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Old 11-18-2004, 01:13 AM   #1
Unsung Boxer
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Default RS DOHC vs SOHC heads

anyone got statistical evidence to show which flows better? DOHC or SOHC heads. ive heard conflicting arguments w/o numbers.

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Old 11-18-2004, 01:31 AM   #2
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hehe
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Old 11-18-2004, 01:39 AM   #3
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DOHC all the way
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Old 11-18-2004, 02:00 AM   #4
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DOHC has many advantages but mostly in two areas. Valve angles are not limited and can be placed wider apart. Also since the cam can be made to directly depress the valves (bucket over valve) reciprocating mass is reduced allowing for greater rpm before valve float.

SOHC has one advantage alone, reduced cost through reduced number of parts.
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Old 11-18-2004, 02:03 AM   #5
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I'm lazy so I just cut and pasted my reply from the other board.


I don't remember Cobb posting any tests about DOHC vs SOHC. But I do know they tested the SOHC vs the old Legacy Turbo SOHC heads. The 99+ SOHC heads even flowed better than stock LS1 heads, IIRC.
Here is the link to their writeup : http://www.cobbtuning.com/tech/sohc/index.html

I dunno about DOHC being better than SOHC, who knows, they probably flow the same. But, I do know that the internal workings of the SOHC heads are MUCH better since it uses roller rockers, and has screw type adjustment, instead of HLAs.

Here is an excert from their site :
Quote:
And as we saw before, the late model Phase II SOHC cylinder head uses a high-tech mechanical roller rocker design. This means the end of the rocker arm that comes in direct contact with the camshaft has a rolling cylinder on it. Other methods, like the type used in the Subaru DOHC heads, utilize a direct metal to camshaft contact. One advantage to using a roller style camshaft is that we can take the valve from fully closed to fully open much quicker as the roller will not dig into the camshaft. This helps by allowing more air into the motor without necessarily keeping the valve open for a long period of time. By doing so, we get power but we keep drivability, good idle, etc. Definitely a plus for this particular project.
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Old 11-18-2004, 05:13 AM   #6
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I'm not comparing particular heads since the original post didn't refer to particulars. I was referring to overall design potential. How often to you see SOHC on any sort of competitive race that allows the use of DOHC ?
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Old 11-18-2004, 08:22 AM   #7
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Good power can be made with both heads, in the long run. However, adjustments to the internals may also be necessary. i.e. cams or stiffer springs. To say that one is better than other out of the box is just a bit arbitrary and there is not a whole lot of evedence out there to support one or the other. Besides COBB, who has done a real intensive analysis of either head flow rate? And COBB only did the SOHC. For something that flows better than an LS1 (is that 5.8L, ) head bone stock, there is not much to complain about.

But, most EJ22T who have undergone a swap seem to prefer using the DOHC configuration, but many high HP SOHC RS's still abound.

The real debate about heads on the RS is the issue of what to do about a cam.....

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Old 11-18-2004, 10:18 AM   #8
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have u picked up some cams yet graham?
i ordered my twes the other day...i had to do it
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Old 11-18-2004, 12:39 PM   #9
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http://www.wrxforum.com/cgi-bin/ulti...=000917#000000
Read this. If we are talking fully built and ported heads, then the potential of the two designs are equal. They each have advantages and liabilities. And one of the things you will discover by reading that thread is that the roller rocker design of the SOHC boxer heads allows quicker opening and closing of the valves than the shim over bucket design of the DOHC heads...
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Old 11-18-2004, 02:24 PM   #10
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Awesome link Matt, thanks!
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Old 11-18-2004, 02:37 PM   #11
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Quote:
Originally Posted by Matt Monson
http://www.wrxforum.com/cgi-bin/ulti...=000917#000000
Read this. If we are talking fully built and ported heads, then the potential of the two designs are equal. They each have advantages and liabilities. And one of the things you will discover by reading that thread is that the roller rocker design of the SOHC boxer heads allows quicker opening and closing of the valves than the shim over bucket design of the DOHC heads...
Quicker opening yes... quicker closing no. The only thing that closes valves is spring pressure and the more mass it has to overcome (more things it has to push to open) the slower it becomes. The DOHC design with no rocker arm will always be able to rev to a higher rpm before valve float.
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Old 11-18-2004, 03:03 PM   #12
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unless u get stronger springs and
u dont need a shim under bucket conversion
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Old 11-18-2004, 05:22 PM   #13
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Quote:
Originally Posted by White 2.5rs
unless u get stronger springs and
u dont need a shim under bucket conversion
Stronger springs can also be put on a bucket over cam and it will always have the advantage of not having to push a rocker. SOHC really only has the low cost advantage and low maintinance (can be used with hydraulic valve lash adjustment). Every performance area using equal design levels it will loose out to DOHC.
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Old 11-19-2004, 12:26 PM   #14
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revving to higher rpms before valve float is a discussion in mental masturbation. How high are you gonna rev? A SOHC EJ25 head with upgraded springs can rev to 8000rpms, no problem. Sure, a built DOHC head can rev to 9000rpms without float, but unless you spend the big $$$ on a bucket over shim conversion, you are gonna spit shims at that rpms. Otherwise, with upgraded springs the max I would personally rev the shim over bucket heads to is about the same 8000rpms.

AND, you had better have a monster freakin turbo on there if you are going up there or there is just no point. Most of the turbos that most people are using around here don't blow enough cfm's up there to be efficient up there and 7500 tends to be the number where most people redline around here.

If you have enough $$$ you can build anything, but I prefer to keep my tech discussions grounded in the world of mortal working man and human horsepower desires...
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Old 11-21-2004, 04:11 PM   #15
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Quote:
Originally Posted by Matt Monson
revving to higher rpms before valve float is a discussion in mental masturbation. How high are you gonna rev? A SOHC EJ25 head with upgraded springs can rev to 8000rpms, no problem. Sure, a built DOHC head can rev to 9000rpms without float, but unless you spend the big $$$ on a bucket over shim conversion, you are gonna spit shims at that rpms. Otherwise, with upgraded springs the max I would personally rev the shim over bucket heads to is about the same 8000rpms.

AND, you had better have a monster freakin turbo on there if you are going up there or there is just no point. Most of the turbos that most people are using around here don't blow enough cfm's up there to be efficient up there and 7500 tends to be the number where most people redline around here.

If you have enough $$$ you can build anything, but I prefer to keep my tech discussions grounded in the world of mortal working man and human horsepower desires...
The parameters of the discussion were already set. It was simply SOHC vs DOHC. Did you read ANYTHING from the original post mentioning anything about cost? If you are going to inject your own opinions and put your own limitations to the discussion you don't have to sound like a JERK.

Learn to participate in a discussion without putting your own narrow mindedness into it. READ THE ORIGINAL POST. Point out to me where it said anything outside of dohc vs sohc.
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Old 11-21-2004, 04:19 PM   #16
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jesus angry
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Old 11-21-2004, 06:04 PM   #17
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I hope this answers the question in the original post...

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Last edited by klug; 11-21-2004 at 06:29 PM.
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Old 11-21-2004, 06:11 PM   #18
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Quote:
Originally Posted by White 2.5rs
jesus angry
what can I say? doesn't help to keep it all in right?
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Old 11-22-2004, 12:13 AM   #19
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Bump...for edited previous post with good info.

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Old 11-22-2004, 12:30 AM   #20
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Nice find klug!

Even extra special since I have Stg. 1 heads.
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Old 09-10-2005, 06:39 PM   #21
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I'm in the process of buying the TWE stage II SOHC setup and they are bascially telling me that it's not ideal due to SOHC combustion chamber size (99MM) in realtion to the EJ22T combustion chamber size (97MM) and should go with DOHC. That's more work than I care to do with this project, but has anyone heard anything similar?

Quote:
Originally Posted by hartage
The parameters of the discussion were already set. It was simply SOHC vs DOHC. Did you read ANYTHING from the original post mentioning anything about cost? If you are going to inject your own opinions and put your own limitations to the discussion you don't have to sound like a JERK.

Learn to participate in a discussion without putting your own narrow mindedness into it. READ THE ORIGINAL POST. Point out to me where it said anything outside of dohc vs sohc.
Incorrect... it was RS DOHC vs SOHC heads specifically as stated in the title not just simply SOHC vs DOHC. Also, this is the aftermarket forced induction forum also implying a relation to older model Subaru heads. Just thought I'd state this for no particular reason since your tone and this paragraph arenít necessary anyway.
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Old 09-10-2005, 07:04 PM   #22
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Unless you swap out the pistons, your compression ratio will be below 8... using a thinner head gasket is not even an option and hence if you have the stock EJ22T and you mate it to the RS SOHC head, you get something like 7.7:1 compression which would make the low end gutless. I believe the only DOHC head worth using is the JDM STi heads, otherwise it's not worth the trouble - even JDM heads are just too pricey to come by itself.
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Old 09-10-2005, 09:07 PM   #23
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I ran mid 12's at 5000 ft above sea level with a stock block ej22t and stock SOHC heads. It was hardly "gutless"
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Old 09-11-2005, 03:11 PM   #24
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I have Pauter Rods and CP pistons designed for 8.0:1 compression based on stock SOHC RS heads and longevity isn't a critical priority since it won't be driven that much.

RS heads are not that far behind the big port STI heads in stock form from what I found on this board. Itíll be interesting to see what the setup does with a GT35.
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Old 09-11-2005, 06:56 PM   #25
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Quote:
Originally Posted by seven881
I'm in the process of buying the TWE stage II SOHC setup and they are bascially telling me that it's not ideal due to SOHC combustion chamber size (99MM) in realtion to the EJ22T combustion chamber size (97MM) and should go with DOHC. That's more work than I care to do with this project, but has anyone heard anything similar?
Yes, I have heard this from someone who has done the swap, but he did it with the DOHC heads, and yes, the two are different bore sizes, and this in one form or another seems to disturb the airflow, unless you get some machining in there to smooth out the differences.

Also, the coolant and oil passages don't seem to perfectly mate up.

Finally, on the potential debate. This is definitely an old debate, and cobb has written articles about this on their website. Apparently, from their point of view, the SOHC has just as much potential, if not more, than the DOHC. Valvetrain mass is not so different, since you have all the shims and buckets going on all the valves, and an extra pair of cams, with an extra pair of cam sprockets and bearings. Also, the lobes have to be bigger on the DOHC cams, because it is a 1:1 ratio to the valve. The rocker arms on the SOHC have a higher ratio, like 1.4:1 or something similar (don't quote me on this number....I just know it's more than 1:1). The larger lobes mean less centralized mass on the cam, meaning, more relative valvetrain mass.

Basically, just use whatever you already have, ha!
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