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Old 02-09-2009, 09:39 AM   #1
Vlad
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Default 2.5 WRX internals vs Sti internals

I always wanted to know:
As far as engine internals (pistons, rods, crankshaft, block) are the 2.5 engines very similar or identical? (Sti and WRX)
The Sti has the AVCS that I don't think the WRX has, which will probably make the heads different, even as castings, I would suspect.

A consequence could be that if you're swapping an Sti engine into a WRX but cannot make provision to operate the AVCS, you have a car with the same engine features and potential that a 2.5 WRX has. Am I wrong?
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Old 02-09-2009, 10:00 AM   #2
blackdub06
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Default

They are nearly identical. They have different pistons and they both have AVCS but the heads are slightly different due to the air pump. Other than that I'm pretty sure they are the same.
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Old 02-09-2009, 10:18 AM   #3
the suicidal eggroll
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The wrx also has slightly higher compression (8.5 vs 8.2)
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Old 02-09-2009, 10:20 AM   #4
nhat
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http://forums.nasioc.com/forums/show....php?t=1667168
http://forums.nasioc.com/forums/show....php?t=1594587

Quote:
Originally Posted by sponaugle View Post
As I suggested in my original post, it is not typical for Subaru to have different part numbers for the same part, especially when it forces a cascade of part changes.

As for the local shops, hundreds of people on the forums, and your eyes, they might need glasses. Just kidding.

I picked up a brand new 2006 WRX pistons, and well as 2006 STI piston. (Of course I am $70 lighter just for the sake of clearing up this discussion. I don't like having questions about the accuracy of data hanging around. )


See the picture below.



WRX on the left, STI on the right

If you look carefully you can see that the dish on the WRX piston is deeper then the STI. In person it is very obvious, but I took that picture at that angle to make it eaiser to see.

The pistons are indeed different.

The STI pistons has a dish volume of 22ccs, while the WRX one is 25.5ccs, and difference of 15.9%. Given that the WRX EJ255 has a higher overall compression ratio, this would suggest that the EJ255 heads (in the WRX) must have a smaller combusion chamber by quite a bit.

This discovery leads to a few other interesting conclusions as well. More to come on that.

Cheers,

Jeff Sponaugle
Quote:
Originally Posted by sponaugle View Post
Don't be too hard. I have made mistakes in the past. They key is recognizing the missteps and correcting them when possible. This does open up an interesting suggest: If you are doing a hybrid EJ257 SB + EJ205 WRX Heads you should consider instead using the 06-08 WRX EJ255 SB, which will net you lower (closer to normal) compression.

Consider the following:



I highlighted the 50cc number for the 06+ WRX heads as that is a guess. I have not actually measured them myself. If anyone else has made an accurate measurement, let me know. You can clearly see that if you are doing an EJ205 WRX head + EJ25X SB Hybrid, you are better off starting with the 06+WRX EJ255 block instead of the 04-07 STI EJ257 SB.

Cheers,

Jeff Sponaugle
Surgeline Tuning
and there you have it. search! took me less than a minute to come up with all that...

Last edited by nhat; 02-09-2009 at 10:25 AM.
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