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Guest Message by DevFuse

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Frankenmotor Compression Ratio with Piston Comparison

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3 replies to this topic

#1 ShawnW


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Posted 07 September 2013 - 03:11 PM

Attaching a photo of 4 different EJ25 Piston options we have here at the shop.  All of them would obviously have their benefits and are doable but wondering what compression ratios these would make with 1996 EJ22 Single port heads that the owner of the car has available to him and an accompanying harness and intake manifold make that choice rather easy.  We could switch to dual port heads and swap the front of the ypipe but would rather not as a cost measure.


PIstons shown:

1996 EJ25D

1997 EJ25 D single dot 

2004 Subaru Baja Turbo (ej255 I believe)

2004 Ej253 2 dot (sohc piston)


I could also go for a 2006 Ej25 sohc piston set we have around here that looks even higher compression visually to me than the 2004 model.


Does anybody know what the compression of the engine would be with these types of pistons?  Fuel requirements at altitude?


The engine will be going into a 1984 Subaru Brat with a dual range ea trans so keeping the power reasonable is necessary with the clutch/trans.

#2 Gloyale


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Posted 07 September 2013 - 03:44 PM

If you are using a 25d block/crank, then

I'd use the 96 or 97 25d pistons.


The Turbo ones will yeild lower compression.  But may come proud out of the block when used with the 25d rods.  Would need a thick headgasket so lower comp still.


The 06 sohc pistons will not work with the 2.2 heads.  They have a semi dome section that protrudes into the heads on the Variable lift engines.


If you are using a 251-3 block/crank, you could use the 04 SOHC pistons.

#3 ShawnW


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Posted 07 September 2013 - 07:27 PM

We are using the block and crank from a 2001 RS to have the larger rod bearings and #5 rod journal.  



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Posted 07 September 2013 - 09:34 PM

The extreme dome pistons in the 06 era engine won't even clear the huge combustion chamber of the 96 ej25 heads. iI know it's not relevant to what you're doing, but it can't hurt to have the info out here somewhere.

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