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Old 04-01-2012, 01:56 PM   #1 (permalink)
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Default Torque Curve b/w 2.5 and 3.6

Hey all,

As i have done in my past cars, i always like to see just how flat the torque curve is. (With the 4runners, you can see here: Torque Curve b/w V6 vs. V8 - Toyota 4Runner Forum)

Based on the brochure graph of the two engines, i made some extrapolations: http://www.elitemotors.ca/brochure/O...0_Brochure.pdf

Unfortunately, i do not have the new FB (2.5 DOHC) engine's torque curve. If anyone does, then please show me!!

Disclaimer: this is all based on me drawing a line down and across (with some extrapolation)...so, the torque figures are not 100% accurate, but should give you an idea of torque curve differences between the two engines. With both, i tend to favor a little bit on the generous side in extrapolating. So, as always, take it with a GRAIN OF SALT!

torque % = torque at specific RPM divided by max torque

2.5 Liter SOHC Boxer (max torque 170 lb-ft at 4000 rpm)

1000 rpms = 137 lb-ft divide by 170 lb-ft = 80.6% of max torque
2000 rpms = 156.25 = 91.9%
3000 rpms = 162.5 = 95.6%
4000 rpms = 170 = 100%
5000 rpms = 164 = 96.5%
6000 rpms = 140 = 82.3%

3.6 Liter DOHC Boxer (max torque 247 lb-ft at 4400 rpm)

1000 rpms = 178 lb-ft divide by 247 lb-ft = 72.1% of max torque
2000 rpms = 225 = 91.1%
3000 rpms = 237.5 = 96.2%
4000 rpms = 244 = 98.8%
5000 rpms = 242 = 98%
6000 rpms = 225 = 91.1%

Conclusion: At least based on my primitive math and extrapolation, the 2.5 engine is designed to favor low RPM. It suffers at higher RPMs where it runs out of breath, esp. after 5500 rpms. (I would assume that the new DOHC will improve on this across the board.) The 3.6 engine is excellent throughout, maybe a reflection of the DOHC design. The torque curve shifts ever so slightly more toward the upper RPM end. Regardless, at any speed, the 3.6 produces more power to move the OB than the 2.5 because there is "no replacement for displacement" nor is there a replacement for superior cylinder counts. The 2.5 does hold its own in that the torque curve is flat where it counts for most owners...and certainly, with me, my OB 2.5 can get out of its own way at low revving. (And this may explain why most 2.5 owners find the engine more than adequate, even at high altitude.)

What does this all mean? I have no idea. It is more of a fun exercise for me.

**See page 3 of this thread (specifically: http://www.subaruoutback.org/forums/416036-post30.html) to see the torque curve of the Kia's Theta II 2.4D-CVVT gasoline engine found in their SUVs.
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Old 04-01-2012, 03:10 PM   #2 (permalink)
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Thai, thanks for posting this.

There was a thread about a month or two ago on this topic, but it resulted in more of a controversial discussion about what torque is and how its figures are derived, rather than a posting of the actual figures at stated RPMs. The bottom line - - the engines, whether a 2.5 / 4 cyl., or a 3.6 / 6 cyl. do what they are supposed to do and make power / torque in ranges which are expected for their particular type. End of story!
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Old 04-01-2012, 04:49 PM   #3 (permalink)
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Add to this that the 3.6 has VVT and is one reason the torque curve is almost flat from 2000 RPM to 5000 RPM.
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Old 04-01-2012, 06:57 PM   #4 (permalink)
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Quote:
Originally Posted by OutbackUFO View Post
Add to this that the 3.6 has VVT and is one reason the torque curve is almost flat from 2000 RPM to 5000 RPM.
So does the 2.5..
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Old 04-01-2012, 07:27 PM   #5 (permalink)
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The boxer diesel 2.0D:
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Old 04-01-2012, 08:05 PM   #6 (permalink)
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Quote:
Originally Posted by RobMunach View Post
So does the 2.5..
Didn't know that..
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Old 04-01-2012, 10:43 PM   #7 (permalink)
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Only the 3.6R and the new FB series engines (Forester, Impreza, 2013 outback and 2013 legacy) have dual VVTI (requires 4 angle adjustable cams). The current 2.5 in the 2012 outback and legacy have a rather primitive variable lift device on the intake valves --- not at all the same.
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Old 04-01-2012, 10:59 PM   #8 (permalink)
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Quote:
Originally Posted by Thai View Post

At least based on my primitive math and extrapolation, the 2.5 engine is
designed to favor low RPM. It suffers at higher RPMs where it runs out of
breath, esp. after 5500 rpms. (I would assume that the new DOHC will
improve on this across the board.)
Here are some numbers based on published Subaru specs that seem to
suggest a very different conclusion. Specifically, the current 2.5i SOHC
engine (EJ253) is very competitive with any/all of the others in terms
of specific torque (lb-ft per liter) and specific power (hp per liter).

Also, the SOHC maintains an impressive 93.8% of max torque all the
way out to its horsepower peak at 5600 rpm. In this respect, it beats
both the DOHC 3.6R (EZ36D) and 2.5x Forester (FB25) engines. The
latter is expected to be very nearly the same as the 2013 OB 2.5 --
but AFAIK, Soobie hasn't published all the details yet (i.e., rpms for
peak torque and horsepower, and exact displacement). Notice that
the 2013 OB gets an additional 3 hp, but no increase in torque over
the 2012 Forester/FB25 -- so, it's not exactly the same.

Anyhoo, the current OB's 93.8% torque @ 5600 rpm is indisputable
evidence that it has NOT "run out of breath ... after 5500." In fact,
it handily beats the DOHC Forester (88.5% @ 5800) in that respect.

Conclusion: I find it rather disappointing that Mother Soobie has
"upgraded" the 2.5L engine from SOHC to DOHC -- while gaining
little or nothing in terms of specific output. The very small gains
in power and torque can be attributed almost entirely to the 1.7%
increase in displacement (2498 vs. 2457 cc). So, why bother?

Code:
Subaru Engines   2012 Outback   2012 Forest.   2013 Outback   2012 OB 3.6R
Engine code      EJ253          FB25           FB25x   (?)    EZ36D
displacement     2457 cc        2498 cc        2498 cc (?)    3629 cc

max horsepower   170 @ 5600     170 @ 5800     173 @ ????     256 @ 6000
max torque       170 @ 4000     174 @ 4100     174 @ ????     247 @ 4400

specific power   69.2 hp/L      68.1 hp/L      69.3 hp/L      70.5 hp/L
specific torque  69.2 lb-ft/L   69.7 lb-ft/L   69.7 lb-ft/L   68.1 lb-ft/L

torque @ max hp  159.4 @ 5600   153.9 @ 5800   xxx.x @ xxxx   224.1 @ 6000
% max torque     93.8% @ 5600   88.5% @ 5800   xx.x% @ xxxx   90.7% @ 6000


Equations:
power:   hp    = lb-ft x rpm  / 5252
torque:  lb-ft = hp    x 5252 / rpm
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Old 04-01-2012, 11:11 PM   #9 (permalink)
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There is more to it than just peak numbers. You would need to compare the full curves as well as mpg numbers.
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Old 04-02-2012, 12:19 AM   #10 (permalink)
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Quote:
Originally Posted by Seabass View Post
There is more to it than just peak numbers. You would need
to compare the full curves as well as mpg numbers.
You're correct that none of the posts (so far) in this thread have
addressed mpg. There could be significant differences that are
not predictable from the curves or xxx @ rpm specs. However,
the Soobie press releases don't suggest that there are.

OTOH, the peak power @ rpm and torque @ rpm specs reveal
pretty much everything worth knowing WRT performance.
The general shapes of engine output curves are well known
and predictable -- and highly similar among similar engines.

The power @ rpm and torque @ rpm peak numbers locate two
points on each curve (although there's really only ONE curve).
Since there's very little room for differences in curve shape
among engines as technologically and geometrically similar as
those discussed above, nailing down two well-spaced points on
each curve is enough to rather accurately predict and compare
engine performance.

.
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