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Old 03-16-2005, 10:21 PM   #1
RS-Flyboy
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Default Part compatible: STi -> 1998 2.5 RS DOHC

So, I'm trying to figure out what parts are compatible for my 1998 2.5 RS, DOHC, pulled from an STi, or even WRX. For example: Can I pull the valves, pistons, camshafts, etc... I just want to find upgradeable parts that conform to the sizes that I currently have. I don't care about compression ratios, I plan on boosting my baby anyways.

Any help greatly appreciated!
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Old 03-16-2005, 11:07 PM   #2
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Valvesprings and retainers swap right over. I think the STi valves are different size, but the whole heads will bolt on up. Cams are going to be different because of the AVCS. Pistons probably won't since we're dealing with phaseI and phaseII type motors. You should be able to use newer EJ25 pistons in the STi block though.....but since you're gonna boost it anyway nevermind.


Jay Storm
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Old 03-17-2005, 08:37 AM   #3
Section 8
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You can't use any other pistons in the phase one blocks (they will stick out of the bore, you would have to use the matching rods for the pistons, but the journal sizes are different too, so not happening). You can use phase 2 rods and pistons in the STi block, but you have to replace rods and pistons at the same time, rod length and pin placement are different, with teh same deck height. THE heads will swap over, but your 98 intake manifold will not bolt on to it (DOHC and SOHC have different bolt patterns).

USDM WRX, STI and the SOHC 2.5 heads all have the same intake manifold bolt pattern, so any of the heads will work with any of the intake manifolds. However, the STi and the WRX manifolds have larger internal diameter runners, and you will have to gasket port the SOHC heads for it to be worth the swap. You could use WRX heads with a WRX manifold, obviously.

There isn't much that you can do with that engine and spare parts lying around...
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Old 03-17-2005, 11:46 AM   #4
Matt Monson
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Section 8,
You can actually put phase II EJ251 pistons into a phase I block. They do not stick up over the top like the phase I pistons did. It is actually a good way to easily bump the CR for someone with a phase I NA motor. But putting the STi pistons in would require changing to the STi rods as well. The wrist pin location is different and you can't mix and match them as a result.

As for swapping in WRX heads? Absolutely no reason if someone already owns a set of DOHC EJ25 heads. Those heads are made from the same cores as the '97-98 WRX's and they have much larger ports than the current USDM EJ205 heads. And obviously, the valves wouldn't be any better either. And the stock cam profile of the cams on the DOHC EJ254 heads is fine for boosting.

Do a search over in the Aftermarket Forced Induction forum and you will find plenty of info. Look for Xephyr's build. He has done a variety of blocks over the years. Currently he runs an EJ257 STi shortblock(bone stock purchased new from the dealer for $1750) with his stock EJ254 NA heads with stiffer springs. It is everything else he has incorporated that makes the big power. Last time on the dyno he put 452HP to the wheels on pump gas. He is currently (as in the new piping was welded yesterday) installing a a bigger/more efficient turbo, and we'll see where that ends him up next week once it is tuned.

Also search for Graham. When he was building his car (EJ257 block with SOHC heads) we had a long discussion about the cams. He opted to go with the stock cams and just upgrade his springs as well.

In short, there is no reason to try and pull those random parts from STI's and WRX's. Just get the block and go from there...
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Old 03-17-2005, 02:55 PM   #5
Whitie
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so...... section 8 did you say the 98 2.5 heads will bolt on to the sti block (if i am reading that right)
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Old 03-17-2005, 03:44 PM   #6
Section 8
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Quote:
Originally Posted by Matt Monson
Section 8,
You can actually put phase II EJ251 pistons into a phase I block. They do not stick up over the top like the phase I pistons did. It is actually a good way to easily bump the CR for someone with a phase I NA motor. But putting the STi pistons in would require changing to the STi rods as well. The wrist pin location is different and you can't mix and match them as a result.
I have never done the phase 2 pistons in the phase 2 block, but 8Complex has done it or vice versa (phase 1 pistons in a phase 2 block) and his stuck up. If the reverse is true, then the pistons would be lower and the compression would also be lower. You can't use STi piston/rod combination in the phase 1 blocks because the rod journal are different diameter. THe rod journals are to big for the crank.

Quote:
Originally Posted by Whitie
so...... section 8 did you say the 98 2.5 heads will bolt on to the sti block (if i am reading that right)
In short, you can put any head on any block, the question is if you want to, and if you have the correct manifold for the heads.

cheeRS,

Greg
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Old 03-17-2005, 04:46 PM   #7
Matt Monson
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Greg,
I didn't state it correctly. If you put the phase 2 pistons in a phase 1 block, they do stick up, but the exact same amount as the original phase 1 pistons did. The CR increase is based on the piston dish itself. There is less volume on the phase 2 pistons, thus the CR bump. We built one of these recently, and you must use the original thickness HG with it...

On the other part about the crank, I didn't realize the rod journals were smaller on the phase I cranks. I thought all the EJ25 cranks had the same journal diameters...
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Old 03-17-2005, 04:53 PM   #8
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ahh... gotcha
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