The world's first boxer diesel engined-passenger car has arrived, offering outstanding refinement, throttle response and fuel economy coupled with ultra-low emissions and exceptional handling thanks to the light weight and low center-of-gravity of its horizontally-opposed powerplant.

This is Subaru's first diesel engine - developed in-house using over 40 years' expertise in 'boxer' engine technology coupled with turbocharging know-how gleaned from the fierce World Rally Championship arena.

The Japanese manufacturer's Boxer Diesel engine will initially only be fitted to European-market cars, with the yet-to-be introduced new Forester receiving the engine in September and new Impreza three months later.

Class-Leading Figures

The 150 PS Boxer Diesel Legacy and Outback not only have the best fuel efficiency in the 4WD passenger car class but beat all main 2.0 liter two-wheel drive rivals including the new Ford Mondeo TD and VW Passat- both in lower-powered 140 PS formats.

The Legacy Sports Tourer 2.0D R, for example, returns 49.6 mpg on the Combined Cycle with 151 g/km of CO2.

Its 2WD Ford Mondeo estate equivalent returns 47.9 mpg with 156 g/km, while the VW Passat 2.0 TDI estate gives 47.1 mpg and 158 g/km.

Comparing the new Legacy Sports Tourer with the 4WD version of the Passat - called the 2.0 TDI 4Motion - shows the Subaru to be 8.7 mpg better on the Combined Cycle with emissions down by 31 g/km compared to the VW.

Nor are these figures achieved at the expense of performance. The Legacy Sports Tourer has a 126 mph top speed and 0-60 mph time of 8.5 seconds while the Outback is only slightly behind at 124 mph and 8.8 seconds for the 0-60 mph dash.

Refinement is also a Boxer Diesel strongpoint, with official moving sound levels of 70.5 dB (A) compared to the Mondeo's 71.0 and Passat's 73 dB (A). Even a Rolls-Royce Phantom is higher at 72.0 dB (A).

Thoroughly-Developed Package

The new Boxer Diesel Legacy and Outback are thoroughly-developed models in their own right with several unique features to suit their diesel application.

For example, the engine has liquid mounts, with the body benefiting from extra sound-proofing to enhance refinement, while the five-speed manual gearbox has higher ratios to suit the diesel's stronger torque and lower engine speed.

The power-steering is now electric to improve fuel economy and carefully tuned for strong feel and response. Front spring rates have also been tuned to suit both the diesel engine and new steering system.

The Boxer Diesel's front brakes have also been uprated and the gear-lever length reduced by 10 mm for a sportier, shorter throw.

Even the air-conditioning unit has been modified to provide hot air earlier in the engine's warm-up cycle, diesel engines having different characteristics to petrol powerplants.

Visual changes include new cloth seat trim for Legacy and Outback R models and a discreet bonnet air intake scoop.

Dedicated to the horizontally-opposed 'boxer' engine layout for over 40 years, Subaru engineers were determined to use this format for their first diesel engine.

The reasons were to create a light, compact, strong unit with reduced vibration, noise and internal friction compared to an in-line engine and for an ultra-low center-of gravity.

This enhances handling agility and roadholding - especially when combined with Subaru's renowned symmetrical all-wheel drive system.

In fact, the entire engine and transmission are not only mounted very low down but also within the wheelbase which further boosts chassis dynamics.

Canceled Out Vibration

The 2.0 liter Boxer Diesel has a class-leading light weight with the horizontally-opposed pistons effectively canceling out the secondary harmonic vibration (vibration with twice the frequencies of the engine speed).

The crankshaft is short and highly rigid, minimizing the vibrational noise of many diesel engines.

And the superior balance of the horizontally-opposed engine means internal rotational inertia and friction are drastically reduced compared with an in-line engine.

Reduced internal friction means throttle response is exceptionally lively and in keeping with Subaru's sporty heritage.

Compared to Subaru's 2.0 liter engine fitted to the Impreza, Legacy and Forester, the 2.0 liter Boxer Diesel has a 11 mm longer stroke and 6 mm smaller bore for a capacity of 1,998 cc versus 1,994 cc.