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Old 08-23-2006, 07:53 AM   #1
kaisersouse
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Default Build Theories 1: Piston questions

The engine in question is an SOHC 2.5 from 98RS.

I want to run as close to 10:1 compression as possible. I understand that pistons is the only sure way to acheive this. Are there existing OEMish pistons that will do this and if so from which model subie? If OEMish ones do not exist...who makes a good piston that doesn't cost an arm and a leg?
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Old 08-23-2006, 09:11 AM   #2
Patrick Olsen
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Well, first of all, a '98 2.5RS doesn't have a SOHC, so you've got me a bit confused there. Unless you got some really, really weird car from the factory, you've got a DOHC 2.5L.

http://wac.addr.com/auto/obs/turbo/ejcalcs.html has some information, but I think the gasket thicknesses are old (the original gaskets that Subaru was using a few years back). I haven't seen compressed gasket thicknesses for the new(er) gaskets. But I would think you'd be close to 10:1 with the newer gaskets.

Just out of curiousity, why do you want to be as close to 10:1 as possible?

Pat Olsen
'97 Legacy 2.5GT sedan
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Old 08-23-2006, 09:26 AM   #3
kaisersouse
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This is an enginre I'm waiting for someone to swap out of their car (they are doing WRX swap, im buying leftover). Maybe it was a 00/01. I'd have to go back and look. Sounds like I should look so I know what I'm tlaking about lol.

I want to teach myself engines by building up an NA 2.5. I figured 10:1 would give more power but the compression wouldnt be so high it would still make a good daily driver on 87oct.

I thought I had seen people post that gasket swap wont do the trick, but if it in fact does...I'm a happy camper. I would think that swapping gaskets would be easier than swapping pistons.

And i thought the NA 2.5s ran 9.5 or 9.8:1 or something liek that so I didnt think going to an even 10 would throw the planet out of orbit.

Quote:
Originally Posted by Patrick Olsen
Well, first of all, a '98 2.5RS doesn't have a SOHC, so you've got me a bit confused there. Unless you got some really, really weird car from the factory, you've got a DOHC 2.5L.

http://wac.addr.com/auto/obs/turbo/ejcalcs.html has some information, but I think the gasket thicknesses are old (the original gaskets that Subaru was using a few years back). I haven't seen compressed gasket thicknesses for the new(er) gaskets. But I would think you'd be close to 10:1 with the newer gaskets.

Just out of curiousity, why do you want to be as close to 10:1 as possible?

Pat Olsen
'97 Legacy 2.5GT sedan
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Old 08-23-2006, 11:47 AM   #4
Whiteghost 2.5
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SOHC engines in 99 has just under 10:1 c/r and 00-01 SOHC engines had 10:1 c/r stock.
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Old 08-23-2006, 11:48 AM   #5
kaisersouse
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Quote:
Originally Posted by Whiteghost 2.5
SOHC engines in 99 has just under 10:1 c/r and 00-01 SOHC engines had 10:1 c/r stock.
I thought that was the 2.2 tho. The manuals I've been reading all say 2.5s were lower.
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Old 08-23-2006, 11:51 AM   #6
ballitch
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ya hate to break it to ya bud, but the stock motors had that much compression. now if you dont mind buying premuim......there's another option for you..........FRANKENSTEIN!!!!!!


~Josh~
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Old 08-23-2006, 03:47 PM   #7
Master2192
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Quote:
Originally Posted by kaisersouse
I thought that was the 2.2 tho. The manuals I've been reading all say 2.5s were lower.
Nope, 98 DOHC and 99 SOHC has 9.7:1 compression and all RSs after those years have 10:1

Putting the new thinner SOHC gaskets in the 98 DOHC yields a 10.5:1 compression ratio. I had this done to my car and have no problems running 87 octane.
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Old 08-23-2006, 04:07 PM   #8
azn2nr
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mill the head surface???
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Old 08-23-2006, 05:06 PM   #9
Matt Monson
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Quote:
Originally Posted by azn2nr
mill the head surface???
Bad idea. The amount that you can mill and still stay within spec is neglible at best. I recently purchased a set of SOHC Ej25 heads that show what happens when you overmill them. EVERY single intake valve on both heads was bent... I bought them for spare parts, but somebody didn't know what they were doing...
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Old 08-23-2006, 07:41 PM   #10
kaisersouse
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hate to break it to me??? this is GREAT!!!

This is less **** I have to buy and mess with

thank you!!
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Old 08-23-2006, 07:59 PM   #11
blacktalon606
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But I thought you wanted more power? If you already have 10:1 and aren't happy with the ponies your putting down... doesn't that now mean you want 11 or 12:1?
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Old 08-23-2006, 08:19 PM   #12
kaisersouse
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Quote:
Originally Posted by blacktalon606
But I thought you wanted more power? If you already have 10:1 and aren't happy with the ponies your putting down... doesn't that now mean you want 11 or 12:1?
I have a 2.2 and Im getting a 2.5. While displacement is the best way to make more power, I thought the 2.5s had lower compression and that raising it would require new pistons...and possibly wear the motor down quicker. Now that I know I'm getting both power and higher-than-i-thought compression I dont have to worry about it.

This is a family daily driver that gets a lot of miles. I'm looking for a balance of power and reliability that fits my driving style (faster than normal but not being an asshat when i pass people). I'm sure the upgrade from 2.2 to 2.5 will be sweet...but since i will have it apart i want to think about what i need/could do with the internals. The money saved on pistons gets to go somewhere else
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Old 08-23-2006, 08:52 PM   #13
redwagon
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Put the 2.2 heads on your 2.5 block and switch to premium.
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Old 08-23-2006, 09:10 PM   #14
kaisersouse
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Quote:
Originally Posted by redwagon
Put the 2.2 heads on your 2.5 block and switch to premium.
bzzzt no. im not spending that much for gas. I know it'll go higher but still...
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