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Swapping a 2.5 SOHC for a 2.5 DOHC?


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11 replies to this topic

#1 FSRBIKER

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Posted 18 April 2006 - 11:53 AM

Yes I searched and read alot of posts but didn't find the answers i need.

My 97 Legacy Outback motor is shot and i have found a few GenII 2.5RS(SOHC) engines locally for sale to replace my Gen I 2.5 DOHC. What problems should I look out for? Should I keep my original ECU and swap my old intake and harness onto the new motor? Will the brand new clutch setup I have still work with the RS 2.5?
I pissed a bunch of money away on the old engine thinking it was a head gasket problem and it's not, the head must be cracked but with 170,000 miles I don't want to invest any more into it. Money is real short now with our 4th child on the way so I need to know any pitfalls or added expenses which might prevent thsi swap from going smoothly.

Thanks in advance

PS If anyone has a 2.5DOHC or SOHC engine for sale let me know.

#2 TheYeti

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Posted 18 April 2006 - 02:06 PM

If you are going to use the stock ECU, my guess it that you probably need to stay with a set of DOHC heads. I recently rebuilt my '97's motor. It uses a '99 phase2 block with '96 heads and it runs very well and also ups the compression a bit. This build also allows you to use your existing intake, header, and whatever else you may already have.

Some other options would be to use your existing 97 heads with a phase2 block, or just find a used 2.2 and put it in.

#3 FSRBIKER

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Posted 18 April 2006 - 02:25 PM

Thanks for the info, I can get the RS motors complete with harness and ECU if I need to swap...I was assuming the stock exhaust should bolt up though. I am not sure what is wrong with the original engine hence the reason I want a complete different engine. Anyone know if the DOHC intake will bolt to the SOHC engine?
Used 2.2, turbo would be nice but I need to keep things simpler right now.

If you are going to use the stock ECU, my guess it that you probably need to stay with a set of DOHC heads. I recently rebuilt my '97's motor. It uses a '99 phase2 block with '96 heads and it runs very well and also ups the compression a bit. This build also allows you to use your existing intake, header, and whatever else you may already have.

Some other options would be to use your existing 97 heads with a phase2 block, or just find a used 2.2 and put it in.



#4 Dr. RX

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Posted 18 April 2006 - 04:56 PM

I can imagine too much difference in the two engines electronically. I'd start by trying to hook up the exsisting harness to the new engine with the old ECU. If that works, then you've save yourself some time, if not, THEN swap out the harnesses and ECUs.

#5 970subaru

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Posted 18 April 2006 - 05:33 PM

there are issues with the cam sensor, the DOHC heads read differently than the SOHC heads. there may also be differences in the TPS and different size injectors.
I believe using a SOHC shortblock(phase II) will bump up the compression on the motor, so you could consider that.

#6 ballitch

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Posted 18 April 2006 - 06:59 PM

the phase II pistons dont go past the deck surface like the phase I's, so your CR would go down. but only if you used the phase I heads on phase II block. are your heads toast? if so then just swap your intake harness over to the new RS motor, and change/modify any sensors that are different. take a look over on nasioc.com, the onfo HAS to be there of all places.



~Josh~

#7 Dr. RX

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Posted 18 April 2006 - 07:58 PM

there are issues with the cam sensor, the DOHC heads read differently than the SOHC heads. there may also be differences in the TPS and different size injectors.
I believe using a SOHC shortblock(phase II) will bump up the compression on the motor, so you could consider that.

Not usre that I'm following this, are you trying to tell me that the DOHC turns faster then the SOHC? I think not, they should turn the same speed, therefore the senor would pick up the same signals from either set up. Why would the TPS and injectors be different. The intake manifold might be slightly different, but I can't see why they would change the TPS or the injectors (the engine displacement is the same). I think that I remember that someone locally did this, I'll see if I can find out who did it and get back with you.

#8 Dr. RX

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Posted 18 April 2006 - 08:00 PM

the phase II pistons dont go past the deck surface like the phase I's, so your CR would go down. but only if you used the phase I heads on phase II block. are your heads toast? if so then just swap your intake harness over to the new RS motor, and change/modify any sensors that are different. take a look over on nasioc.com, the onfo HAS to be there of all places.

~Josh~

Josh, he mentioned using the whole short block, not just the pistons. A SOHC intake is different then a DOHC intakem they don't just switch.

#9 FSRBIKER

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Posted 19 April 2006 - 05:48 AM

Thanks for the technical info guys, keep it coming. I don't want to use the block or heads from the DOHC, just swap anything over to the new motor I might need.
I posted on Nasioc.com but haven't got any info, I just changed the subject like I did here though so maybe that will help.

#10 grossgary

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Posted 19 April 2006 - 06:47 AM

i have not done this swap, but have pulled some EJ motors. i am not positive but will offer my oppinion.
i find it hard to believe this isn't a drop in swap. swap the entire engine assembly into your existing car. it should be plug and play, no need for an ECU change. swap the entire long block, intake and everything.

the only issues you may have are the exhaust. if they are different, just get the exhaust manifold for the SOHC engine you install and it should bolt right up.

a long shot - is the wiring harness on the engine different? i highly doubt it, but if it was you could likely swap your old DOHC engine harness onto the SOHC engine.

2.2 swaps into the 2.5 cars are done regularly without much issues, they plug and play, retain the same ECU and have no issues. certain years require bringing the 2.2 exhaust manifold with it, but those bolt right up to the rest of the "2.5" vehicle exhaust after the headers. bolt on swap. and they plug right in as well.

if the 2.2 swaps are easy, i can't imagine going from 2.5 DOHC to SOHC is that big of a deal.

#11 TheYeti

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Posted 19 April 2006 - 07:02 AM

The exhaust manifold is the same for both phaseI and phaseII 2.5 motors.

#12 grossgary

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Posted 19 April 2006 - 07:13 AM

there you go, this swap sounds easier and easier the more i hear.
the broader the year gap between the SOHC and DOHC, the more i'd look into it.
once you find a suitable SOHC, you should verify the year and any changes between 97 and the year you have.




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