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… 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 larger motor is quite prolific, with increased torque all through the rev range, making squirting through the mountains south of Palm Springs, California, a brisk affair. – Car and Driver
This entry was posted
on Thursday, April 26th, 2007 at 9:58 am and is filed under SUVs, Subaru, technical.