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2.5 DOHC heads on a 2.5 SOHC block?


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9 replies to this topic

#1 FSRBIKER

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Posted 21 July 2006 - 12:44 PM

I am asking since the wiring swap to install the 2.5 SOHC(2002 RS) engine/harness/ECU I have I think is too much for me to handle but swapping my heads onto the newer block should get me back on the road.

FYI: I have a 97 Legacy Outback with the 2.5 DOHC

#2 merkatroid

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Posted 21 July 2006 - 01:37 PM

The heads will bolt on fine. The dohc heads have a smaller combustion chamber, this will raise your compresion ratio. I dont know what the sohc head cc is but the dohc is 46.6cc. The cam signal should work also. You should be able to correct the compresion ratio with thicker headgaskets, cometic makes good ones.

#3 Qman

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Posted 21 July 2006 - 09:12 PM

It will work. The only difference is the coolant passages of the 2.5 DOHC heads are slightly larger than the sohc block. You will need to use the gaskets for the DOHC heads to make sure that you do not create interference when you put everything together.

Good luck!!

#4 970subaru

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Posted 22 July 2006 - 01:42 AM

sounds like a HOT NA combo. DOHC flow, compression out the rump roast, should be pushing 200hp with a good exhaust and some cams.
SOHC heads are 50cc.
CR calculation, just because I am bored. all #'s taken from ejcalcs
http://wac.addr.com/...bo/ejcalcs.html

with thin gaskets:
616+46+4.5+6=672.5cc@BDC
46+4.5+6=56.5cc @TDC

11.9:1:eek: :eek:

with thick gaskets:
616+46+11.8+6=679.8cc@BDC
46+11.8+6@TDC=63.8
10.65:1 a little safer... but where's the fun in that

#5 merkatroid

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Posted 25 July 2006 - 07:07 PM

Thanks for the link. Great info

#6 970subaru

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Posted 27 July 2006 - 12:23 AM

I dont know how great it is, some of the numbers are known to be slightly off. I would use it as a rough guide more than anything.;)

#7 offroadsubaruguy

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Posted 27 July 2006 - 06:04 AM

so let me get this right, if i were t use a dohc block with sohc heads the cr rises?

#8 970subaru

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Posted 27 July 2006 - 04:28 PM

no, sohc heads are 50cc whereas dohc heads are 46cc. that would drop some, it is 9.3:1 or so with the thick DOHC gaskets IIRC.

#9 ballitch

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Posted 29 July 2006 - 05:04 PM

i ran your CR and got about 9.8:1 with the phase I ej25 heads. with the 50 cc phase II heads it goes down to 9.3:1. i know the phase I is supposed to have a 9.6:1 CR, but i think some of my numbers are off, thanks to ejcalcs. but you can bet that the new heads and dohc gasket will reduce your CR by about 0.5 so your looking at about 9.1:1 - 9.2:1 or so is a safe bet. which isnt too high for turbo. but not the ideal CR for turbo.



~Josh~

#10 ShawnW

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Posted 03 August 2006 - 07:00 PM

I am asking since the wiring swap to install the 2.5 SOHC(2002 RS) engine/harness/ECU I have I think is too much for me to handle but swapping my heads onto the newer block should get me back on the road.

FYI: I have a 97 Legacy Outback with the 2.5 DOHC


The heads are a direct bolt on.

The block for a 99 DOHC is EXACTLY the same as a 00 SOHC.




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