Welcome to the North American Subaru Impreza Owners Club Sunday April 20, 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 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 03-29-2009, 05:53 PM   #1
subadrew99
Scooby Newbie
 
Member#: 204755
Join Date: Mar 2009
Default EJ25 SOHC vs DOHC

Hey just wondering what the pros and cons are to the SOHC EJ25 and the DOHC EJ25.
* Registered users of the site do not see these ads.
subadrew99 is offline   Reply With Quote
Old 03-29-2009, 06:46 PM   #2
Smoke
Scooby Guru
 
Member#: 172486
Join Date: Feb 2008
Chapter/Region: NWIC
Location: Cheney, WA
Vehicle:
95 V4 STi-RA swap
Stage 49.8

Default

The DOHC were known to have head gasket difficulties.

Edit: For the 2.5rs ones of course

Last edited by Smoke; 03-29-2009 at 08:03 PM.
Smoke is offline   Reply With Quote
Old 03-29-2009, 07:30 PM   #3
RaceFaceXC
NASIOC Supporter
 
Member#: 165749
Join Date: Dec 2007
Chapter/Region: South East
Location: stupid Florida
Vehicle:
2007 No YELLING
on the bus!

Default

MY05-07(08-09?) SOHC's have great intake manifolds
RaceFaceXC is online now   Reply With Quote
Old 03-29-2009, 09:49 PM   #4
Back Road Runner
Scooby Guru
 
Member#: 60082
Join Date: Apr 2004
Chapter/Region: MWSOC
Location: Minnesota
Vehicle:
2004 Forester STI
Silver

Default

There some dyno graphs floating around directly comparing the DOHC and SOHC engines. Cobb also wrote some tech articles a few years back discussing both engine types. You can't find them on Cobb's site now, but Archive.org's Wayback Machine can give you them from older, stored pages. I know there's been various discussions on both engines. My impression is that the SOHC is basically superior, but there may be cases arguing both ways. With some modifications, the SOHC engine has no problem with top end power.
Back Road Runner is offline   Reply With Quote
Old 03-30-2009, 12:38 AM   #5
PA-Outback2000
Scooby Specialist
 
Member#: 161832
Join Date: Oct 2007
Chapter/Region: Tri-State
Location: PA
Vehicle:
2000 Outback Limited
Black

Default

da risin smoke... 2.5rs, aka N/A's, have the SOHC, not DOHC. i have a 2000 2.5L and had the HG changed at around 110,000 miles.

SOHC are fine within the normal 6500rpm redline, but the DOHC flow more b/c they have 2 camshafts doing the work of one. there really isn't anything "wrong" with the SOHC, but i don't think it would be worth it to change to a DOHC if u r currently running a SOHC.

i am keeping the car SOHC throughout all the mods i plan on doing.
PA-Outback2000 is offline   Reply With Quote
Old 03-30-2009, 01:15 AM   #6
subadrew99
Scooby Newbie
 
Member#: 204755
Join Date: Mar 2009
Default

I have the choice between a 98 and 99 2.5rs so I'm just weighing out the pros and cons between the two. Basically I'm just wondering which is better performance wise and which is more reliable and overall which is better
subadrew99 is offline   Reply With Quote
Old 03-30-2009, 07:09 AM   #7
Storm
Scooby Guru
 
Member#: 5218
Join Date: Mar 2001
Chapter/Region: MWSOC
Location: SAUL'S Motorsports
Vehicle:
96L Most Over-
Developed Beater

Default

You've never seen the 96 Outback with an EJ25D or 98RS with the EJ25D, or any of the few 99RS' that also got the DOHC heads?


Quote:
Originally Posted by PA-Outback2000 View Post
da risin smoke... 2.5rs, aka N/A's, have the SOHC, not DOHC. i have a 2000 2.5L and had the HG changed at around 110,000 miles.

SOHC are fine within the normal 6500rpm redline, but the DOHC flow more b/c they have 2 camshafts doing the work of one. there really isn't anything "wrong" with the SOHC, but i don't think it would be worth it to change to a DOHC if u r currently running a SOHC.

i am keeping the car SOHC throughout all the mods i plan on doing.
The difference between the DOHC and SOHC is not all that great. SOHC has more torque while the DOHC has more top end pull. The difference is so small that it could boil down to port design or just a matter of ECU tune that was put on the car. Nothing really to do with the number of cams or anything like that.

The SOHC is simple to work on, where the DOHC requires a bit more attention to detail during timing belt changes or headgasket changes. I would go with a SOHC motor for that reason alone.

Jay
Storm is offline   Reply With Quote
Old 03-30-2009, 10:15 AM   #8
subadrew99
Scooby Newbie
 
Member#: 204755
Join Date: Mar 2009
Default

Thanks for the info that was what I was looking for
subadrew99 is offline   Reply With Quote
Old 03-30-2009, 04:43 PM   #9
PA-Outback2000
Scooby Specialist
 
Member#: 161832
Join Date: Oct 2007
Chapter/Region: Tri-State
Location: PA
Vehicle:
2000 Outback Limited
Black

Default

99rs switched back to SOHC for N/A's. my bad about the others.
PA-Outback2000 is offline   Reply With Quote
Old 03-31-2009, 01:29 AM   #10
ballitch
Scooby Specialist
 
Member#: 74907
Join Date: Nov 2004
Chapter/Region: NWIC
Location: salem, OR
Vehicle:
98 cam'd EJ251
Wrx Tranny,slowr than ish

Default

Quote:
Originally Posted by Storm View Post
You've never seen the 96 Outback with an EJ25D or 98RS with the EJ25D, or any of the few 99RS' that also got the DOHC heads?



The difference between the DOHC and SOHC is not all that great. SOHC has more torque while the DOHC has more top end pull. The difference is so small that it could boil down to port design or just a matter of ECU tune that was put on the car. Nothing really to do with the number of cams or anything like that.

The SOHC is simple to work on, where the DOHC requires a bit more attention to detail during timing belt changes or headgasket changes. I would go with a SOHC motor for that reason alone.

Jay
I'm with you on that one, thats why I stuck with SOHC heads. Those graphs have to be here somewhere. Those alone made my mind up for me.

~Josh~











my woman wants me to put this in.......... she loves it......
ballitch is offline   Reply With Quote
Old 03-31-2009, 08:28 AM   #11
STiluv55
Scooby Newbie
 
Member#: 206102
Join Date: Mar 2009
Location: SE WI
Vehicle:
1999 2.5 RS
Black

Default

Quote:
Originally Posted by Storm View Post
You've never seen the 96 Outback with an EJ25D or 98RS with the EJ25D, or any of the few 99RS' that also got the DOHC heads?



The difference between the DOHC and SOHC is not all that great. SOHC has more torque while the DOHC has more top end pull. The difference is so small that it could boil down to port design or just a matter of ECU tune that was put on the car. Nothing really to do with the number of cams or anything like that.

The SOHC is simple to work on, where the DOHC requires a bit more attention to detail during timing belt changes or headgasket changes. I would go with a SOHC motor for that reason alone.

Jay
I see you have spoken of a tune. Do you mean aftermarket performance tune or the stock one? Would a tune be worth the money if you had CAI short ram and full exhaust with high flow cat on a SOHC N/A? Or would the increase in hp you get from the tune be so little it's a waste of money?
STiluv55 is offline   Reply With Quote
Old 03-31-2009, 03:44 PM   #12
Storm
Scooby Guru
 
Member#: 5218
Join Date: Mar 2001
Chapter/Region: MWSOC
Location: SAUL'S Motorsports
Vehicle:
96L Most Over-
Developed Beater

Default

I meant the OEM tune on the ECU. Even the base timing is more advanced according to the SOHC FSM ('00). If you have the means to do it, an optimized tune is ALWAYS worth it. There is a substantial amount left on the table compared to the stock maps.

Jay
Quote:
Originally Posted by STiluv55 View Post
I see you have spoken of a tune. Do you mean aftermarket performance tune or the stock one? Would a tune be worth the money if you had CAI short ram and full exhaust with high flow cat on a SOHC N/A? Or would the increase in hp you get from the tune be so little it's a waste of money?
Storm is offline   Reply With Quote
Old 03-31-2009, 03:52 PM   #13
VWVancouver
Scooby Newbie
 
Member#: 14507
Join Date: Jan 2002
Location: Surrey, B.C.
Vehicle:
1984

Default

Hey guys, I'm looking at getting a 2003 or 2004 Forester. Just wondering when the head gasket was upgraded? Was is 2004 they fixed the problem? TIA.
VWVancouver is offline   Reply With Quote
Old 03-31-2009, 07:19 PM   #14
Storm
Scooby Guru
 
Member#: 5218
Join Date: Mar 2001
Chapter/Region: MWSOC
Location: SAUL'S Motorsports
Vehicle:
96L Most Over-
Developed Beater

Default

It was updated well before then. It will be a SOHC EJ25 and should hold up just fine.

Jay
Quote:
Originally Posted by VWVancouver View Post
Hey guys, I'm looking at getting a 2003 or 2004 Forester. Just wondering when the head gasket was upgraded? Was is 2004 they fixed the problem? TIA.
Storm is offline   Reply With Quote
Old 04-01-2009, 07:30 PM   #15
VWVancouver
Scooby Newbie
 
Member#: 14507
Join Date: Jan 2002
Location: Surrey, B.C.
Vehicle:
1984

Default

Thanks Jay.
VWVancouver is offline   Reply With Quote
Old 04-01-2009, 07:58 PM   #16
FuJi K
Scooby Guru
 
Member#: 3687
Join Date: Jan 2001
Chapter/Region: MWSOC
Location: St. Paul, MN
Vehicle:
MY99 GF4 JDM 6spd
NF Performance

Default

The 2.5 DOHC has larger intake ports, larger intake manifold runners. The exhaust however is a tad choked where the valve guide support is compared to the 2.5L SOHC. The 2.5L SOHC has decent intake ports, straight shot, but smaller intake manifold runner. Also the SOHC runs a higher compression ratio compared to the DOHC. This gives it more power to start.

If they ran the phase2 shortblock on the DOHC heads, that'd make more power than the SOHC no doubt. If BOTH where fitted with an after market turbo, the DOHC would make more TOP END power because of the better flowing heads and intake manifold. I see a good amount of SOHC turbo's RS's fall off in power above 6000rpms all the time and it falls fast unlike the USDM WRX/STi where the intake manifold is larger. What also plays a roll is the camshafts too.. but... if you ever seen the intake manifold, and heads of the SOHC/DOHC NA and WRX/STi DOHC you'll know wassup.

BOTH motors has leaks of their own kind. The DOHC with overheating issues, the SOHC with oil leak issues.
FuJi K is offline   Reply With Quote
Old 04-02-2009, 12:28 AM   #17
subadrew99
Scooby Newbie
 
Member#: 204755
Join Date: Mar 2009
Default

to do a swap from DOHC to SOHC would you need to do a complete wiring harness swap?
subadrew99 is offline   Reply With Quote
Old 04-02-2009, 03:40 PM   #18
Storm
Scooby Guru
 
Member#: 5218
Join Date: Mar 2001
Chapter/Region: MWSOC
Location: SAUL'S Motorsports
Vehicle:
96L Most Over-
Developed Beater

Default

Not really.......but the (lack of) EGR and IAC becomes an issue as the DOHC used an externally mounted solenoid. Depending on the year EJ25 you use, the IAC will be a 3 wire solenoid (but still not compatable), or 6 wire stepper motor which is just out of the question.....unless you're good at building microcontroller circuits. Only certain year and emission spec. SOHC motors got an EGR system too.

Otherwise, the motor will start and run fine off the EJ25D ECU.....it'll just have a permanent CEL.
Storm is offline   Reply With Quote
Old 04-02-2009, 04:02 PM   #19
bubba2533
Scooby Newbie
 
Member#: 175483
Join Date: Mar 2008
Chapter/Region: MAIC
Location: Pittsburgh PA
Vehicle:
13 WRX 5-Door
SWP

Default

Some EJ25D motors had EGR

or did you mean EJ25 SOHC didn't have EGR? (what you said is kind of confusing)
bubba2533 is offline   Reply With Quote
Old 06-04-2010, 07:51 PM   #20
calgearhead
Scooby Newbie
 
Member#: 133745
Join Date: Dec 2006
Location: NorCal Gold Country/ helL A
Vehicle:
1998 Lifted Outback
Green + Dirt

Default

Sorry to drag this back from the past but as Storm mentioned about the DOHC to SOHC swap on a Phase one (1997) ride, it will run and drive fine without an ECU swap or anything but with a perma-CEL.
calgearhead 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
Sohc Vs Dohc Ej25 BlckBeautyVTEC Southern California Impreza Club Forum -- SCIC 3 09-22-2006 04:33 PM
EJ25 - SOHC vs DOHC 2.5rsDOHC Normally Aspirated Powertrain 8 08-04-2004 11:38 AM
EJ25, SOHC to DOHC Honkey3k Normally Aspirated Powertrain 7 07-22-2004 02:03 PM
Subaru EJ25 SOHC & DOHC head components (valvetrain, cams etc) iON Performance Inc Vendor 'For Sale' Classifieds 21 03-09-2004 02:34 AM
EJ25 SOHC to DOHC and back again? JoShYo Normally Aspirated Powertrain 16 11-14-2003 11:24 AM


All times are GMT -4. The time now is 04:43 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.