Exploring the Subaru Legacy Tourer’s Boxer Diesel Engine
Posted on June 3, 2011 by Cathal Doyle
I’ve been spending some time with our long term test car, the Subaru Legacy Tourer, including a 600 km round trip this week for what was its first caravan towing test. That will be featured in full in the forthcoming edition of Caravan Cruise Ireland magazine, but suffice to say that the Legacy has been more than a pleasant revelation in every respect so far. And hooked up to a Bailey Unicorn via the Malcolm’s Towbar Services supplied Witter towbar, it certainly looks a well balanced and purposeful outfit – at least I think so!
Today though, I want to focus on the two litre Boxer diesel engine that powers the Legacy Tourer. Indeed apart from the 1-litre petrol unit found in the Justy, this is the only engine option available across all of Subaru’s models presently sold in Ireland, a reflection once again of how dominant oil burners have become here in the past couple of years.
Subaru is a brand that does things its own way, one where engineering prowess comes first and foremost. For years the company has been a champion of the boxer engine format, a flat-four system whereby the four cylinders are horizontally opposed. That format isn’t the easiest to design or manufacture, which is probably why other manufacturers haven’t followed Subaru down the flat four route, but it has two major advantages over conventional straight or V-designed engines where the pistons operate up-down rather than in a horizontal plane. Firstly, it allows for a lower centre of gravity for the car, thereby improving handling, and secondly, because each pair of pistons moves simultaneously in and out instead of alternatively, the engine is better balanced and has less inherent vibration than ‘straight’ engines.
Subaru was relatively late to embrace the diesel revolution, for a long time maintaining faith with its petrol flat-four engines in the face of a growing diesel tide. Ultimately it became clear that without such an offering the brand was going to fail, at least in Europe, but whereas the easy option would have been to buy in a diesel powerplant from another manufacturer, this just wasn’t the Subaru way. As a relative minnow among motor manufacturers, it is to the brand’s credit – and determination to remain true to its engineering principles – that, in 2008, it introduced the world’s first, and thus far only, diesel boxer engine.
But enough of the history, what’s it like? Taken in isolation the figures, while respectable, don’t stand out particularly. The 1998 cc Euro 5 compliant DOHC flat-four produces 150 PS at 3,600 rpm, with a maximum torque figure of 350 Nm at 1,800 – 2,400 rpm. That doesn’t give the full picture though, as it is how the Boxer Diesel drives that makes it a rather special powerplant.
Start it up, and while from the outside it is unmistakably an oil burner, it does sound ‘different’ to other diesels. Inside you’ll be harder pressed to know what fuel the car is powered by, with little or no diesel rattle to be heard. Rev it past 2,000 rpm, and there is a hint of that boxer growl often associated with rally bred Imprezas. Not that you would be mistaken for a boy racer in this refined estate car, but the absence of a typical diesel clatter is certainly a pleasing characteristic of the engine.
The second thing you quickly notice about the Subaru Boxer Diesel is how free-revving it is, and in fact, its characteristics are more akin to petrol engined cars than other diesels. For starters, despite the healthy Nm output, the car doesn’t feel particularly torquey at low revs, and I did notice when towing that it needed a reasonable application of the accelerator pedal to get moving. On the other hand, unlike other diesels which both run out of grunt and sound horribly harsh if you rev them much beyond 2500 rpm, the Subaru encourages you to stay in gear rather than change up early. Even beyond 3,000 rpm it continues to surge forward with enthusiasm, while the balanced layout of the flat-four design can be noticed in the linear power delivery throughout the rev range.
On the move, whether it be motorway or country lane, the Subaru Legacy Diesel scores highly for refinement, with little engine noise permeating through to the cabin.
There’s a six speed gearbox which on initial impressions felt a little notchy, but after a short time has becomes second nature to use. Considering the car has little over one thousand kilometres under its belt, it may well loosen up over time. I also feel that even though the gear ratio figures look well spaced out on paper, in reality there is something of a gap between second and third, which can cause the engine to bog down somewhat if you change up too early. Again though the free revving nature of the engine helps, and it is just a case of adapting one’s driving style slightly more towards how you would drive a petrol car versus a diesel to overcome this.
I’ve been pleased with the fuel consumption figures so far as well, with the Subaru returning a worst figure of 6.6 l/100 km (42.8 mpg) fully laden on a motorway drive, but overall I’m averaging between 5.7 and 5.9 l/100 km (47.8 – 49.5 mpg). Considering the size of the car and the fact that there is a full time four wheel drive system to be propelled along, it is impressive that the Legacy can match and beat many competitors in this regard.
In summary, the Boxer Diesel is a very impressive offering indeed, matching the fuel efficiency of a normal diesel powerplant with the driveability of a petrol engine. I’m looking forward to spending much more time with it this summer.