Under square vs. Over square engines

19,815

Asked by Nov 18, 2015 at 09:34 PM about the 2010 Subaru Outback 2.5i Limited

Question type: General

The changes that Subaru made from the EJ 25 series 2 motor to the new FB
series engine introduced first on the Forrester in 2011 and later on the Outback
in 2013,  had some significant changes.  The bore and stroke of these engines
changed from the old   EJ  design of 99.5 bore and 79 stroke to the new FB
specifications of 94 bore to 90 stroke.  While this may not seem significant at
first blush, it means that the engine design went from being over square to
under square.   See this summary of technology talk about these different
engine designs and layouts.  Could this possibly be one of the reasons that
some newer models are burning oil.   See attachment,

7 Answers

65,195

They both were over square Mark. The new engine does have a longer stroke which will give more torque. The new engine is almost square but still over square.

65,195

Square is when the bore and stroke are the same. Both engines have a larger bore than stroke.

65,195

Many engines have similar bore/stroke ratios and don't use oil. The reason Subaru's use oil is low tension oil rings and 0 -20 oil. It really is not a mystery.

19,815

I had read somewhere that they went from hugely over square to square. You're right , in the case of the 2.0 Four, the engine became an under square engine. See this ,

19,815

Here's the information on the 2.0 Four

4,570

FoR and I are in full agreement. Stop using skinny synths.

65,195

What will fix this problem is an aftermarket manufacturer to make a new piston for these engines that will take standard style rings. It would take a full re-build but it would solve the oil consumption problem. You might lose 0.1 mpg but who would care?

Your Answer

Add photo

Related Questions

Outback

Looking for a Used Outback in your area?

CarGurus has 42,349 nationwide Outback listings starting at $1,100.

ZIP:

Search Subaru Outback Questions

Subaru Outback Experts

#1 Markw1952
Markw1952
Reputation 7,000
#2 TheSubaruGuruBoston
TheSubaruGuruBoston
Reputation 2,200
#3 F_O_R
F_O_R
Reputation 1,840
View All

Related Models For Sale

Used Subaru Forester
413 Great Deals out of 29,349 listings starting at $995
Used Subaru Impreza
203 Great Deals out of 19,393 listings starting at $1,995
Used Subaru Legacy
225 Great Deals out of 14,432 listings starting at $999
Used Honda CR-V
924 Great Deals out of 59,345 listings starting at $1,990
Used Toyota RAV4
892 Great Deals out of 67,125 listings starting at $1,990
Used Toyota 4Runner
260 Great Deals out of 19,017 listings starting at $2,500
Used Toyota Highlander
362 Great Deals out of 26,007 listings starting at $2,800
Used Toyota Tacoma
473 Great Deals out of 41,586 listings starting at $1,995
Used Honda Accord
946 Great Deals out of 77,735 listings starting at $899
Used Ford Escape
1,438 Great Deals out of 90,835 listings starting at $900
Used Jeep Grand Cherokee
1,022 Great Deals out of 72,251 listings starting at $700
Used Toyota Camry
1,104 Great Deals out of 90,757 listings starting at $1,599
Used Honda Pilot
390 Great Deals out of 22,299 listings starting at $1,500

Used Cars For Sale

2017 Subaru Outback 2.5i Limited For Sale
22 Great Deals out of 13,203 listings starting at $25,787
2016 Subaru Outback 2.5i Limited For Sale
26 Great Deals out of 407 listings starting at $18,913
2015 Subaru Outback 2.5i Limited For Sale
20 Great Deals out of 575 listings starting at $15,990
2014 Subaru Outback 2.5i Limited For Sale
21 Great Deals out of 478 listings starting at $11,999
2013 Subaru Outback 2.5i Limited For Sale
10 Great Deals out of 334 listings starting at $8,950

Content submitted by Users is not endorsed by CarGurus, does not express the opinions of CarGurus, and should not be considered reviewed, screened, or approved by CarGurus. Please refer to CarGurus Terms of Use.