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Old 06-06-2006, 05:01 PM   #1
James@RPM
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Have a Nice Day? 2.0 RA JDM vs 2.5 USDM Shortblock

I have a wrx and looking to swop out the usdm 2.0 with one of the two motors in the title bothe are never turned motors which would you choose and why it will be with wrx heads and RA gear box.
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Old 06-06-2006, 05:20 PM   #2
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I don't know the exact differences from the JDM RA, but picking my block was easy. USDM STI shortblocks are easily available, cheap, and have more torque. JDM motors are designed to rev higher, but as a daily driver I prefer the torque. Ver. 7 and ver. 8 jdm sti motors have forged internals, so handle more boost without swapping internals. However, finding a jdm shortblock isn't easy, and the cost of drop-in forged pistons on usdm sti motors is easy and cheap.

Using your WRX heads on the sti motor will raise your compression if you don't get a thicker headgasket or different pistons. If you plan on running close to 350whp or higher, I'd just get forged pistons with lower compression to mate with the 2.0 heads. If you plan on those numbers, you also would be better off getting upgraded cams. USDM STI cams is the cheapest, easiest upgrade. If you're under 400whp, your gearbox will be fine unless you drive hard on your gears, as there are several 300-400whp users on RA's with no problems.
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Old 06-06-2006, 06:12 PM   #3
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Both motors are aval ,price would be about the same as well.
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Old 06-06-2006, 06:25 PM   #4
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What's your power goals and vehicle usage?
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Old 06-07-2006, 11:01 AM   #5
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From everything I have read the v8 has the same cast pistons as the USDM does.

IMO do the USDM 2.5l with a set of 100mm forged pistons. That will drop the compression down to ~8:1. DO NOT USE "Drop in" pistons.

TMS

Quote:
Originally Posted by keaniegenie
I don't know the exact differences from the JDM RA, but picking my block was easy. USDM STI shortblocks are easily available, cheap, and have more torque. JDM motors are designed to rev higher, but as a daily driver I prefer the torque. Ver. 7 and ver. 8 jdm sti motors have forged internals, so handle more boost without swapping internals. However, finding a jdm shortblock isn't easy, and the cost of drop-in forged pistons on usdm sti motors is easy and cheap.

Using your WRX heads on the sti motor will raise your compression if you don't get a thicker headgasket or different pistons. If you plan on running close to 350whp or higher, I'd just get forged pistons with lower compression to mate with the 2.0 heads. If you plan on those numbers, you also would be better off getting upgraded cams. USDM STI cams is the cheapest, easiest upgrade. If you're under 400whp, your gearbox will be fine unless you drive hard on your gears, as there are several 300-400whp users on RA's with no problems.
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Old 06-07-2006, 11:46 AM   #6
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Quote:
Originally Posted by TheMadScientist
From everything I have read the v8 has the same cast pistons as the USDM does.

IMO do the USDM 2.5l with a set of 100mm forged pistons. That will drop the compression down to ~8:1. DO NOT USE "Drop in" pistons.

TMS
I second that. 2.5L is where its at, its cheap, easy to find parts for, and better for the street.

Eric
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Old 06-07-2006, 11:49 AM   #7
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Quote:
Originally Posted by TheMadScientist
From everything I have read the v8 has the same cast pistons as the USDM does.

IMO do the USDM 2.5l with a set of 100mm forged pistons. That will drop the compression down to ~8:1. DO NOT USE "Drop in" pistons.

TMS

Are you saying don't use drop in pistons because his heads? Compression?
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Old 06-07-2006, 12:06 PM   #8
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Quote:
Originally Posted by keaniegenie
Are you saying don't use drop in pistons because his heads? Compression?
he's saying that because its the wrong way to do things. when putting new pistons in you should always bore or hone the cylinders. i'm pretty sure he's not saying anything bad about the pistons themselves more just you need to size them up one and do the proper machining. Compression can go up if he plans on using aftermarket cams but if not a head gasket or lower compression pistons should be put in.

Eric
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Old 06-07-2006, 12:09 PM   #9
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Quote:
Originally Posted by ejsportcom
he's saying that because its the wrong way to do things. when putting new pistons in you should always bore or hone the cylinders. i'm pretty sure he's not saying anything bad about the pistons themselves more just you need to size them up one and do the proper machining. Compression can go up if he plans on using aftermarket cams but if not a head gasket or lower compression pistons should be put in.

Eric

It sounds like the author to this thread is getting a brand new shortblock though.

TMS also advised for lower compression pistons, which I'm assuming is related to the fact the author has WRX heads so the compression is normal, no?
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Old 06-07-2006, 12:26 PM   #10
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Quote:
Originally Posted by keaniegenie
It sounds like the author to this thread is getting a brand new shortblock though.

TMS also advised for lower compression pistons, which I'm assuming is related to the fact the author has WRX heads so the compression is normal, no?
People have run it with the compression that the stock pistons would give you and its ok, but like i said if he does aftermarket cams then the higher static compression would be better because the dynamic compression would go down. But you are right the compression will be a lil higher then normal.

Eric
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Old 06-07-2006, 12:48 PM   #11
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IMO any piston that is labled as "Drop in" WILL give you a car that sounds like a desiel truck. I have drop in pistons right now in my car, I have heard a few other motors that have them. Never ever again will I be using them.

If you do some searching you will find a few people that have had a very hard time tuning the WRX heads on a stock 2.5 short block. The people that replace the pistons to lowwer the compression and match the quench area of the piston to the WRX head have had much better results.

TMS

Quote:
Originally Posted by keaniegenie
Are you saying don't use drop in pistons because his heads? Compression?
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Old 06-07-2006, 01:05 PM   #12
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Quote:
Originally Posted by TheMadScientist
IMO any piston that is labled as "Drop in" WILL give you a car that sounds like a desiel truck. I have drop in pistons right now in my car, I have heard a few other motors that have them. Never ever again will I be using them.
Me too.
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Old 06-07-2006, 01:10 PM   #13
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Quote:
Originally Posted by PeteDucati
Me too.
your axis 2.5 has some slap?
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Old 06-07-2006, 01:21 PM   #14
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Quote:
Originally Posted by keaniegenie
I don't know the exact differences from the JDM RA, but picking my block was easy. USDM STI shortblocks are easily available, cheap, and have more torque. JDM motors are designed to rev higher, but as a daily driver I prefer the torque. Ver. 7 and ver. 8 jdm sti motors have forged internals, so handle more boost without swapping internals.
Not true. Version 8 has Hypereutectic cast pistons. That said, there have been several 400+ AWHP V8 JDM motors.
My experience: I have a Version 8 JDM motor mated with V5 Type RA JDM heads w/ a 3 angle valve job. I run a 9:1 compression ratio and run 15PSI on 93 octane, or 19PSI on 100 octane by swapping a hose around on the stock boost control solenoid. Everything on the motor is ported and polished, minus the heads. The (stock) redline is 8000RPMs, controlled by the V5 JDM WRX Type RA ECU (STOCK ECU, stock boost control, etc). It is very fast, loves to rev, and pulls hard to the redline. But compared to a stock STI the low end torque just isn't there. This is on a 2840lbs car, mind you. My car feels like a cavalier or a dodge neon until 3100RPMs or so when the VF22 turbo spools, whereas a stock STI seems to pull a lot harder from lower in the RPM range. Of course, once my turbo spools its game over for the STI, but as nice and fast as my car may be, on daily driving I'd have preferred the 2.5L block.
The reason why I went JDM is because it was a *much* cheaper option for me. $1000 for a Version 8 shortblock + the ability to run it off my stock ECU VS $1600 for a 2.5L shortblock + standalone + dyno tuning...

For the record, on a race track I would rather go with the JDM block. But my car is not a race car
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Old 06-07-2006, 01:40 PM   #15
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Quote:
Originally Posted by TheMadScientist
IMO any piston that is labled as "Drop in" WILL give you a car that sounds like a desiel truck. I have drop in pistons right now in my car, I have heard a few other motors that have them. Never ever again will I be using them.

While I agree that the correct way to install pistons is to bore and re-hone the cylinder walls for the specific pistons, a couple of my customers have simply not had the money available to go through with the boring and honing, and so we dropped the Mahle powerpak 99.5mm pistons in. The Mahle's are forged from a much lower expansion alloy than any of the others out there. In both circumstances the engines ran very quietly...as quiet as if not quieter than a stock EJ257.

This being said, I still recommend match boring each cylinder to the specific piston that is going in it.
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Old 06-07-2006, 02:08 PM   #16
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Quote:
Originally Posted by Templar
While I agree that the correct way to install pistons is to bore and re-hone the cylinder walls for the specific pistons, a couple of my customers have simply not had the money available to go through with the boring and honing, and so we dropped the Mahle powerpak 99.5mm pistons in. The Mahle's are forged from a much lower expansion alloy than any of the others out there. In both circumstances the engines ran very quietly...as quiet as if not quieter than a stock EJ257.

This being said, I still recommend match boring each cylinder to the specific piston that is going in it.
Glad to hear that at least one company got it right. The Mahle's look very nice, but when my next motor gets built it will have the Mahle 100mm with the block bored to match.

TMS
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Old 06-07-2006, 02:25 PM   #17
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I used the Mahle drop in 99.5 and it is pretty quiet I can hear slap for about 30 seconds and then it is gone and it is not really loud in the first place most people dont even hear it.
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Old 06-07-2006, 03:00 PM   #18
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Quote:
Originally Posted by InfamousDX
your axis 2.5 has some slap?
No. My Axis 2.5 has ALL the slap. It NEVER goes away. People laugh and point and call me names - but I don't care - it's t3h faStAR!

It's loud as hell but doesn't burn any oil - and my tuner didn't have any issues with the slap or the thicker gaskets while it was on the dyno.

Ps. That particular Subaru ej257 came with "A" pistons all around - which are the larger of the two pistons used. That may be another reason behind the audible slap.
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Old 06-07-2006, 06:03 PM   #19
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Thanks for all your input from what Robert at Forced air tech was telling me it is the only motor out of Subaru that is fully balanced and is the only one that runs a speacial hardend black crank he states it is different motor than just the plain STI JDM can anyone confirm?
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Old 06-07-2006, 07:11 PM   #20
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Quote:
Originally Posted by James@RPM
Thanks for all your input from what Robert at Forced air tech was telling me it is the only motor out of Subaru that is fully balanced and is the only one that runs a speacial hardend black crank he states it is different motor than just the plain STI JDM can anyone confirm?
I have been told that it is blueprinted and balanced to spin at 9000RPMs and that the crankshaft has extra oiling holes to support it as well.
Now if only I had the valvetrain to sustain that
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Old 06-07-2006, 08:04 PM   #21
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there was another thread where a reputable tuner responded the extra oiling holes were worthless. what gives? anyone? --keith
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Old 06-07-2006, 09:17 PM   #22
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i'm also trying to decide between an 06 WRX swap or a ver3-4 STI swap... its really hard, especially considering the stock/stock comparisons of power but i feel way better with a drivetrain that only has 900 miles.... almost like having a new car.. lol wonder if the warranty still applies!!! hahaha
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Old 06-07-2006, 09:18 PM   #23
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Quote:
Originally Posted by kheff46
there was another thread where a reputable tuner responded the extra oiling holes were worthless. what gives? anyone? --keith
That's funny; a lot of engine builders offer to drill extra oiling holes as an optional service for motors that are being designed to rev high...
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Old 06-07-2006, 09:56 PM   #24
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supposedly the 2.5l crank has zero prob's with 9k. i personally have zero idea if this is true. thats not to say the heads, valvetrain, etc. can handle it.
thats definately a good idea. high hp aircraft engines use bigger holes, so it seems like a worthwhile idea. i just remember some tuner(can't recall) stated that the 257 crank has no problem with these revs. 10-11k is a whole 'nother story, like any engine theres a redesigning that takes place whether its balance issues, or lubricating prob.'s. just wanted to make sure no one is speculating as to wheher the crank is able to rev to the range posted. bearings, heads, etc. all have to be taken into account obviously when in streetbike territory. no hating here! --keith
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Old 06-08-2006, 03:50 PM   #25
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Quote:
Originally Posted by kheff46
supposedly the 2.5l crank has zero prob's with 9k. i personally have zero idea if this is true. thats not to say the heads, valvetrain, etc. can handle it.
thats definately a good idea. high hp aircraft engines use bigger holes, so it seems like a worthwhile idea. i just remember some tuner(can't recall) stated that the 257 crank has no problem with these revs. 10-11k is a whole 'nother story, like any engine theres a redesigning that takes place whether its balance issues, or lubricating prob.'s. just wanted to make sure no one is speculating as to wheher the crank is able to rev to the range posted. bearings, heads, etc. all have to be taken into account obviously when in streetbike territory. no hating here! --keith
I think its simpler than that. You look at how much your rotating assembly weights, and how much it moves. With the 2.5L there is a bigger stroke so there will always be more stress at the same revs, oiling issues aside.
And I don't think any subaru valvetrain will allow more than 8500RPMs, and even then only the later JDM STI RA/R/Spec C will do it. USDM 2.5L STIs and 2L WRXes will float valves at 7700 - 7900or so from what I hear.
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