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DOHC vs SOHC any advantages?

15748 Views 19 Replies 15 Participants Last post by  4wdFury
I'm wondering if there are any advantages of DOHC vs SOHC? I believe the power ratings where pretty much the same between the two on the RS but there must be a difference in some situations. Does one react to mods better? what about under forced induction situations? I'm looking more for theory then actual application here as I believe the mods for the DOHC RS are fairly limited.
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I know a guy that is getting a DOHC motor built by Cobb and they are just amazed at the results of it compared to the previous results they've seen from SOHC buildups. Supposedly they just changed the intake cam, left the exhaust the same, and are pushing about 350hp to the wheels (yeah, nuts). My money is on the DOHC heads, but SOHC are good for lower money situations, I think.
From the what I've been told by engineers over the years and read, the main advantage of DOHC is that you have a lot more flexibility with valve angles because you can have the cam lobe acting dircetly on the valve instead of through a rocker. You will also have less mechanical power loss with direct valve actuation, and it reduces the tendency for valve float.

The big advantage is the valve angle thing though. The valves can be set at angles that are very close to the intake and exhaust ports, therby reducing the bend into the combustion chamber, and increasing flow. It also allows the valves to be angled in towards the center of the cylinder, which places the head of the valve further away from the cylinder wall and increases flow.

For a street engine though, the additional cost of DOHC usually isn't worth the small power gains that are available over SOHC if you are still meeting emissions.

You want to see really wild, read up Ducati's desmoquatro valve train... no valve springs!
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8Complex said:
I know a guy that is getting a DOHC motor built by Cobb and they are just amazed at the results of it compared to the previous results they've seen from SOHC buildups. Supposedly they just changed the intake cam, left the exhaust the same, and are pushing about 350hp to the wheels (yeah, nuts). My money is on the DOHC heads, but SOHC are good for lower money situations, I think.
is this an NA RS motor from 98 or a turboed application?
From what I've read, the SOHC has a touch more power/torque than the DOHC, but the delivery is different. The SOHC has a much flatter torque curve that delivers more torque at lower RPM's as compared to the DOHC. I just recently read that in a back issue of Road&Track, but i can't quite find the right issue right now, but I'll post it when I do. But for a turbo application, the DOHC would work better, as they do have a better flow rate, and that will certainly help with with a FI system.
are any parts interchangable between the two engines?
I think the engines are basically identical with just the heads and ECU being different. The compression ratio is a little different as well I think, so maybe the pistons are different. That could also be explained away by the heads though.
There are also some differences in the lower block, like placement of the thrust bearings, I think basically the motors are very similar but have few or no interchangable parts.
That seems like a lot of work to change for only one year. Was the DOHC used in other models for longer?
From what I understand.... 98's and early 99's had a weaker lower end because of the thrust bearing location or something like that. These are called Phase I blocks, and they were found in the legacies and forresters as well. On the plus side they had a decent DOHC design. Most 99's got the stronger Phase II block, but they had moved to a hot film MAF which has problems w/ intakes and forced induction, I have heard, instead of the previous hot wire MAF which was more robust. 00's and 01's got SOHC which is still a good setup, but maybe not as flexible as the DOHC. They also got redesigned pistons which are sturdier, and a MAP which is alot more reliable, just not as accurate. Come to think of it, the 99 pistons might be the newer design too. If you don't mind a frankenstien, from this, I'd say 98 heads and MAF(or 00 MAP, this is debatable) w/ an 00 block. Even the newer design of pistons have a reputation for failing in turbo charged situations, so if that's your route, go after market and you can lower the compression for higher boost. if staying NA the 00 pistons are probably good.
that's what I know, it could be wrong... good luck

oh, yeah, congratulate me on my first post:sunny:
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mrbell... congratulations on your first post.
mrbell, how dare you come in here and make all sorts of good points on your first post! You should be saing stupid things, and making a nuisence of yourself! :lol:
Stock SOHC Imprezas are quicker off the line than DOHC ones. Once you're rolling though, there is no difference. One plus for DOHC is that you can retard or advance your torque curve (with adustable cam gears). And all those damned fool WRX' have DOHC heads.
If you are planning on heavily modifyning your cylinder heads, then the DOHC is going to be the way to go. Otherwise, the SOHC heads have much better flow characteristic. But, unless you are doing a complete ECU, wiring harness, and so on swap, you probably don't want to switch from one set of heads to the other. The ECU has a cam position sensor, and I am certain that throwing four cams into an ECU looking for two, or vice versa would really cause some limp-home mode problems.
The DOHC heads will fit on a SOHC block with no fitment problems (I am unsure about the ECU though - you will probably have to also swap it out). If you are looking towards the internals of your motor for modifications, it is highly unlikely you will leave the cams stock. If you are pursuing this route, then the head swap is a good idea.

One of the main reasons Subaru chose a SOHC is lack of complexity. It is cheaper and easier to build. The main benefit is lower parasitic losses from the valvetrain.
Here's my first post on the board.

Templar, the use of a SOHC ecu with a DOHC motor and vice versa is VERY POSSIBLE, actually. The reason being that I have a 01 SOHC motor from a legacy GT in my 97 GT . . . all using the stock 97 ecu!! It's true that the ecu's use cam angle sensors, but for both models, the ecu only monitors one cam. The trick is to use the sensor with the correct amount of teeth. The same holds true for the crank sensor. That being said, let me list a few things I've found when making the swap:

1. 00-01 SOHC motors use a 2-tooth crank sensor, 98 DOHC engines use a 7-tooth sensor. Not sure about the 99's. Same holds true for the cam sensors.

2. 98's and (I think 99's) have an egr valve, 00-01's do not.

3. 00-01 engines have relocated the main thrust bearing, to minimize the amount of crankshaft resonance at higher rpms.

4. 00-01 engines "supposedly" have stronger internals, but I believe the only difference was really increasing the size of the piston skirts to prevent the infamous "piston slap" found on the DOHC engines.

5. DOHC engines are NOTORIOUS for blown headgaskets. I believe this fact as I blew my headgaskets at 100,000 miles. This was my motivation for swapping in a 01 engine.

6. 00-01 engines had a 10.0:1 compression ratio, 98-99's had a 9.7:1 compression ratio.

7. 98's used a hot wire MAF, 99's used a hot film MAF, and 00-01's used a MAP sensor.

8. DOHC intake manifolds bolt up to DOHC heads, SOHC intake manifolds bolt up to SOHC heads. You CAN NOT swap them. But all Subaru heads bolt up to all Subaru blocks.

I could document the actual engine swap, if you guys want, but I'll save that for a second post, just in case you guys are actually interested in it.

Hope this helps,

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A correction to mrbell's post on Phase I and Phase II engines: Phase I was DOHC, Phase 1.5 was DOHC heads mated to a SOHC block, and Phase II was SOHC.

In the middle of '98, Subaru installed the Phase 1.5 engines in the 2.5 RS (not sure about Legacies and Forresters). They switched to the Phase II beginning in '99. There are ridges on the engine block (as well later production dates) to distinguish Phase I from Phase 1.5 engines.

I had my '98 DOHC engine replaced under warranty. They mated the DOHC heads with the SOHC block, giving me a Phase 1.5 engine. If you're thinking of turboing your RS, this is the best engine to do it on, like others in this thread have said.

go here if you want to see a good comparson of the DOHC and SOHC heads and cams.

basically the DOHC heads have better flow but the camshaft design in the SOHC heads are more efficient which allows the SOHC to allow more air fow into the cylinders than the DOHC.
so for turbo charging it's better to buy the late model 98 motor? if this is true, can u take those DOHC heads off of that motor and swap it to the 00 block, and then slap a WRX turbo on it? if this is true or false plz e-mail me at [email protected] i really appreciate it. i'am going to put a wrx turbo, etc. on my 00, but i'am not sure weather to switch to the DOHC or keep my SOHC? like i said what should i do? or.... should i build up the cams, pistons, injectors, valve springs, etc... and then turbo it WAY down the road after i do all of that, will that be way more dependable, or what? right now i'am in the middle of research, i need to know these things plz give me some feed back i would really appreciate it.
god damn this thread is old
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