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Old 03-04-2002, 04:11 PM   #1
Patrick Olsen
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Default Dyno graph comparison - stock SOHC vs. stock DOHC

A little while back a group of us NESIC types got together for a dyno day at Adrenalin Motorsports in Massachusetts. I dyno'd my '97 Legacy 2.5GT 5-speed sedan in stock configuration (well, stock + K&N drop-in filter and synthetic fluids), and Marc Sawaya dyno'd his '00 2.5RS 5-speed also in stock configuration.

We all know that the SOHC is supposed to have better mid-range than the DOHC, and I wanted to see how true that was. So, I finally got around to reading the numbers off of my graph and off of Marc's graph to create a combined DOHC vs. SOHC graph. I did actually cheat a bit on my graph - the day that Marc dyno'd his car (and hit 104.x hp) my car dyno'd at 100.6hp or something like that. On a previous day, also in stock configuration, and on the same dyno, I had dyno'd at 104.x hp. Knowing that (theoretically) the peak power should be the same on the two engines, I used my graph from an earlier day so that the graphs would "match". Who knows, maybe in real life the SOHC cars actually do make a few more HP than the DOHC engines, as was indicated on the day we dyno'd at the same time. Or, perhaps the few extra HP that Marc's car laid to the rollers was due to the fact that my engine has twice (?) the mileage and hence is a bit tired? I dunno. I just figured for a "neater" comparison I'd use the graphs that had virtually matching peak power outputs.

So, here's the graph:


It's pretty clear to see that the SOHC has significantly better mid-range torque. You can also see that, at least in this case with these specific engines, the DOHC doesn't drop off as severely at high RPM. (Plus we DOHC-kids can actually rev to redline! )

Pretty cool, eh?

Edit: I apologize for the light blue SOHC Torque curve, which is kinda hard to see - you'll have to thank Microsoft Excel for that!

Also, the dyno used for both cars was a Dyno Dynamics Low Boy AWD eddy current type dyno.

Pat Olsen
'97 Legacy 2.5GT sedan

Last edited by Patrick Olsen; 12-06-2006 at 01:23 PM.
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Old 03-04-2002, 04:31 PM   #2
ColinL
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that's pretty cool Pat, good job even if it's not scientifically robust it's interesting stuff.

cobb tuning street cams will reduce the tendency for a 2.5L sohc to die above 5k.

the other thing that was touted by FHI in 1999 when the Phase II SOHC first replaced the 2.5L DOHC in impreza and forester models was that it had significantly more torque at part-throttle. that's a little hard to dyno though.
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Old 03-04-2002, 04:38 PM   #3
Patrick Olsen
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Quote:
Originally posted by ColinL
the other thing that was touted by FHI in 1999 when the Phase II SOHC first replaced the 2.5L DOHC in impreza and forester models was that it had significantly more torque at part-throttle. that's a little hard to dyno though.
Yeah, I remember some stat like the SOHC had twice as much torque at 30% throttle at 2500rpm or something like that. Having ridden in a couple of SOHC 2.5RSs, and more recently a '00+ SOHC 2.5GT, it's definitely a noticeable difference.

Pat
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Old 03-04-2002, 06:04 PM   #4
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Based on this information, does anyone want to swap heads? have SOHC 2.5 heads.

Jeff
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Old 03-04-2002, 06:18 PM   #5
Midwayman
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Very cool stuff. Thanks for sharing!
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Old 03-04-2002, 06:18 PM   #6
ColinL
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jeff-- IMO that's a mistake, given the SOHC's narrower valve angle and higher flowing ports.

unless you plan to heavily modify the heads, appropriate aftermarket cams in your SOHC heads is much smarter.
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Old 03-04-2002, 06:24 PM   #7
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Thanks for your recommendation on keeping the SOHC heads.

I have several motives for wanting to swap heads, including the ability to use more aftermarket ECU's. I also am craving anything that will give me a higher redline. If I remember correctly the DOHC had a higher stock redline.

jeff
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Old 03-04-2002, 06:38 PM   #8
inpreza kid
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thats pretty intresting. i can understand why the DOHC had a 6500 rpm redline now. i would still take the SOHC because with minor fuel/ignition tuning i bet the power won't drop off as bad after 5200rpm.
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Old 03-05-2002, 12:09 AM   #9
Farfrumgerman
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Talking HAHA

That's funny. I actually printed these two dyno's and manualy imposed one on top of the other to see the difference back when Pat first spread the word of the MRT, etc... dyno day.

I do like the rev-happyness of the dohc, but man is that torque band FAT with the sohc. In "heated" driving I supose it would be about even (keeping it on the boil) in terms of acceleration - but around town, and cruisin' up the mnt that Tq would be a welcomed addition.

anyway - this just furthers my cause of sourcing a wrx motor to replace the dohc in me GT! (there is no stoping me now - I was trounced by a silver rex wagon on the street, and my wife , who was in the car, said "baby, we gotta get you a TURBO!")

sweeeeeeeeet she is soooo coool!

-Chad
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Old 03-05-2002, 11:49 AM   #10
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Default What about a HYBRID?

Pat,

I'd be interested to determine whether or not my using a DOHC ecu had any bearing on performance of my SOHC engine? I remember reading something on the Cobb Tuning website that the MAF based ecu's can compensate for variations in environmental conditions better than MAP based ecu's due to more accurate fuel mapping. That being said, I have noticed that the SOHC engine in my GT seems to pull fairly strong all the way to the 6500rpm redline. I'm not sure if this is due to the MAF ecu or any other additions made to the car . . . I just have an intake and exhaust . . . nothing major.

Also, I noticed that you compared your GT to an RS . . . are there more driveline losses associated with a larger driveshaft, or are the differences negligible.

I'm not trying to poke holes in your comparison (I'm saving it on my computer as we speak), I just wanna know if I made the right decision in swapping out engines . . .

Jason
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Old 03-05-2002, 01:36 PM   #11
Patrick Olsen
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I'm curious about the SOHC engine running off the DOHC ECU myself, Jason, so I really don't know what to tell you there. I'd be really curious to see HP/Tq graphs and an AFR graph for your car compared to stock DOHC and stock SOHC graphs - those would really let you see how well the ECU was adapting to the "wrong" engine.

As for the drivetrain losses, I would assume the differences are negligible, but that's just my feeling - it's not really based on any engineering fundamentals.

Pat
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Old 03-05-2002, 02:11 PM   #12
ColinL
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actually based on the fundamentals, I would say not to place too much stock in the pure numbers. tire & wheel weight, gearing, rolling resistance of the tires, environmental conditions, etc. prevent chassis dynos from being a good measure of one engine purely against another. honestly given the same model of car, trap speed at the drag strip would tell you more about peak HP and area under the torque curve in the powerband.

but clearly we can safely draw some conclusions about the SHAPE of the curves. no problem there.
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Old 03-05-2002, 02:22 PM   #13
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Quote:
I'm curious about the SOHC engine running off the DOHC ECU myself
The original reason this question popped into my head was after reading about SOHC engine owners using S-AFCs to lean out their A/F mixtures at higher rpms. Could this be an aspect of the "less than stellar" performance of the MAP-based ecu? I remember somebody posting dyno runs of pre- and post- S-AFC tuning and he gained something like 7-8hp!! This would place the SOHC engine in similar territory as the DOHC engine WRT hp numbers above 5000rpm.

Anybody know how much a AWD dyno session at that shop in Gaithersburg, MD is? I know, it wouldn't be a TRUE comparison considering differences in dynos, etc . . . but it would give a better understanding as to whether the ecu is to blame for those high rpm power losses with the SOHC engine.

Jason
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Old 10-17-2004, 12:50 PM   #14
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Whats the HP/QM specs for the stock 00 2.5RS 5spd. I just picked one up, and ive been looking around for the specs, and so far have been unsuccessful.
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Old 10-17-2004, 01:34 PM   #15
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iirc, 15.3 1/4 from magazine testing, something like 8.3 0-60

hp, depends on the dyno, 165 stock crank hp, cobbs dyno shows 121.x whp, another type shows right around 100
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Old 10-17-2004, 09:03 PM   #16
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I've seen 0-60 as low as 7.7, stock. Your Impreza is faster than the new generation w/ the EJ25.
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Old 10-18-2004, 12:23 AM   #17
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Man, I loved reading this info back then. It's a shame more discussion on this didn't take place back then in this thread as this is a cool dyno comparison. Oh well!
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Old 10-18-2004, 12:16 PM   #18
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FWIW, I went to Mastro Subaru (DynoPack) for a dyno run this morning. 137.48 HP(~@4900rpm) and 148 lb-ft(~@4200rpm). This are labeled as "Corrected HP SAE" and "Torque Flywheel".

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-nils

Last edited by nils; 10-18-2004 at 12:27 PM.
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Old 10-18-2004, 05:51 PM   #19
Matt Monson
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Quote:
Originally Posted by Kevin Thomas
Man, I loved reading this info back then. It's a shame more discussion on this didn't take place back then in this thread as this is a cool dyno comparison. Oh well!
Me too. If you want more discussion, go here:
http://www.wrxforum.com/cgi-bin/ulti...=000917#000000
I reintroduced this discussion on Cobb's site one day when I was trying to decide if I was staying SOHC or going to DOHC. I even bastardized Pat's dyno sheet.
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Old 10-19-2004, 04:49 AM   #20
Patrick Olsen
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Holy old thread, Batman!!

Pat

PS - I fixed the image link in the original post so people can still see the dyno curves.

Last edited by Patrick Olsen; 10-20-2004 at 04:47 AM.
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Old 10-19-2004, 04:50 PM   #21
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98RS fo life!!
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Old 12-09-2005, 09:37 PM   #22
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Which cars came with the DOHC and SOHC engines?

(sorry to dig this thread from the grave)
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Old 12-09-2005, 10:20 PM   #23
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The 98 2.5RS was the only Impreza to have the DOHC. The Legacy had them up to 99. 99+ N/A 2.5L Impreza's and 00+ N/A Legacy's all have the SOHC engine.
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Old 12-09-2005, 10:40 PM   #24
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I believe some '99 N/A Imprezas were DOHC...
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Old 12-10-2005, 12:13 AM   #25
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anyone know what gear those cars were dyno'd in?
100hp max? so there was a 60some hp loss from the crank? or am i misreading.
can someone knowledge this noob?
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