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2014 Forester XT CVT
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Subaru Drive Performance Magazine : Twin-Scroll Turbo (Blueprints dp 10.3)

In today’s auto industry, there’s no such thing as “business as usual” when it comes to building engines. Tighter regulations in fuel economy demand that a next-generation engine delivers not only outstanding performance, but also impressive fuel efficiency. The quick, lean, and clean new 2.0-liter DOHC SUBARU BOXER twin-scroll turbocharged engine exceeds those demands.

Serving up 250 horsepower and 258 lb-ft of torque, the twin-scroll turbo meets the needs of two distinct types of drivers – those who delight in the sense of swelling acceleration that a turbo engine offers and those who enjoy the smooth, linear power delivery of large-displacement engines.

Two Are Better Than One

The use of turbocharging has become commonplace in the industry because it allows efficiency without sacrificing power. Typically, turbocharging’s challenge has been complexity and a delay in power delivery called “turbo lag.”

The use of two distinct turbochargers can reduce turbo lag, but increases complexity and cost.

A twin-scroll turbocharger reduces both complexity and turbo lag.

Two Paths to Power

On the Forester 2.0XT engine, the twin-scroll turbocharger connects to the exhaust manifolds via two input paths. Each exhaust path runs from sets of opposing cylinders to one of the impeller turbine’s two “scrolls” – sets of vanes, both built into the same wheel. The paths have individual wastegates that control the gases from each input.

Using two paths for the exhaust gases separates cylinders with firing sequences that would cause gases to interfere with one another during the four-stroke cycle. This allows a smoother, more effective flow to the turbo, improving response.

The Benefits of Twin-Scroll Turbo Technology

Preview: Subaru Lineartronic CVT (Continuously Variable Transmission)

Subaru mated the 2.0-liter turbocharged engine to its responsive Lineartronic CVT with an 8-speed manual shift mode via paddle shifters.

Watch for more about this version of the Subaru Lineartronic CVT in a future issue. (For background information, see “It’s What Makes a Subaru, a Subaru: Lineartronic CVT.”)

Twin-scroll technology offers a number of benefits over a traditional turbocharger:

*Slightly offset, the two paths alternate delivery to help keep the turbo spooled up during gear changes, improving engine response.
*Increased pressure spins the turbocharger faster, which adds to boost and resultant power.
*A larger quantity of fresh air or air/fuel mixture can be forced into the cylinders on each stroke, which increases power.

The benefits of traditional turbocharging have been proven. Twin-scroll technology enhances those benefits with improved dynamics, increased performance, and reduced fuel consumption.

Complementary Engine Technology

The centerpiece of the Forester turbocharged engine is direct fuel injection. This technology allows for small-engine fuel efficiency with big-engine power through the use of high-pressure injectors located in the combustion chambers. The engine computer precisely controls the amount of fuel delivered during the intake stroke. The result is very little wasted fuel and increased power delivery.
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