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Old 10-14-2008, 01:03 AM   #1
WoodsWagon
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Default Stock Sti pistons in phase 1 2.5?

My question is, can you put the stock pistons out of a sti shortblock onto the rods on a phase 1 2.5, have it fit the bores on said engine and drop the comp ratio low enough to put a turbo on.

I've got a phase 1 shortblock on a stand, then another complete phase 1 in the shed and a phase 2 out there as well. Both of those are torn down to the basic bits. Some EJ22's out there to. The phase 2 has gouges in the cyl wall from a piston breaking, the phase 1's all have degrees of pitting on cyl bores due to blown headgaskets.

The plan is to put some combonation of parts in my 86 GL-10. I've got a nice header, sundry turbo's, superchargers and intercoolers and whatnot in my other shed, and a n/a EJ22 wiring harness.

It's not my first EJ swap into the EA chassis, and if I just did a basic EJ25 N/A swap it would be easy. But the GL-10 has the turbo crossmember and it seems a shame to waste that. It moves pretty good with the 111hp turbo 1.8 in it... so even a n/a ej25 would be a big improvement.

Since I already have a mostly OK shorblock on the stand, it would be easy to swap in pistons. I'm cheap, so using stock cast-off's from someone building a sti block would be nice, if they fit.
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Old 10-14-2008, 01:15 AM   #2
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You "can" install STI pistons on the 2.5 phase 1 rods and crank. I know, cause I did it. I made 350ish whp... But the piston sits 1-2 mm below the deck. LOW compression.

But it worked!
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Old 10-14-2008, 03:55 PM   #3
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Quote:
Originally Posted by reddevil View Post
...LOW compression.

But it worked!
Any idea of the compression numbers?

Sounds like a good build for high-boosting turbos.

sotc
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Old 10-14-2008, 04:00 PM   #4
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I hear the best way to go about it is get a phase 2 crank from an sti or newer legy with sti rods. Then the pistons from the sti will sit properly and your NA engine will be able to hadle a lot more psi. I have not done this myself, but I have been doing a lot of searching around and found that this is the most reliable setup for turboing the phase 1 NA blocks. It's definetly worth looking into. If you would like to do some reading you should go to 2.5rs forums and search for posts by skully. He had a phase 1 block from a 97-98 rs (I think) and he was safely running more than 20psi. Best thing is, if this really does work, is that this doesn't come with the $6000 price tag

.... and sti head gaskets... and of course you need supporting mods

Last edited by paurio.tt; 10-14-2008 at 04:02 PM. Reason: woops forgot something
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Old 10-14-2008, 05:03 PM   #5
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Quote:
Originally Posted by reddevil View Post
You "can" install STI pistons on the 2.5 phase 1 rods and crank. I know, cause I did it. I made 350ish whp... But the piston sits 1-2 mm below the deck. LOW compression.

But it worked!
ANd how many lbs of boost did it take to hit those #'s ?
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Old 10-14-2008, 05:27 PM   #6
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22-24 ish...

Hey, I said it worked, not that it was the best!
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Old 10-14-2008, 06:05 PM   #7
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I have a phase 2 crank out of the SOHC 2.5, but the thrust bearing is on the #5 main rather than the #3, and it doesn't look like they machined the crank to accept a #3 thrust...

I suppose we could chuck up the crank on the lathe and machine it to fit. The phase 2's cranks have a different throw than the phase 1's? Or is it just the rods are longer? If that was the case, could I use the phase 2 rods on the phase 1 crank?

I'm feeling a bit phased...
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Old 10-15-2008, 02:09 AM   #8
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The only differences are:

Phase I EJ25 crank: 79mm stroke, 48mm rod journals, thrust bearing located on #3 main bearing.

Phase II EJ25 crank: 79mm stroke, 52mm rod journals, thrust bearing located on #5 main bearing.

So no, you wont be able to use the Phase II rods on the Phase I crank.


IIRC, you need to have the block machined for the Phase II crank, not the crank machined for the block.



~Josh~
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Old 10-15-2008, 07:22 PM   #9
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So the phase 2 connecting rods are longer than the phase 1's, and the pistons are shorter by the same amount? Mabe to offset the piston slap issues that the phase 1's had.

So... If i put my phase 2 SOHC pistons on my phase 1 rods, the installed height wouldn't come all the way up the bore... Which would lower my comp... But completely fubar any quench that's supposed to happen in the heads.

Hmmm.
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Old 10-15-2008, 10:58 PM   #10
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Good thread what are the years of the phases? I got a impreza rs 2.5 sohc motor i was going to use the ej20 heads and gt30r turbo, but its good to get all this info
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Old 10-15-2008, 11:14 PM   #11
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Quote:
Originally Posted by WoodsWagon View Post
So the phase 2 connecting rods are longer than the phase 1's, and the pistons are shorter by the same amount? Mabe to offset the piston slap issues that the phase 1's had.

So... If i put my phase 2 SOHC pistons on my phase 1 rods, the installed height wouldn't come all the way up the bore... Which would lower my comp... But completely fubar any quench that's supposed to happen in the heads.

Hmmm.
NO!

The rods have different crank end journals.
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Old 10-17-2008, 09:20 PM   #12
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Quote:
Originally Posted by quikcobra View Post
Good thread what are the years of the phases? I got a impreza rs 2.5 sohc motor i was going to use the ej20 heads and gt30r turbo, but its good to get all this info
Most late 90's 2.5 DOHC engines are phase 1. I think the LGT used it from 97-99, and the RS used it 97-98(not 100% sure). I'm not sure about the forester, but I'm sure it's around those years. One thing I'm sure of is that the RS SOHC engines are stage 2.
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Old 10-18-2008, 12:32 AM   #13
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Quote:
Originally Posted by WoodsWagon View Post
So the phase 2 connecting rods are longer than the phase 1's, and the pistons are shorter by the same amount? Mabe to offset the piston slap issues that the phase 1's had.

So... If i put my phase 2 SOHC pistons on my phase 1 rods, the installed height wouldn't come all the way up the bore... Which would lower my comp... But completely fubar any quench that's supposed to happen in the heads.

Hmmm.
I put Phase II pistons in my Phase I EJ25, with the stock rods. Piston is below deck about .20-.25 mm. Not really enough to screw you up too badly, but it also lets you run a thinner HG to keep your quench. I used the Thick DOHC MLS gaskets because I was dumb. Only reason I did that was because of the higher lift of my spicy delta cams I am using.

If the stock Phase II pistons in a Phase II block, are flush with the deck, and have valve reliefs. You should be able to run the thin phase II HG with no problem, which is what i'll be doing...


~Josh~
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