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Chances are unless you’ve worked in secretive operations inside the Porsche R&D facility or lived in Europe and paid close attention to the curiously wonky sounds emanating from a Subaru Legacy idling in traffic, you’ve never heard four or six horizontally opposed cylinders suck diesel. 

I’m just sayin’. The sound is as unfamiliar to you as it is to moi.
Nevertheless, two manufacturers are addicted to this cylinder layout. The boxer engine, found in the rear end of the Porsche 911 Carrera, Boxster, and Cayman as well as the Subaru Legacy/Outback and Tribeca in six-cylinder form and in the Subaru Impreza, Legacy/Outback, and Forester in four-cylinder form, is a unique layout.
See, everybody else likes to line up their cylinders in a row or position them in a V, like Canadian geese except without taking turns at the front. True, Volkswagen Group members have been known to configure their cylinders in a W formation, most famously in the Bugatti Veyron and previous-generation Volkswagen Passat. 
The boxer nomenclature comes from obvious engines. Think of Obama and McCain or Biden and Palin who will go back and forth at each other in a few debates, over and over and over. They are diametrically opposed, basically two competitors inside a square circle ring; just without the punches. As for the Subaru and Porsche engines, the cylinders are going back and forth at each other; also without making contact in an uppercut or jabbing fashion.
Because of this strange configuration, Porsches and Subarus export a unique sound. We all know that diesels, even modern ones, still have a bit of a unique note as well. 
Subaru has joined the two. The aluminum block 2.0L boxer four is now in the Forester and makes 145 horsepower. That’s 25 down on the output of the basic 2.5L gasoline-fed four-cylinder in the Forester. However, torque stands at 258 lb-ft, up 32 lb-ft on the turbocharged version of the 2.5L gas engine and 88 lb-ft more than the torque output from the naturally aspirated 2.5L. All that torque is available from a very low 1,800rpm.
With towing capacity of 4,400 pounds, Foresters with the boxer diesel can tow an extra 2,000 pounds. And as you’d expect, the Subaru Forester with the boxer diesel and new 6-speed manual transmission will return between 8-10 more miles per gallon than an equivalent gasoline-powered Forester. Subaru UK claims the Forester diesel has the lowest fuel consumption and emissions in its class.
What must it sound like? Check back soon and The Good Car Guy will post a couple YouTube clips.