EJ25 SOHC vs DOHC

subadrew99 · 03-29-2009, 05:53 PM

Hey just wondering what the pros and cons are to the SOHC EJ25 and the DOHC EJ25.

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Smoke · 03-29-2009, 06:46 PM

The DOHC were known to have head gasket difficulties.

Edit: For the 2.5rs ones of course

RaceFaceXC · 03-29-2009, 07:30 PM

MY05-07(08-09?) SOHC's have great intake manifolds

Back Road Runner · 03-29-2009, 09:49 PM

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.

PA-Outback2000 · 03-30-2009, 12:38 AM

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.

subadrew99 · 03-30-2009, 01:15 AM

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

Storm · 03-30-2009, 07:09 AM

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

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

subadrew99 · 03-30-2009, 10:15 AM

Thanks for the info that was what I was looking for

PA-Outback2000 · 03-30-2009, 04:43 PM

99rs switched back to SOHC for N/A's. my bad about the others.

ballitch · 03-31-2009, 01:29 AM

Quote:

Originally Posted by Storm

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......

STiluv55 · 03-31-2009, 08:28 AM

Quote:

Originally Posted by Storm

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?

Storm · 03-31-2009, 03:44 PM

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

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?

VWVancouver · 03-31-2009, 03:52 PM

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 · 03-31-2009, 07:19 PM

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

Jay

Quote:

Originally Posted by VWVancouver

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 · 04-01-2009, 07:30 PM

Thanks Jay.

FuJi K · 04-01-2009, 07:58 PM

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.

subadrew99 · 04-02-2009, 12:28 AM

to do a swap from DOHC to SOHC would you need to do a complete wiring harness swap?

Storm · 04-02-2009, 03:40 PM

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.

bubba2533 · 04-02-2009, 04:02 PM

Some EJ25D motors had EGR

or did you mean EJ25 SOHC didn't have EGR? (what you said is kind of confusing)

calgearhead · 06-04-2010, 07:51 PM

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.

03-29-2009, 05:53 PM	#1
subadrew99 Scooby Newbie Member#: 204755 Join Date: Mar 2009	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.

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	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.

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03-29-2009, 07:30 PM	#3
RaceFaceXC Scooby Specialist Member#: 165749 Join Date: Dec 2007 Chapter/Region: South East Location: stupid Florida Vehicle: 2007 No YELLING on the bus!	MY05-07(08-09?) SOHC's have great intake manifolds

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	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.

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	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.

03-30-2009, 01:15 AM	#6
subadrew99 Scooby Newbie Member#: 204755 Join Date: Mar 2009	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

03-30-2009, 10:15 AM	#8
subadrew99 Scooby Newbie Member#: 204755 Join Date: Mar 2009	Thanks for the info that was what I was looking for

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	99rs switched back to SOHC for N/A's. my bad about the others.

03-31-2009, 03:52 PM	#13
VWVancouver Scooby Newbie Member#: 14507 Join Date: Jan 2002 Location: Surrey, B.C. Vehicle: 1984	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.

04-01-2009, 07:30 PM	#15
VWVancouver Scooby Newbie Member#: 14507 Join Date: Jan 2002 Location: Surrey, B.C. Vehicle: 1984	Thanks Jay.

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	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.

04-02-2009, 12:28 AM	#17
subadrew99 Scooby Newbie Member#: 204755 Join Date: Mar 2009	to do a swap from DOHC to SOHC would you need to do a complete wiring harness swap?

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	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.

04-02-2009, 04:02 PM	#19
bubba2533 Scooby Newbie Member#: 175483 Join Date: Mar 2008 Chapter/Region: MAIC Location: Pittsburgh PA Vehicle: 97 N/A w/ MS3 Aegean Blue	Some EJ25D motors had EGR or did you mean EJ25 SOHC didn't have EGR? (what you said is kind of confusing)

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	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.