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Old 08-04-2007, 07:24 PM   #1
Spilner
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Default RS SOHC vs RS DOHC for hybrid swap

ok so I've been doing some reading and from what I've seen the rs sohc heads seem to be cheaper to build and can flow just as well as their dohc counter part.

My plan is a ej22t block with either of these heads, a good size turbo with supporting mods and I'd like around 350awhp.

What do you guys think of the sohc head vs the dohc?
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Old 08-04-2007, 08:52 PM   #2
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dohicky heads FTW

you can use jdm sti ra cams in them i believe..but the SOHC flow better then the dohickys

also; the SOHC yields for a lower CR.

im doing the same build pretty much, except i am using an ej22e block, just to show everyone how weak open deck blocks are..lol.
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Old 08-05-2007, 01:03 PM   #3
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dohicky heads FTW

you can use jdm sti ra cams in them i believe..but the SOHC flow better then the dohickys

also; the SOHC yields for a lower CR.

im doing the same build pretty much, except i am using an ej22e block, just to show everyone how weak open deck blocks are..lol.
Like I said I've been reading that the sohc flow better, cheaper to upgrade, etc...

But everyone seems to go with the dohickys, just trying to figure out why...
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Old 08-05-2007, 02:01 PM   #4
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I did a lot of reading also before I went dohc.... If you ask some of the gurus, they will tell you the sohc 2.5 heads are very good heads, but in the end, the dohc heads have better real world flow.
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Old 08-05-2007, 02:12 PM   #5
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I did a lot of reading also before I went dohc.... If you ask some of the gurus, they will tell you the sohc 2.5 heads are very good heads, but in the end, the dohc heads have better real world flow.
all I can find so far as the main difference between the two other than the obvious is that the dohc heads lower compression quite a bit.
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Old 08-05-2007, 02:21 PM   #6
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To be honest - for the amount of boost that you'll need to run 350 whp, you are probably going to want to run much lower compression....

I actually built my ej22e shortblock with the thicker head gaskets and phase I ej25 dohc heads....
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Old 08-05-2007, 02:35 PM   #7
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Originally Posted by gto7419 View Post
To be honest - for the amount of boost that you'll need to run 350 whp, you are probably going to want to run much lower compression....

I actually built my ej22e shortblock with the thicker head gaskets and phase I ej25 dohc heads....
that's what i was thinking for the power I want.

I have also been considering upgrading the springs/retainers with twe's
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Old 08-05-2007, 08:07 PM   #8
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good luck!
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Old 08-06-2007, 11:03 AM   #9
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thanks...
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Old 08-06-2007, 11:51 AM   #10
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The only reason you would need to upgrade springs is because of valve float at higher rpms...

You looking at a 9,000 rpm redline? I would be very concerned about oil problems....

Who is building your shortblock? Using extra bearing clearances? Check out this thread if youre thinking about high rpms...

http://forums.nasioc.com/forums/show....php?t=1062626
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Old 08-06-2007, 11:58 AM   #11
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The only reason you would need to upgrade springs is because of valve float at higher rpms...

You looking at a 9,000 rpm redline? I would be very concerned about oil problems....

Who is building your shortblock? Using extra bearing clearances? Check out this thread if youre thinking about high rpms...

http://forums.nasioc.com/forums/show....php?t=1062626
wasnt really planning on reving to that level. Nor was I planning on building the shortblock at this time, most likely I plan on freshening it up but not "builiding" it.
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Old 08-06-2007, 12:59 PM   #12
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Im a bit interested. How are the DOHC vrs SOHC heads, when both are aggressively ported , if both are run with large cams and high (not low) compression? I would assume that if both heads were ported for max velocity, then the SOHC heads would actually do better with large cams as there would be considerably less work on the valvetrain, especially important given that it wouldnt be run in a low compression engine. Is that true? And how does elevated compression change the relationship of the two? Is the DOHC still considered "better" in that situation in a high flow, high pressure environment?
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Old 08-07-2007, 10:37 AM   #13
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Since a high compression NA motor relies on vaccuum to pull air in it probably won't be such a great idea to aggressively port the heads. Unless your planning to rev really high.

The valvetrain loss argument between DOHC and SOHC heads is minimal because the SOHC heads still have rocker arms. Even though you have another cam to spin in the DOHC its touching the buckets.

Things to think about:

I upgraded the springs to supertech dual springs with titanium retainers because I plan to rev beyond 7000 and am going turbo. I also converted to a shim under bucket setup to keep from spitting shims at higher rpms. I would recommend a shimless bucket setup especially if your running high lift cams at higher then 7000rpms and turbo as well.

To be totally honest, try to find a set of DOHC WRX heads. In stock form the 04 and up came with shimless buckets and have a really nicely shaped combustion chamber. If you want to modify the heads you can easily find high performance cams and springs for $1000. On a side note if you ever need to adjust lash you will need to order the whole bucket ($25-each) since the shim is built into the bucket.
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Old 08-07-2007, 04:25 PM   #14
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Quote:
Originally Posted by Spilner View Post
My plan is a ej22t block with either of these heads, a good size turbo with supporting mods and I'd like around 350awhp.
Just another suggestion, but why not just swap an entire EJ205 or EJ255? Both of them are cheap and are less hassle. You can get the 205 longblock for cheap and I'll bet you any $ that it will make more power and tq than your hybrid (not saying the hybrid CAN'T, but in reality I bet it won't).

The EJ22T blocks, as you know, are pricey considering their age and the fact that they also have those toothpick rods.

Just listing some alternatives.
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Old 08-10-2007, 01:26 AM   #15
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so you're saying that a hybrid motor with 200cc's more dispalcement and better flowing heads than a wrx will make less power....?



~Josh~
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Old 08-10-2007, 12:45 PM   #16
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so you're saying that a hybrid motor with 200cc's more dispalcement and better flowing heads than a wrx will make less power....?



~Josh~
that's what it sounds like to me, I guess he hasnt done his research.
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Old 08-10-2007, 01:10 PM   #17
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"Trade-off to consider:

If you use race pistons with valve cutouts in the dome for non-interference, a DOHC Subaru motor will still bend a valve if you ever lose a timing belt. This is because the separate cams on each head can free-wheel, allowing the valves to collide with each other. This can ruin your weekend and cost lots of time and money. So make sure you inspect your timing belt annually, and replace every 2yrs regardless.

If you use similar race pistons with valve cutouts on a SOHC motor, you won't bend valves if you lose a timing belt. Just simply replace the belt and you're back in the dunes. Losing a belt is not a common problem, unless you don't inspect it annually. But if you like the safety of the SOHC setup, the trade-off of protecting your investment may be worth having a little less HP. But remember, SOHC motors can be built to develop 450HP, and that's nothing to sneeze at.

Last but not least, the slightly smaller intake ports inherent to the SOHC heads will create more velocity of the intake air, so the SOHC heads will develop better low RPM torque than the DOHC heads, handy for duning. Many engine builders like the low end power advantage of the SOHC heads and opt for the trade-off, assuming 400-450+ HP is enough. "

-Quote from outback motors...

I still love my 98 DOHC

you can use jdm sti high lift cams (affordable)

and a nice shiny intake manifold http://www.outbackmotors.com/intake_manifold.htm
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Old 08-15-2007, 01:58 PM   #18
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so you're saying that a hybrid motor with 200cc's more dispalcement and better flowing heads than a wrx will make less power....?



~Josh~
I've stated my point quite clearly. I think he'll be better served (money and time) to either go EJ205 or STi/FXT/LGT longblock swap. It is most probably that his EJ22 block with RS heads isn't going to dyno spectacularily better than an EJ205 dyno (if at all). It certainly won't be any stronger, as it has those same toothpick rods (which is the limiting factor for making power on both blocks). Not only that, the displacement is just bore, so you're not really going to get a big jump in tq like you would with the EJ25's.

I looked into this a lot when I was going to build project "22C" a few years ago.

Again, my $.02

-C

Last edited by TypeC; 08-15-2007 at 02:04 PM.
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Old 08-15-2007, 05:43 PM   #19
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Those toothpick rods can support up to 500 whp I believe.
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Old 08-15-2007, 06:56 PM   #20
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First off, I want to lay out some facts. It is a fact that in stock form, the SOHC heads flow a little bit better through the mid-rpm range (4500-5500) while the DOHC heads flow slightly better up top (5500-6500). It's really pretty close and pretty much a wash.

Another fact, as was mentioned, is that if you overport either of these heads, they will suck. It's about port velocity and not about reaching maximum flow. With the right turbo and a mild port, you can pretty easily hit 350whp on either head. This is on stock cams. If you want to read a good discussion on cams, search for Graham and Xephyr in this forum using cams as the search word. I think BOY was in on the "conversation" as well, as was likely Drac9, No-Coast Punk, and myself.

I think the reason you see more people go with the DOHC Ej25 heads is twofold. The first is that they sell for about $100 cheaper than SOHC Ej25 heads. I think the other reason is that because they are cast on an old WRX casting, they already have the bosses for oil and coolant and are just a drill and a tap away from a very stock set up.

Using the SOHC heads requires custom oil and coolant (which isn't rocket science as many members have seen). The other thing is that a stock WRX uppipe doesn't clear them properly, so you are left to manipulate things quite a bit or do something other than use a turbo crossmember and factory style uppipe and downpipe pieces. But again, for a creative and resourceful person, it's not a bit deal.

On the whole Ej22T thing, that's really a personal choice. 350whp is not record breaking. It's not anything that is going to stress the Ej22T. I wouldn't go around saying that 500whp is well within the design limitations of the Ej22t, but 350 is. There's just no point to argue Ej20 versus Ej22 at those power levels. Hell, even an Ej22E open deck NA engine (with proper pistons) will be fine at 350whp and those "matchstick" rods won't matter. It's closer to 400whp or in situations of crappy tunes where that matters.

If it was my car, I would build it on what you got. It's about the same cost to do it either way. Most of the names I have dropped in this thread have done 350whp AFI builds. Do some searching on the history here and you will see it's not that hard. I just stress one thing: tune and fuel, tune and fuel, tune and fuel. I guess that's 2 things. oh well...
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Old 08-15-2007, 07:38 PM   #21
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Quote:
Originally Posted by Matt Monson View Post

The other thing is that a stock WRX uppipe doesn't clear them properly, so you are left to manipulate things quite a bit or do something other than use a turbo crossmember and factory style uppipe and downpipe pieces. But again, for a creative and resourceful person, it's not a bit deal.
I just cut off the heat shield and used the heat wrap on the stock WRX UP with my SOHC heads. It cleared fine after that.
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Old 08-15-2007, 08:20 PM   #22
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well I decided on using DOHC heads for a couple reasons.

As for my next question should I merge the ej22t harness with my 94L harness or is it much less of a headache to right to a standalone with power I'm looking to make? since either way I will need to get some EM so if I can save time and money by not having the harnesses merged it'll help me out.
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Old 08-15-2007, 09:37 PM   #23
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i would go standalone for your goals. if you're looking to make 300+ hp/tq you're going to want something better than the ej22t harness. and IIRC the 94 imp. doesnt have a knock sensor, but the ej22t does. merging the harnesses together still only gets you the stock ej22t ecu timing and fuel maps. i would go with e-manage or something of that sort. you should look around in the EM forum for advice on this.



~Josh~
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Old 08-15-2007, 10:04 PM   #24
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doooood you got OBD1 ??? stand alone ALL THE WAY

I wish they wouldn't keep trying to stick thier wires into MY plug every time I get inspected....
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Old 08-15-2007, 10:24 PM   #25
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i would go standalone for your goals. if you're looking to make 300+ hp/tq you're going to want something better than the ej22t harness. and IIRC the 94 imp. doesnt have a knock sensor, but the ej22t does. merging the harnesses together still only gets you the stock ej22t ecu timing and fuel maps. i would go with e-manage or something of that sort. you should look around in the EM forum for advice on this.



~Josh~
From what I've read though emanage is more of a piggyback is it not? I would like to do the emanage ultimate if I could cause it's a little less coin and seems to work well for the few people I know who have used it.
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