Welcome to the North American Subaru Impreza Owners Club Wednesday July 30, 2014
Home Forums WikiNASIOC Products Store Modifications Upgrade Garage
NASIOC
Here you can view your subscribed threads, work with private messages and edit your profile and preferences Home Registration is free! Visit the NASIOC Store NASIOC Rules Search Find other members Frequently Asked Questions Calendar Archive NASIOC Upgrade Garage Logout
Go Back   NASIOC > NASIOC Technical > Normally Aspirated with bolt-on Forced Induction Powertrain

Welcome to NASIOC - The world's largest online community for Subaru enthusiasts!
Welcome to the NASIOC.com Subaru forum.

You are currently viewing our forum as a guest, which gives you limited access to view most discussions and access our other features. By joining our community, free of charge, you will have access to post topics, communicate privately with other members (PM), respond to polls, upload content and access many other special features. Registration is free, fast and simple, so please join our community today!

If you have any problems with the registration process or your account login, please contact us.
* Registered users of the site do not see these ads.
Reply
 
Thread Tools Display Modes
Old 10-06-2005, 04:22 PM   #1
cdigerlando
Scooby Specialist
 
Member#: 6070
Join Date: Apr 2001
Chapter/Region: South East
Location: Longwood, FL
Vehicle:
9202 SVX, WRX
92 Sil/Blk, 02 Sil

Default Lower Compression-Thick Head Gasket

Has anyone lowered their compression using a thicker head gasket? If so how much HP are you running? What thickness did you use? How many miles do you have with the mod?

I'm working on a turbo SVX project. I hear that changing the pistons is a better way to go, but if people are having success with this mod on their cars I think I may do this to save some $$$$. I am considering lowering the compression from 10.1:1 to 8.5:1.

Thanks for your input.

Chuck D.
* Registered users of the site do not see these ads.
cdigerlando is offline   Reply With Quote
Old 10-06-2005, 04:35 PM   #2
Master2192
RIP 10/12/83 to 02/10/08
Scooby Specialist
 
Member#: 83254
Join Date: Mar 2005
Chapter/Region: NWIC
Location: Roy, Washington
Vehicle:
98 2.5 RS
04 WRX

Default

You'll NEVER lower the compression that much with a gasket. I would say you could raise or lower the compression by as much as .3 with a gasket.

Definitely look into getting pistons or having the heads milled out to increase the combustion area. Sounds like you are going to try forced induction, stronger pistons would definitely be advisable.
Master2192 is offline   Reply With Quote
Old 10-06-2005, 07:59 PM   #3
no-coast-punk
Scooby Specialist
 
Member#: 49087
Join Date: Nov 2003
Chapter/Region: RMIC
Location: Inside your carnot cycle
Vehicle:
1998 RST V8 STi swap
'05 R6 '95 BMW V8 hotness

Default

EJ22t rods/pistons are a straight drop in for your motor. As for the compression thing... there are many engine math resources out there to see how head gasket thickness effects compression.

Thicker/Thinner head gaskets are more prone to blowing unless they're something fairly exotic. Fuji engineers aren't stupid and spent a great deal of time determining how thick the head gasket needed to be to cope with normal expansion/warping of the head/block mating surface during normal operation.
no-coast-punk is offline   Reply With Quote
Old 10-06-2005, 08:11 PM   #4
ai42
Scooby Specialist
 
Member#: 71348
Join Date: Sep 2004
Chapter/Region: TXIC
Location: Frisco, TX
Vehicle:
1999 EJ207 RS.TI
VW Golf TDI 40mpg baby!

Default

http://wac.addr.com/auto/obs/turbo/ejcalcs.html

That is some good info on the effects of a thick headgasket. In the MY00 EJ25 it goes from 11.2:1 to 10.1:1 (I question the first numbers accuracy), and in a Legacy EJ22t from 8.0:1 to 7.2:1.

So roughtly .6-.8 compression differance is what I would say. I don't know what SVX's stock compression is but if you are not under 9.0:1 (or close) I would say that is risky to run anything above 8psi.

Of course a large intercooler, and water/alcohol injection can offset some detonoation.

Last edited by ai42; 10-06-2005 at 08:50 PM.
ai42 is offline   Reply With Quote
Old 10-06-2005, 09:24 PM   #5
Tats
Scooby Specialist
 
Member#: 5692
Join Date: Apr 2001
Chapter/Region: Tri-State
Location: Lovely Florida, NY
Vehicle:
WTF? YES! I am
better than you!

Default

Quote:
Originally Posted by ai42
http://wac.addr.com/auto/obs/turbo/ejcalcs.html

That is some good info on the effects of a thick headgasket. In the MY00 EJ25 it goes from 11.2:1 to 10.1:1 (I question the first numbers accuracy), and in a Legacy EJ22t from 8.0:1 to 7.2:1.

So roughtly .6-.8 compression differance is what I would say. I don't know what SVX's stock compression is but if you are not under 9.0:1 (or close) I would say that is risky to run anything above 8psi.

Of course a large intercooler, and water/alcohol injection can offset some detonoation.
Stock MY00 ej25 is 10.1:1 so something is off.
Tats is offline   Reply With Quote
Old 10-06-2005, 09:40 PM   #6
Portly
Scooby Specialist
 
Member#: 1197
Join Date: Apr 2000
Chapter/Region: MWSOC
Location: Cedar Rapids, Iowa
Vehicle:
1997 Impreza Wagon
Mystic Blue Pearl

Default

Quote:
Originally Posted by Tats
Stock MY00 ej25 is 10.1:1 so something is off.
Stock MY00 is "factory rated" at 10.1:1 - Wayne actually measured volume and ran the calcs. True compression ratio turns out to be a lot higher than the factory states, as he notes on his site.

FWIW, a thicker headgasket will really mess with your valve timing. By spacing the heads out, it throws off the stock timing rather quickly.

The really bad part is, the geometry of the timing belt means that for every degree of timing change you get on the passenger side head, you get 3 degrees on the driver side.

Back when I replaced the head gaskets on my '99 engine, I thought about using '98 gaskets instead. The reduction in compression ratio was very nice. But when I did some calculations on the timing change, it was pretty scary. So I realized it was a VERY bad idea.

_Jeff
Portly is offline   Reply With Quote
Old 10-07-2005, 12:31 PM   #7
Warp3
NASIOC Supporter
 
Member#: 161
Join Date: Aug 1999
Chapter/Region: South East
Location: WNC
Vehicle:
06 WRX TR (SGM)
89 MR2 (Ice Blue)

Default

I also find it odd that he lists the 99 EJ25 as 10:1 (not in the calculated section), but it was advertised as 9.7:1 (the 99 and 2000 models both have the same SOHC engine, but use different piston designs).

The calculated CR differing from advertised CR is definitely interesting, though. Could this possibly explain why a supposed increase of only 0.3 on the CR (9.7:1 to 10:1), led to the 2000+ engines taking boost so much more poorly than the 99 engines? Too bad the calculated field is blank on the MY99 line.

Shane
Warp3 is offline   Reply With Quote
Old 10-07-2005, 02:34 PM   #8
cdigerlando
Scooby Specialist
 
Member#: 6070
Join Date: Apr 2001
Chapter/Region: South East
Location: Longwood, FL
Vehicle:
9202 SVX, WRX
92 Sil/Blk, 02 Sil

Default Thanks for Replies

Thanks for the replies. You have all confirmed my fears. I will stick with stock, and probably change the pistons.

Thanks
cdigerlando is offline   Reply With Quote
Old 10-07-2005, 05:48 PM   #9
Portly
Scooby Specialist
 
Member#: 1197
Join Date: Apr 2000
Chapter/Region: MWSOC
Location: Cedar Rapids, Iowa
Vehicle:
1997 Impreza Wagon
Mystic Blue Pearl

Default

Quote:
Originally Posted by Warp3
Could this possibly explain why a supposed increase of only 0.3 on the CR (9.7:1 to 10:1), led to the 2000+ engines taking boost so much more poorly than the 99 engines?
I believe I have also read (but have not seen side-by-side photo confirmation) that the '99 piston is built stronger than the '00. More metal at the top before the first ring, apparently.

So it seems the '99 engine has 2 things that make it better for boost than an '00. Well, 3, if you count the intake manifold, since it accepts more-common fuel injectors. But I digress.

_Jeff
Portly is offline   Reply With Quote
Old 10-07-2005, 05:57 PM   #10
Master2192
RIP 10/12/83 to 02/10/08
Scooby Specialist
 
Member#: 83254
Join Date: Mar 2005
Chapter/Region: NWIC
Location: Roy, Washington
Vehicle:
98 2.5 RS
04 WRX

Default

What about the 98s? When i got my car, the company tore down the engine and cleaned out all the carbon, replaced all the seals and gaskets. And when he showed me the Stock Head Gaskets to some thinner ones, i decided on the thinner gasket. If i can find out the two thicknesses, can i figure out my total compression?

I just guessed that it went from 9.7:1 to 10:1
Master2192 is offline   Reply With Quote
Old 10-08-2005, 09:58 AM   #11
Portly
Scooby Specialist
 
Member#: 1197
Join Date: Apr 2000
Chapter/Region: MWSOC
Location: Cedar Rapids, Iowa
Vehicle:
1997 Impreza Wagon
Mystic Blue Pearl

Default

Quote:
Originally Posted by Master2192
I just guessed that it went from 9.7:1 to 10:1
If they used the later SOHC gaskets, I think they're only about 1/3 the thickness of the stock DOHC gaskets.

You'd have to do the math based on the volume numbers given on wayne's site, but you're in the upper 10's now. Looking at his calcs, I'd guesstimate 10.8:1.

http://wac.addr.com/auto/obs/turbo/ejcalcs.html (as posted earlier.)

In addition ot a higher compression ratio, your cam timing is now somewhat screwed up, just like I mentioned in an earlier post. Again, the biggest problem is not the change in timing, it's that the timing change is different from one side of the engine to the other.

_Jeff
Portly is offline   Reply With Quote
Old 10-08-2005, 02:50 PM   #12
nosmo
Scooby Specialist
 
Member#: 53142
Join Date: Jan 2004
Chapter/Region: SWIC
Location: Las Vegas
Vehicle:
06' STI,02WRX,982.5
www.Flat4lv.com

Default

That only works for nitro Rc cars.
nosmo is offline   Reply With Quote
Old 10-08-2005, 05:12 PM   #13
Portly
Scooby Specialist
 
Member#: 1197
Join Date: Apr 2000
Chapter/Region: MWSOC
Location: Cedar Rapids, Iowa
Vehicle:
1997 Impreza Wagon
Mystic Blue Pearl

Default

Quote:
Originally Posted by nosmo
That only works for nitro Rc cars.
Whose post are you referring to? What only works for nitro RC cars?

_Jeff
Portly is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Help on buying a used Legacy Outback. Head Gasket Help. Tirpitz56 Normally Aspirated Powertrain 3 06-03-2010 12:01 AM
Head gasket thickness - Is thicker/thinner more likely to leak? bugeyes Built Motor Discussion 3 07-31-2006 02:00 PM
Thickness of stock SOHC head gasket? jasona Normally Aspirated Powertrain 2 06-08-2005 07:42 PM
Ver 7 Head Gasket thickness DuoMaxwell Subaru Conversions 4 09-15-2004 06:55 AM
Head gasket thickness! 2mm? IA Performance Factory 2.0L Turbo Powertrain 4 11-18-2003 12:21 PM


All times are GMT -4. The time now is 04:54 PM.


Powered by vBulletin® Version 3.7.0
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
Powered by Searchlight © 2014 Axivo Inc.
Copyright ©1999 - 2014, North American Subaru Impreza Owners Club, Inc.