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Originally Posted by BLSTIC
The oversquare bore sizing is purely about revving harder and bigger valves. Unfortunately it has a deleterious effect on fuel economy thanks to flame travel distance.
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It also increases the surface area to dissipate the thermal energy right after the combustion.
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You'll note how pre-DI Subaru Flat 4's use much, much more fuel than their Mitsubishi counterparts despite being otherwise fundamentally similar cars
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I'm not so used to Mitsubishi, only to the 4D56 engine, but anyway, one thing that quite surprised me is the comparatively low compression of the Subaru. My dad had a '10 Impreza with the 2.0L engine and it was quite fuel-efficient despite being AWD.