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Old 10-06-2008, 11:03 AM   #1
jblackwell
Scooby Newbie
 
Member#: 190324
Join Date: Sep 2008
Chapter/Region: SWIC
Vehicle:
2004 STI
World Rally Blue

Question WRX 2.0 heads on STi bottom end... Compression?

OK,

I can not find any source for a comparison between the combustion chamber volume of the 2.0 WRX head and the 2.5 STi head. The engine is assembled and the cylinder pressure is in the mid to high 170's. 173 to 178 was the spread from low to high.

Anyhow, does anyone have any idea what the compression ratio is with this combination? I'm currently contemplating tearing it apart and throwing STi heads on the motor if I'm in the mid 9's with compression.

Anyone?
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Old 10-06-2008, 11:47 AM   #2
sponaugle
Scooby Specialist
 
Member#: 4498
Join Date: Feb 2001
Chapter/Region: NWIC
Location: Portland, Oregon
Vehicle:
WRX H6-3.0 Turbo
www.surgelinetuning.com

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Quote:
Originally Posted by jblackwell View Post
OK,

I can not find any source for a comparison between the combustion chamber volume of the 2.0 WRX head and the 2.5 STi head. The engine is assembled and the cylinder pressure is in the mid to high 170's. 173 to 178 was the spread from low to high.

Anyhow, does anyone have any idea what the compression ratio is with this combination? I'm currently contemplating tearing it apart and throwing STi heads on the motor if I'm in the mid 9's with compression.

Anyone?
This has been covered at length in the 2.5L forums over the last 5 years.

To help out, take a look at the following:




Then read some of these threads:

http://forums.nasioc.com/forums/show...1594587&page=2
http://forums.nasioc.com/forums/show....php?t=1547876

Also, if you search for post in the 2.5L forum dealing with WRX heads and EJ257 you will find a plethora of examples.

To more directly answer your question, indeed the compression is higher. The estimate from the chart above is 8.87, but if the heads were surfaced at all, it would be higher. Also, if you used an aftermarket piston that is slightly lower in dish it would be even higher. Your should also be considerate of the difference between the WRX and STI cams.

Cheers,

Jeff Sponaugle
Surgeline Tuning
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Old 10-23-2008, 08:33 PM   #3
aaronthehic
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Member#: 41577
Join Date: Aug 2003
Location: guelph canada
Vehicle:
2000 impreza rs

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i ran both a 2.5 sti block with cometic 3mm gasket and a stock sti and prefer the stock sti gasket

18psi on v5 head

stock gasket ran smother and more responsive
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