Under square vs. Over square engines

22,335

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

79,735

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.

79,735

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

79,735

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.

22,335

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 ,

22,335

Here's the information on the 2.0 Four

5,710

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

79,735

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?

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