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Old 11-05-2003, 07:03 PM   #1
JoShYo
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Default EJ25 SOHC to DOHC and back again?

Does anyone actually know why the EJ25 in the RS (and possibly other cars, although Im not sure..) went from SOHC to DOHC and back to SOHC again?

Please correct me if Im wrong, but the EJ25 in the 2000 and 2001 RS were DOHC and now they're SOHC again. Any ideas?

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Old 11-05-2003, 07:29 PM   #2
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IMpreza- 1998 RS first year- DOHC
1999 RS SOHC (heard a few of the early 99s were DOHC but i dont knwo for sure)

2000 - SOHC
2001- SOHC
2002- SOHC
2003- SOHC
2004- SOHC

WRX EJ20- DOHC

WRX STI EJ25- DOHC

I dont know when the legacies and foresters switched to SOHC, probably around the same time. So as you can see earlier models were DOHC while all later non turbo EJs were SOHC

I personally like the DOHC better, they breath better higher in the RPM range and you make power higher up. The switch to SOHC shifted the power band lower in the RPM range improveing low end and mainly mid range, but it suffers high end alittle. But its more driveable around town.

The real reason I think they switched over is cost. The DOHC simply costs alot more to produce, they can make the same power with the SOHC and produce it cheaper, meaning more profit for them.
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Old 11-06-2003, 12:22 AM   #3
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From 1996 to 1999, Legacy LSi (thru MY97), GT and Outback had the EJ25 DOHC. Only the 1996 EJ25 equipped Legacies had a lower hp/torque than it's newer counterparts. And only the MY98 Forester had the DOHC.

Another problem was the DOHC's were prone to headgasket failure.

I have mine at 55K and no problems; knock on wood.

I personally think the SOHC design was not only cost effective, but a improvement over the DOHC design.

Repairs don't use as much green. And IIRC, an DOHC design for our type of engine configuration (boxer) suits better than a SOHC. Most DOHC 4 cylinder engines, you have 2 camshafts. For the boxer engine, you have 4. They are not half the length of, lets say an inline camshaft, so you end up with much more rotating mass. For the N/A engine, it's not preferred. With the forced induction engines, they just dust it off with .01 PSI more boost and then it overcomes the issue. N/A engines are built more power from power loss from every place possible.

-paK +12

edit- error

Last edited by Pakin; 11-06-2003 at 08:09 PM.
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Old 11-06-2003, 01:27 PM   #4
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Question

Why would a DOHC version of the EJ25 breath better than a SOHC version? The number and size of the valves is the same - this isn't a 2-valve versus 4-valve issue. It's lobes on a cam.
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Old 11-06-2003, 01:41 PM   #5
Matt Monson
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Quote:
Originally posted by Julian
Why would a DOHC version of the EJ25 breath better than a SOHC version? The number and size of the valves is the same - this isn't a 2-valve versus 4-valve issue. It's lobes on a cam.
The head itself was redesigned and flows better. It has nothing to do with cams, as much as the actual head itself. Also, the SOHC got the shin design for valve actuation, so less mass and wear on the cams/valvetrain. The SOHC is a better head regardless of the idea that a DOHC design gives more control of intake and exhaust valve lift and duration...
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Old 11-07-2003, 09:42 AM   #6
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W/ DOHC you have better control over valve opening and closing timming. That is why it breaths better up top.
AO
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Old 11-07-2003, 04:43 PM   #7
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with DOHC, you have the possibility to have to tune the cam timing, no true, the cam synchronising the way you want. i don't know car that come with oem adjustable cam gear so this is 99% false. however, you don't have more control on valve opening and closing. you can put whatever lobe you want on a single camshaft, this is what control valve opening and closing. Actualy, with camshaft in the center of combustion chamber, you can have ports that are straight for longer before they need to curb and get out, increasing flow. The only advantage I would see to return do DOHC is having so many lobes for the variable valve timing system that it is absolutely unpratical to put them all on a single camshaft.
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Old 11-07-2003, 05:04 PM   #8
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Take a look at this. Both engines have started falling off pretty dramatically at the point that the DOHC engine is producing more power (around 5600rpms). IMHO, this whole DOHC breathing better up top myth is just that, a myth. And what was said in the post before this is dead on about you putting the lobes where you want on a SOHC cam. The Phase II heads are better heads as far as I am concerned.

p.s. Thanks for the dyno Pat Olsen
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Old 11-07-2003, 09:09 PM   #9
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Ah, personally im still a fan of the DOHC, Even though I have a SOHC EJ25 in my car.

Subarus variable valve timing thing... anyone have a good link to how that works.. I though it just changed timing, I didnt think it was like VTEC with another set of lobes...hmm

Also on the DOHC you are pretty much directly pushing the valve down when compared to the SOHC whereas there is a long rocker arm on them and it acutally requires more force to press the valve down if you count in the arm length etc. So although more rotational mass, there is considerably less resistance and less force required to press the valve down on the DOHC.

Last edited by Hawk296; 11-07-2003 at 11:14 PM.
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Old 11-08-2003, 02:55 PM   #10
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it advances the cam timing.
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Old 11-08-2003, 03:39 PM   #11
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thats what i though.
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Old 11-08-2003, 07:56 PM   #12
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if you look at the dyno sheet, the DOHC motor is more useful between 5200 and 6400. to some with short gearing, tahts quite useful.
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Old 11-09-2003, 06:41 PM   #13
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if you look at the dyno curves the SOCH have so much more torque from idle to 5200 rpm, to some who enjoy real life driving, thats quite useful
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Old 11-09-2003, 11:11 PM   #14
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i suppose, driveability is the key in production cars, im never below 4,000 rpm when im actually driving, with my current gearbox if i shift at 6,000 i land at 4200 or so, so it really dosnt matter for me.

the lower end torque differance is significant yes. but on a track, its more ideal for car control to be higher up in the revs so u can modulate the attitude of the car in the corners with your right foot. i guess thats what i care about.

h
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Old 11-14-2003, 10:02 AM   #15
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DOHC Subaru engines blow goats for modifying.

1. Shim over bucket valve actuation = no high rpm, or expensive shim under bucket conversion.

2. Twice as many cams = twice as much to put new ones in

3. DOHC Adjustability. Have you looked at the cost of procurring a set of adjustable cam sprokets for a Subaru? No thanks.

4. DOHC heads have a less common manifold bolt pattern. When APS, or other manufacturers/resellers obtain manifolds, they will likely not be supported. If your lucky, they might decide that the old GC8 WRX could be a viable market, in which case you got lucky. You also cannot use a WRX manifold which would be better for both NA and turbo applications.

5. The DOHC head WILL allow stock/aftermarket GC8 WRX turbo exhausts to fit without farting around with spacers and such for those that care about such things.

Argueing aove a few HP traded over the stock rev range is pointless. Add in the stock electronics that the DOHCs are saddled with and the problems that they give when doing simple upgrades to the DOHC engine, and you have a base to do nothing with compared to the later SOHC. You can do a hodge podge turbo easier with the DOHC electronics, but you should be using programmable management.

The engine is a system, the heads are only a part of it.

cheeRS,

Greg
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Old 11-14-2003, 12:08 PM   #16
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Well everything you said was related to modifying the car. The question was why they switched. I can guarantee you that when subaru put the 2.5 in their cars they did not design it around what could more easily modified.
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Old 11-14-2003, 12:24 PM   #17
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The question might have been that at one point, but the recent posts wandered to this.

And thanks for pointing it out that my post was about modifying the heads. I forgot that it was typed in the first line.
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