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autoweek article on the 08 beca

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Not much news, but more detail on the engine

2008 Subaru Tribeca - Auto Shows
Subaru’s slow-selling mid-sizer gets a new 3.6-liter engine and less polarizing styling.
March 2007

Although the availability of a “real” SUV in the form of the B9 Tribeca bumped overall Subaru sales past the 200,000-unit mark in 2006, sales of the controversially styled mid-size SUV were considered mediocre. In response, Subaru is bringing out a hastily revised version. The new Tribeca (the B9 suffix will be dropped for the 2008 model) features fairly comprehensive front and rear styling revisions, replacing the controversial Alfa Romeo–like cues with more conventional treatments. At the same time, the flat-six powerplant is being boosted in capacity from 3.0 liters to 3.6 liters in the interests of better performance and economy.

But here’s where it gets interesting. The flat-six engine (or H6, as Subaru calls it) is already a tight fit in the Tribeca’s engine bay. Since the H6 is a horizontally opposed layout, any significant increase in crankshaft stroke would result in engine-block deck-height increases that would make the engine too wide to fit in the existing structure.

The problem was exacerbated by typically thrifty computer-aided engine design, which left minimal space between the engine’s cylinder bores. Subaru calculated that it could safely increase the bore size from 89.2mm to 92.0mm, but that change would have bumped the displacement to only 3191cc. Subaru needed more than that for the performance improvements it was seeking, so it came up with a novel way of increasing piston stroke without raising the cylinder deck height. The answer was an asymmetrical connecting-rod design (which looks somewhat like a lamb chop) that permits the piston to travel farther downward into the crankcase and increases the effective stroke from 80mm to 91mm.

Increasing both bore and stroke has resulted in a 3.6-liter engine, a growth of 0.6 liter, with substantial gains in power and torque. The new engine produces 256 horsepower at 6000 rpm versus 250 at 6600 in the previous engine. But torque now peaks at 247 pound-feet at 4400 rpm versus 219 at 4200, and about 180 pound-feet are now available at engine speeds as low as 1200 rpm.

Various tuning revisions have altered the engine’s octane requirement from premium to regular, promising substantial cost savings to the Tribeca’s owners, and the new engine is also expected to yield something like a 10-to-15-percent fuel-economy improvement, putting its EPA numbers somewhere around 20 mpg city and 25 highway.

During the engineering redesign process—which pared about 10 pounds off the engine’s weight—Subaru altered the cooling system from one where cylinders receive the coolant flow sequentially to one where separate ducts provide each cylinder with its own fresh supply. This more-equal cooling strategy allowed the engineers to advance the spark timing by one degree, yielding six pound-feet of torque in the process.

The overall improvement in torque delivery permitted revisions to the transmission torque converter and shifting strategies, greatly reducing the amount of hunting and downshifting that is considered annoying by current Tribeca owners. The result, says Subaru, is a more flexible and tranquil drivetrain, with improved throttle response, a more flexible operation, and better performance with lower operating costs. It sounds like a tall order, and we’ll see how well the new Tribeca performs when it arrives later this year as a 2008 model.
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