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Subaru to Introduce Boxer Diesel in Geneva

The boxer turbo diesel. Click to enlarge.

Subaru will introduce a 4-cylinder horizontally-opposed (boxer) turbo diesel engine at the Geneva International Motor Show next month. This is not only its first diesel, but the first boxer turbo diesel.  Subaru had announced at the 2006 Paris motor show that development of the turbo diesel was almost complete.

Due for its first vehicle application early next year, the Subaru boxer turbo diesel is a highly rigid unit with low levels of noise and vibration.

This eliminates the need for a balancer shaft which counters uneven combustion pressures and general roughness. Subaru’s first diesel is as compact as its gasoline units and combines unusually strong pulling power at low engine speeds with high-rev throttle-response.

Subaru will unveil both the engine and a symmetrical all-wheel drive drivetrain as part of its press conference at the show on 6 March.



Wow. I'd rock one of those in a Baja for $25k.


The cutaway diagram suggests the cylinder block may be a single casting and there is one turbo. Having the weight low down is good but possibly more vulnerable to concrete divider strips and rocks.


Good for Subaru. The article states that it is the "first boxer turbo diesel". I understand this is Subaru's first foray into diesel engines, but is the article saying that this will be the first diesel to ever be built in the horizontally opposed format?

Rafael Seidl

Aussie -

I think your fears are unfunded. For one thing, gasoline boxer engines are just as exposed. For another, the most exposed portion of an engine is always the oil carter, regardless of how the cranktrain is arranged.

The one exception would be engines with dry sump lubrication, but afaik that is pretty much only used on cars built for smooth, clean racetracks. Subarus are built for off-roading, so the odd pebble shouldn't cause any serious damage.

Schmeltz -

Subaru and Porsche are the only manufacturers that mass-produce boxer engines these days. They tend to be more expensive and suffer from gas pulsation losses in the crankcase. Oil consumption may be higher as well. But mostly, their form factor requires a chassis designed specifically to accomodate their great width.

So far, Porsche has stuck exclusively with gasoline engines. With Audi winning LeMans with a diesel, Subaru coming out with a diesel boxer and the EU Commission proposing very low fleet average CO2 emissions, perhaps Porsche will reconsider - at least for the European version of their SUV.


Diesel's torque in an AWD passenger vehicle...sounds mighty tasty as long as it is 50 state emissions legal.

Rafael Seidl

Patrick -

I believe Subaru developed this engine at the European market only. Hopefully it will come with a DPF as standard rather than optional equipment. The engine would not meet EPA emissions - let alone California's - without an additional, expensive NOx store catalyst or SCR system.

Note that most AWD vehicles sold in Europe feature diesel engines. That's why Subaru had to come up with one in order to grow sales.


It is obvious that no engineer or anyone truly familiar with engines wrote the report.

"This eliminates the need for a balancer shaft which counters uneven combustion pressures..."

Balance shafts do absolutely nothing of the sort. They only balance mass forces and moments relating to the reciprocating motion of the piston (first order) and the articulating motion of the connecting rods (second order).


...and typically balance shafts are more about NVH than actual engine performance.


"It is obvious that no engineer or anyone truly familiar with engines wrote the report."

That entire release is directly from Subaru.



Nope. Subaru’s PR talks only about NVH, not uneven combustion pressure (that would be silly of them, but I’ve read even more illiterate things from PR people, and even from engineers from Porsche).

Uneven pressure is about uneven torque delivery, and depends on number of cylinders. 6 cylinders (or 12) is the best, and also 6-badger has balanced first and secondary order forces. In line six (Mercedes, BMW, Skyline…) and boxer 6 (Porsche, Subary) are the best, with 60 degree V6 only slightly off.


As any rotational mass connected to crank, balance shafts slightly impede revving-up of the engine and hence reduces performance. I bet only dyno could detect the difference. However, I bumped couple of times on instructions for performance enthusiasts for how to remove balance shafts for better performance.



(Note this post is from Patrick, look at the name in the bottom right hand corner not the top)
WHAT are you talking about? I SAID: "Balance shafts are more about NVH"

Anything dealing with combustion will be about engine performance and nearly ALL balance shafts are sucking away several horsepower from the additional mass and a slight amount of friction with the gearing and bearings used to support the balance shafts.



We are talking about same things, only use different wording.


Porsche designed a protype diesel boxer engine for the original beetle in the 50's. It did not make it into production. see:

Rafael Seidl

Just for completeness' sake, not that that an inline flat twin is also perfectly balanced. It's the minimal crank-slider configuration with that property.

I don't know of anyone who builds them, though. Three cranks and a split connrod are hard to fit unless your connrod is rather long and bore-to-stroke ratio high (i.e. a high RPM concept). Cooling the piston crown is a little tricky in any inline design, because gravity does not help you get rid of the excess oil you spray up there - especially in a sharp turn.

Alfred Roelli

Another opposed cylinder Diesel engine...


A turbo-diesel engine in the Forester would be fantastic!!!

Thanks for the link - a 647 kW TD Boxer engine - wow!


opposed cyl. is 1 piston in each a boxer engine. Jumo 205 has 2 pistons in 1 cyl.pistons opposed to each other, not cylinders opposed. for infernal combustion piston engines, it probably is the best of all designs. Thx Ron

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