heads machined? that's gonna be a big problem I only have a couple days to do the swap.
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post #4 of 23 Old 11-17-2015, 06:53 AM
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That hybrid build requires a certain plug to keep detonation from occurring. Are you getting rod knock or is it detonation knock? Rod knock you'd hear pretty much all the time and it would get worse as the rpms climb.

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post #5 of 23 Old 11-17-2015, 07:14 AM
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take more than a few days to research this project. while it's a pretty straight forward process, its not something that should be rushed.

the proper way is to machine the heads, but you can get away with a thicker head gasket instead to drop the compression ratio back to ~8.2:1. its not an overnight engine swap like dropping in some sti long block. changing the shortblock takes alot more time & effort.

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post #6 of 23 Old 11-19-2015, 01:13 AM Thread Starter
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take more than a few days to research this project. while it's a pretty straight forward process, its not something that should be rushed.

the proper way is to machine the heads, but you can get away with a thicker head gasket instead to drop the compression ratio back to ~8.2:1. its not an overnight engine swap like dropping in some sti long block. changing the shortblock takes alot more time & effort.
I'm planning now to do the swap Feb of next year so I still have 2 months to prepare and research everything. that is if my knocking engine now can survive the next 2 months wait lol! I give it a week to do the swap I have no choice I really have to finish it in a week..

right now im also thinking of getting used cylinder heads and have that one machined instead. i really cant afford to wait for my current cylinder head to be machined when i start to taking down the engine. my issue now is looking for a machine shop here at the bay area norcal that have done this before machining a wrx 2.0L heads for 2.5L block.

and lately i have been looking for a short block that have been rebuild already and i found this at craiglist .. will this work for me??

1999-2004 2.5 SUBARU SOURCE short and long blocks

btw. do i really need high end high performance internal parts like pistons, rods, bearing etc. when i do the hybrid swap? coz sorry for saying this i think i'll be the 1st to say it here.. I'm not actually looking for a huge power gain. i wont be racing my car and rarely go up 80mph on the freeway i'm dead scared of the cops here in California they don't smile ..im from jersey originally and cops there smile 1st before they shoot you lol! anyways enough with the drama. my goal right now is to have a running engine to replace my knocking one. If that engine block posted at craiglist survive 2 to 3 yrs of 15min drive to work and back under 80mph then im okay with its current internal parts setup.
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post #7 of 23 Old 11-19-2015, 01:29 AM Thread Starter
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Quote:
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That hybrid build requires a certain plug to keep detonation from occurring. Are you getting rod knock or is it detonation knock? Rod knock you'd hear pretty much all the time and it would get worse as the rpms climb.
its actually my current EJ20 that is knocking ..that's why im thinking of doing a hybrid swap and yeah I thinking its the rod bearing.. I've attach a knock reading from my accessport this morning. the reading sometimes gets -6.xx something
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post #8 of 23 Old 11-19-2015, 01:30 AM Thread Starter
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its actually my current EJ20 that is knocking ..that's why im thinking of doing a hybrid swap and yeah I thinking its the rod bearing.. I've attach a knock reading from my accessport this morning. the reading sometimes gets -6.xx something
haha! sorry forgot to rotate it!
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post #9 of 23 Old 11-19-2015, 06:53 AM
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all you'll need is a good set of forged pistons only and then OEM everything else for 400bhp. get the 2011+ sti oem crankshaft. its nitride treated.

to do the swap without chamber maching your heads then youll need...

Je Hybrid Pistons. Specifically made for this swap so the pistons dont hit the valves: JE hybrid pistons opinions - NASIOC

Cometic .051 Head Gasket to bring your compression ratio back down to a moderately safe 8.5:1 : Head Gaskets for 2.5L WRX Hybrid - NASIOC

there may be other ways but this is the safest i know of if not machining heads.

and my bad, i misunderstood what you said. when u said you only have a couple of days i didnt know you meant shop time. i thought you had to swap motors by this weekend.

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post #10 of 23 Old 11-19-2015, 07:54 AM
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I had knock like that for a year. Literally same #s.

When I tore the engine apart I expected to see something wrong and it was fine. Do you have a pro tune with your current set-up? You might find all you need is for that to be corrected.
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post #11 of 23 Old 11-20-2015, 12:24 AM Thread Starter
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all you'll need is a good set of forged pistons only and then OEM everything else for 400bhp. get the 2011+ sti oem crankshaft. its nitride treated.

to do the swap without chamber maching your heads then youll need...

Je Hybrid Pistons. Specifically made for this swap so the pistons dont hit the valves: JE hybrid pistons opinions - NASIOC

Cometic .051 Head Gasket to bring your compression ratio back down to a moderately safe 8.5:1 : Head Gaskets for 2.5L WRX Hybrid - NASIOC

there may be other ways but this is the safest i know of if not machining heads.

and my bad, i misunderstood what you said. when u said you only have a couple of days i didnt know you meant shop time. i thought you had to swap motors by this weekend.
sorry titter pls. bare with me ... so your saying if I don't machine my heads i'll need to get the Je hybrid pistons? but if I go ahead with machining my heads I can use OEM or STD design piston? just the forged once?
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post #12 of 23 Old 11-20-2015, 12:38 AM Thread Starter
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I had knock like that for a year. Literally same #s.

When I tore the engine apart I expected to see something wrong and it was fine. Do you have a pro tune with your current set-up? You might find all you need is for that to be corrected.
yeah this knock is killing me! funny thing .. when I go to work in the morning you wont hear any knock .. but when I go home at night same speed sometimes over 3000rpm sometimes below you can hear a ticking noise.. and when I pop the hood open rev the engine through the throttle no ticking noise at all ..crazy!

no I don't have a pro tune right now sad to say.. anyways ill try to check the tensioner maybe next week ..I haven't replace my timing belt since I got my car Aug of last year 2014. some say it might be the tensioner..
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post #13 of 23 Old 11-20-2015, 06:19 AM
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What tune do you have on there? Cobb OTS Stage 2?

The ticking noise is normal. That's a Subaru for you.

I am going to tell you that this is all because your current part set-up needs to have a protune. Although you will survive with this OTS tune your car will run much better, no knock and live longer.

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post #14 of 23 Old 11-20-2015, 06:53 AM
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its probably piston slap and not knock.

Rod knock/piston slap.
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post #15 of 23 Old 11-20-2015, 10:15 AM
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I run a 2.17L stroker and since day one and after my pro-tune I daily it with FBK between -4.55 and +3.75, check out Cobb's info on knock control and what to look for. I was told by my builder that it should be worry free unless you see it spike around 8. Also I would watch your DAM, that's a great monitor for knock feedback.

Why do you want a 2.5 bottom? More TQ all around, faster spool, better top end? I'm running the stock TD04 (eBay replacement came on it when I got the car) with WRX Blue injector (565cc) and they're clogged, and the 2.17l and I still keep up and pass SRT4 neon tuned. I'm thinking of a 18g for a responsive DD with nice top end.

Keep in mind Hybrids don't last as long as a full 2.5 or 2.0 build, and you lose about 4-5 MPG with the 2.5 over the 2.0 in the city.
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