Subaru Develops More Powerful, Efficient Boxer Engine

PALM SPRINGS, CA – Subaru of America Inc. introduces a more powerful engine for the ’08 Tribeca cross/utility vehicle that is equally compact as the first-generation mill.

For ’08, SOA replaces the Tribeca’s 3.0L 6-cyl. boxer with a 3.6L version that produces 256 hp and 247 lb.-ft. (335 Nm) of torque, up from 245 hp and 215 lb.-ft. (292 Nm).

Powertrain engineers were tasked with two difficult objectives:

In order to keep the new engine roughly the same size as its predecessor, engineers increased its bore and stroke while reducing the amount of space between cylinders. This left the overall width unchanged, and added only an incremental 0.83 in. (21 mm) to the engine’s length.

The extra length was due to a change in the cam drive layout, David Sullivan, product car line manager for SOA, tells Ward's at a media event here.

“Increasing the bore was achieved by basically keeping the bore pitch the same (as the 3.0L),” he says. “However, we’ve decreased the distance between the edges of the cylinders. We were able to do that by redesigning the outer liners.”

The iron cylinder liners were made thinner, allowing for a better aluminum flow when the block was cast, Sullivan says.

Subaru raises horsepower in ’08 Tribeca without increasing engine size.

Using an asymmetrical connecting rod design also helped engineers to squeeze the cylinders closer together without widening the engine, Sullivan says.

Subaru decided to go with intake and exhaust variable valve timing, whereas the earlier mill only employed VVT on the intake side.

“This year we’ve got variable valve timing on both the intake and exhaust side, and it makes for a more flexible engine,” Sullivan says. “You can alter the timing to increase the torque characteristics on low- to mid-range engine speed, and it also allows us (greater) cylinder efficiency in higher engine rpms to help with fuel economy.”

Subaru combined VVT technology with a new engine-cooling system in order to allow the 3.6L to run on regular-grade gasoline without knocking.

The new cooling system decreases by 50% the difference in temperatures from one cylinder to another, Sullivan says, allowing engineers to advance timing in the low- to mid-rpm range of the engine and increase torque without inducing knock.

“That cooling system allowed us to get that big increase in the lower end of the rpm range to beef up the engine’s torque curve,” Sullivan says.

Meanwhile, Sullivan says Subaru has not ruled out the creation of a high-performance version of the 3.6L boxer engine. “There is some room for development in this engine, and we’ll continue to look at that,” he says.

“Subaru engines tend to have long lives, and we tweak them a lot over the life of development,” he notes, adding turbocharging the 3.6L mill has been discussed.

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