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Old 11-19-2006, 12:25 AM   #1
Turbogt
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Default STI short block/JDM wrx heads?

I know there, alot of threads on this, but none talk about the use of JDM heads with a USDM 2.5L short block. I have heads from a 97 JDM EJ20T I'm using on my swap. Jon a outback told me this will yeild me a 8.5 to 1 compression ratio. Can anyone confirm this?
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Old 11-19-2006, 07:49 AM   #2
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http://forums.nasioc.com/forums/showthread.php?t=630797 To be on the safe side you should have someone CC your combustion chamber. But it looks like you'd be at 8.7:1 if they're JDM V7 Spec C heads with a ~50cc chamber.
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Old 11-19-2006, 10:52 AM   #3
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you need the CC of your combustion chamber...and also head gasket thickness when compressed.
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Old 11-19-2006, 11:44 AM   #4
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Right now I have stock sti head gaskets. How can I find the cc of combustion chamber? Thanks for replys.
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Old 11-19-2006, 12:40 PM   #5
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good question, needing this info as well
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Old 11-19-2006, 12:47 PM   #6
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Are these the heads with fins on the top and bottom of the valve cover? What do your heads look like? Hyd. or Mech.?
Did you measure the thickness of your old gaskets?
Were the ones you took off .060?
Sti are .040
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Old 11-19-2006, 01:01 PM   #7
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Here's the meat and potatoes of the link in the 2nd post.

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Old 11-19-2006, 01:14 PM   #8
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Here's an online calculator that seems to work well: http://www.csgnetwork.com/compcalc.html
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Old 11-19-2006, 01:22 PM   #9
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No fins on valve covers, three protruding horizontal lines over 4CAM 16VALVE . Pretty sure it's mechanical. Has gold colored metal cam gears, but that's all that looks different to me. Not sure what old gaskets were, already thrown away. I don't plan to run big boost(aiming for 300whp) so is 8.7 to 1 CP a problem?
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Old 11-19-2006, 01:42 PM   #10
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Quote:
Originally Posted by Turbogt View Post
..is 8.7 to 1 CP a problem?
More and more people are running with a higher CR without any problems. If anything your motor will have more torque and spool faster. You do NOT want your total quench height to be more than 1mm.. issues with detonation are more likely to occur. Using thicker head gaskets will give you a quench height of 2mm (twice the desired amount) and throw off your cam timing from one side of the motor to the other.
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Old 11-19-2006, 01:57 PM   #11
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Quote:
Originally Posted by PeteDucati View Post
Here's the meat and potatoes of the link in the 2nd post.

the EJ257/USDM sti heads seems off...
who did the numbers/measurements on that?
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Old 11-19-2006, 02:36 PM   #12
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Jeff Sponaugle of PDXTuning.com with thanks to Ron at Axis for a wealth of information on this topic.

You could always read this thread (the one in the 2nd post). http://forums.nasioc.com/forums/showthread.php?t=630797
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Old 11-19-2006, 06:26 PM   #13
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The gasket thickness is wrong.
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Old 11-19-2006, 06:55 PM   #14
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Quote:
Originally Posted by Hopper View Post
The gasket thickness is wrong.
I'm sorry, but your statement contradicts Crawford, Axis, and someone who MEASURED a crushed STi headgasket with a micrometer. Please back up your findings.
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Old 11-19-2006, 09:00 PM   #15
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On your spread sheet, gasket thickness is shown as 0.60mm that = .023
All of the ones that I have measured = .040 or 1.016mm

Like I said before itís wrong!
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Old 11-19-2006, 10:21 PM   #16
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You can always work it out for yourself here: http://www.dunegoon.org/compression.html
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Old 11-19-2006, 10:43 PM   #17
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solving for compression ratio isn't hard...getting the right values is where it always get's f'ed up
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Old 11-19-2006, 11:11 PM   #18
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While we're on the subject... I'm aiming for a similar setup...

Is it agreed that going with the hybrid EJ257 shortblock with EJ207 heads (and EJ207 ECU) is beneficial over a more straightforward EJ257 longblock and EJ257 ECU?

Just looking for a little reassurance

Bryan
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Old 11-20-2006, 01:07 AM   #19
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Quote:
Originally Posted by Wrinkleboi View Post
While we're on the subject... I'm aiming for a similar setup...

Is it agreed that going with the hybrid EJ257 shortblock with EJ207 heads (and EJ207 ECU) is beneficial over a more straightforward EJ257 longblock and EJ257 ECU?

Just looking for a little reassurance

Bryan
Yes, the 207 heads are better.
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Old 11-20-2006, 12:18 PM   #20
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The heads you have are early (MY92-96) EJ20G heads with hydraulic lash adjusters. These run pretty good cams. The head CC is 46-47cc.

The have smaller ports than the later EJ207 heads, but ports are about the same size as USDM EJ205 heads.

With a standard USDM 2.5 STI bottom end and head gasket CR will be 8.7:1 which is OK if you don't plan on running a lot of boost (above 18 psi).
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Old 11-20-2006, 01:09 PM   #21
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You'll find the CR will be more around 9.1:1 with a 46cc chamber and the 2.5 sti headgaskets.
It'll be around 8.4:1 if you use the 2.5n/a 1.6mm gaskets.
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Old 11-22-2006, 03:22 PM   #22
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Quote:
Originally Posted by Hopper View Post
On your spread sheet, gasket thickness is shown as 0.60mm that = .023
All of the ones that I have measured = .040 or 1.016mm

Like I said before itís wrong!
That's not my spreadsheet.. it's a cut and paste from the thread started by Jeff Sponaugle of PDXTuning. For some reason Ron Williams, Quirt Crawford, and Jeff all think the gasket is .6mm crushed. When using that value to calculate the CR with the rest of the engine dimensions that are not disputed you get the factory specified CR. If the gasket were 1mm the CR would be 7.8:1, and we know that's off. I'm just relaying info I found in another thread that's backed up by three people who are all well respected.
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