RexNuts Subaru Forums

Gday guys,

Quick question, I have a MY08 wrx and I'm in the need to do a bit of a rebuild as I have dropped a big end.
Now, from what i understand it is an EJ255 in there and I am looking at getting a new short motor (as rebuilding it is stupidly expensive) and do a swap over. So, what im wondering is, can i get an EJ257 short motor and use that with the standard heads etc ? I have access to a new EJ257 short motor for a very very good price through a friend at subaru and if i can use that it will potentially save me many thousands of dollars versus a full rebuild (Which would be fantastic!).
Now i have seen that the block is exactly the same between the 2 but the pistons in the 257 are domed versus the "flat tops" in the 255, does this mean that i will have compression issues? I can't find anyone who actually knows what they are talking about and a few guys referred me here as a good source of knowledge.

In short - I have an ej255, i want to use an ej257 short motor with everything else from the 255 bolted onto it (including ecu of course).

Any help would be greatly appreciated as i need to get this sorted asap as i desperately need the car back on the road.

Thanks all

Hi Simon and welcome to Rexnuts.

I did a quick search and found this link which may have some answers for you

http://forums.nasioc.com/forums/showthr ... ?t=1667168

They are the same except for pistons, effectively you'll be changing the compression but you can counter this with an STI head gasket and the compression will even out to an STi Standard as the heads are virtually the same. The STi has a thicker head gasket Im pretty sure.

From my understanding its something like this
Higher Compression Ratio = More power @ the Cost of knock
Lower Compression Ratio = Less power but more reliable with less knock

Can I get a second person to confirm?

Thanks Francisco!

Tony - The gaskets for the standard rex and the sti from my08 are the same. Even the same part number from subaru.

From what i can see, the compression ratio doesn't look it is going to change all that much with a difference of 2cc's across the face of the piston.

The biggest problem im having is finding something definitive. I am starting wonder if its worth going to a 257 at all and just getting a 255 short motor and doing a straight swap?
Anyone reckon its worth going for a 257 over the standard 255?

I think maybe you can ask Michael @ msr as I believe he builds engines.

viewtopic.php?f=103&t=1383

I goes like this Higher Compression = More power less boost
Lower compression = More boost to get to the same power as a higher comp motor,

No engine will knock if tuned correctly

Cheers for that Francisco.
Ive shot Michael quick message now. Hopefully i can get this sorted and get back on the road asap! I miss the ugly bugger of a thing

Pretty sure the compression does not change? only when you use 2.0L heads on a 2.5 block it increases?

I sell 4 255's to every 257 i sell, I allways recomend for build ups the 255's to save some coin.

And yes same headgasket for the hole 2006+ EJ2.5T range.. forester,wrx and liberty use the same gasket.

Yes higher comp due to the smaller CC of the 2L heads. This is fixed by using the right head gasket.

We are doing them atm for trade price.. viewtopic.php?f=2&t=2072

Kris