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00 BRP RS Coupe
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Discussion Starter · #1 ·
This has always kinda nagged at me and I've searched and searched, but can't seem to find a definitive answer...

First, picture in your mind the "top of the line" Impreza for each year mentioned below for North America.

Why was it that Subaru used DOHC in the 98 RS, SOHC in 99 - 01 RS, and then back to DOHC in 02+ WRX? What happened in those 3 years? And why only in North America? I've always been curious...
 

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02RS RIP, 91Stealth, 05STi
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Ummmm..... I am lost now what is your question why Subaru used a DOHC in the 98RS then a SOHC in a 99-01 RS and a DOHC in a 02+WRX? You are comparing totally different motors.(by that I mean turbo motor to NA motor)
I know the JDM WRX/STi from 93+ had a DOHC motor
if you are going to compare something above 02+ to the RS compare the 02-05RS which also used a SOHC motor and the 06-11 2.5i still uses the SOHC motor.

Its a very hard question to answer.

My view on why they used a SOHC on the EJ251 and not DOHC like the EJ25D is probably the flowrate of each head and its probably cheaper to manufacture SOHC then DOHC.
 

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98 Black RS GM6
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843 Posts
Well they used the DOHC engine's on the Legacy's too.
I guess maybe it was easier to move that market into the
US for the first year of the 2.5's if they already had a strong
engine that they could simply drop in there rather than having
to redesign one in their first year of production/introduction.
 

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2000 Impreza 2.5RS, Blue Ridge
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105 Posts
Well they used the DOHC engine's on the Legacy's too.
I guess maybe it was easier to move that market into the
US for the first year of the 2.5's if they already had a strong
engine that they could simply drop in there rather than having
to redesign one in their first year of production/introduction.
^This.

Keep in mind the problems those early DOHC 2.5's had with blowing headgaskets too. I'm with the OP though in the sense that I prefer DOHC for tuning potential. Especially in an N/A car, having the opportunity for different duration cams for exhaust and intake is almost worth the lost reliability (of using a first-gen DOHC 2.5 motor).
 

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05 STi WannaBE
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COBB actually did a flowbench test on both early DOHC heads and SOHC ones. They showed that the SOHC heads flowed just as good or even better than the DOHC.

Why do you even want a EJ25D? The SOHC is a superior setup. The SOHC makes gobs more midrange torque and cams make the top end difference between the two disappear.



A copy of Cobb's old tech article. This is a ZIP file, not a PDF. Just relabel it to .ZIP after downloading. I just did this due to the file size limitations over on RS25. Once unzipped, it's a Word document copy and paste of Cobb's online tech articles that they used to have up on their website from when they were doing measurements and flow testing the heads.



http://www.rs25.com/forums/attachment.php?attachmentid=46570&d=1307648934



At the same time, you can head over to the Naturally Aspirated section of this forum and search for dyno plots. There's a number of plots of the DOHC and SOHC motors, including direct comparisons. The tiny bit of top end is overshadowed by the massive lack of midrange. Plus a simple set of cams on the SOHC magically give it top end.
I know thats not the OPs question but I thought I'd throw this article out there :)
 

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97 2.5 obs, 98 Legacy GT Daily
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The Turbo engines have always been DOHC, they just didnt come to the US till 2002.

The new SOHC for the N/A cars flows about the same, is probably cheaper to build, and at least could be used to "fix" the reliability issue.

My guess is they kept the dohc in the legacy in 99 to get rid of their surplus since the new model was coming out the next year, wheras the 99 rs was a year of a new model (sort of). 98 rs was sort of an odd ball test subject. < what Im hoping the 2012 BRZ will be too :naughty:
 

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The Turbo engines have always been DOHC, they just didnt come to the US till 2002.

The new SOHC for the N/A cars flows about the same, is probably cheaper to build, and at least could be used to "fix" the reliability issue.

My guess is they kept the dohc in the legacy in 99 to get rid of their surplus since the new model was coming out the next year, wheras the 99 rs was a year of a new model (sort of). 98 rs was sort of an odd ball test subject. < what Im hoping the 2012 BRZ will be too :naughty:
the EJ22 turbo from the OG Legacy is a SOHC turbo dont forget

but yes, for the sure the 98RS was a parts bin special - using up tons of existing inventory to create a car for marketing purposes and to start priming the pump for what was to come down the road
 

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'17 Impreza Sport, '15 OB 3.6R
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I recall there being issues with the DOHC in the late '90s which is why they switched back to SOHC. RE: Any EJ25 1997-1998 Legacy/Outback.

DOHC being a more modern design though, obviously, they used it later on and in the new engine types.
 

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I recall there being issues with the DOHC in the late '90s which is why they switched back to SOHC. RE: Any EJ25 1997-1998 Legacy/Outback.

DOHC being a more modern design though, obviously, they used it later on and in the new engine types.
though keep in mind the GD chassis RS are still SOHC

what are current non turbo Impreza's? Havent paid attention beyond the turbo models in recent years
 

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'17 Impreza Sport, '15 OB 3.6R
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Well, I don't blame them for switching. The 2012 has a lot of haters, but every new car does. Frankly I think it was a smart move, on Subaru's part. The loss in horsepower in the base model is irrelevant to its buyers - it's the big increase in MPG that's important.

I also think it looks a lot better than the 2008-2011, though the rear/hatch does look a bit too much like a Dodge Caliber. And I hate playing the "it looks like" game. :) The new WRX and STi should be awesome.
 
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