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Old 10-20-2000, 11:45 AM   #1
Thug
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Post DOHC heads onto a '00 2.5 block?

Can I put '98 DOHC heads onto my MY00 block and still run the stock fuel sytem? How about with Cobb's cams? Id eventually like to go with different engine management so I could raise the rev limiter.
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Old 10-20-2000, 02:03 PM   #2
Sunrise City Rider
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If you were going to go all out and turn your 2000 RS into a full race car, then I would see the point in having seperate cams. But if you are gonna keep your car on the street, then you are not gonna see big numbers with the DOHC. In fact you would have to work the heads over to get the performance to meet the SOHC. Plus, what will the different cams do if you have no way of adjusting them...more money for adjustable cam gears and I don't think adjusting them will be as easy as a Honda. Your idea may make sense but not for a street RS.
Zee
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Old 10-20-2000, 03:09 PM   #3
bill harvey
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ask trey cobb he will tell you that the dohc is a better head design also even with the stock cams there will be no more flat spot at hoigh rpms like the sohc heads have when they run out of air.
Nate this is one you need to talk to trey himself about this because there is so much propaganda about how much better the sohc is which is being expeled by alot of people who have never driven the dohc never mind seen the heads off the engine.
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Old 10-20-2000, 03:11 PM   #4
doripreza
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the US 98 DOHC heads are totally different from the wrx head design.
The piston dome is also a much crappier design on the MY98 us motors which explains why everybody dislikes my98 heads.
if you really want DOHC so bad then get some WRX heads from japan. a totally different and much better head design.
the SOHC heads have a much higher tolerance for high revs than the DOHC's anyway because of the reduced amount of friction points..

why do you think you can make better compression?
why better drivability in SOHC? more torque=more drivability?

ohh and to answer your question
the entire fuel rail will have to be changed if you attempt this switch. and the injectors are different also.
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Old 10-20-2000, 04:19 PM   #5
jackwei
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really? SCC said the 98 heads = WRX heads - wrx cams + 98 cams
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Old 10-20-2000, 06:19 PM   #6
bill harvey
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the compresion will be higher because the volume of the compresion chamber in the dohc heads is less than in the sohc heads therefore put the dohc heads onthe pistons from the 99 or 00 and gues what more compresion.
as far as the combustion chamber being "crappy"
well trey cobb said to me that it is beautiful.
why did subaru change to sohc you ask, simple cheaper, if you think they are better why did they have to lower the redline? and why do the sohc run out of breath at high rpms?
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Old 10-20-2000, 06:45 PM   #7
Andre Vandenberg
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It all depends on what you want to do with the car, and how “unique” you want your car to be. Trey, will correct me if I’m wrong here, but I think MOST of the torque increase gained by going with the SOHC design was accomplished with simple cam design, and had very little to do with head design!

If you think going with DOHC design is a “step backwards”, better rip those heads off of the WRC car and slap a set of those “superior” SOHC jobies on there. BUT, Don’t bitch if the Suby’s can’t even crack the top 20 at a regional event, or are being beat by most of the “private” teams in WRC! As for 50% more torque at half throttle….. I refer back to my age old statement of: “What do you wipe your a$$ with after you go to the bathroom? ……If you can put THAT on paper, you can put any old S%*& on paper.” Get it out there on the asphalt and lets do some roll-ons, DOHC against SOHC….. I bet you, stock for stock there will be NOTHING in it, and as soon as you start scratching around in the respective engine compartments, the DOHC will out perform the SOHC!!

MY2C
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Old 10-20-2000, 10:54 PM   #8
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And no, it wont be a street car. It will be race car. And yes, I do like being original. And I believe Trey's bench flow test of heads showed the RS heads had higher flow then the older WRX ones.
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Old 10-20-2000, 11:34 PM   #9
Andre Vandenberg
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Which are NOT the same as EJ25 heads...
Andre
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Old 10-21-2000, 12:21 AM   #10
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Cobb doesn't make cams for the DOHC heads, yet.

It should swap over but the details are unknown to me.
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Old 10-21-2000, 12:41 AM   #11
Thug
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The DOHC setup will make more power once it's tuned and I can raise the redline. Also, the '98 heads on the '00 block will make higher compression. Subaru only switched to SOHC for easier driveability. I believe all the 2.0L turbo motors use DOHC heads. So would I REALLY be taking a step backwards? I dont think so.
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Old 10-21-2000, 04:08 PM   #12
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I'm not taking sides here, but I'm wondering why SOJ has never (to my knowledge) implemented the SOHC design to any of the Japanese spec Imprezas. Can anyone explain?
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Old 10-22-2000, 12:50 AM   #13
doripreza
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if i'm not mistaken the my98 DOHC heads have a different valve angle from the WRX heads.
among other things

as for running out of breath at high rpm... well it is a totally different cam
you have seen the flowbench results don't tell me its running out of breath due to head design..

and i never compared the us SOHC to the WRX DOHC.. i think the WRX DOHC is much nicer
but the SOHC has never had problems before and its run 400+ps on other peoples cars. why you think it will break down in anytype of race i don't know.

if i was going all N/A i would for sure go with SOHC rather than DOHC.. why? i think a milled SOHC would perform better thats all.
but they both could handle high revs just fine i think.. wrx's have been setup to rev to 9000 and the MY99 (only saw one) could rev to 8000 with new springs.. sounded good and its still running to my knowledge
hell i'm setting mine up to rev to 8500 i havn't run into any brick walls so far
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Old 10-22-2000, 05:49 PM   #14
Andy_T
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2yLiTe -

good question. You'd have to ask Subaru that, but here's some ideas:

driveability; the SOHC motor has reasonable mid-range grunt and goes like your average Joe (not that there's any on this board!) likes a car to go. Also we have emissions, fuel economy and quality, and market positioning to think about (can't have a SOHC if everyone else has DOHC, regardless of the pros and cons).

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Old 10-22-2000, 07:22 PM   #15
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Ok... not to argue subaru specifics here, but the advantage of DOHC is tuning. That's really it. it's easier to tune a DOHC system because you can tune intake to exhaust w/o milling both cams. Subaru probably decided they had found a good combo and then could combine them into cam after what amounts to years of road tests on the DOHC, combined them to reduce manufacturing costs, and viola. The problem I have w/ my own theory here is that I have always been told(mechanics, engineers, etc) that DOHC was actually cheaper because the tolerances didn't have to be as tight. ...even tho DOHC has more parts, and more complex design.
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