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Blueprints: Twin-Scroll Turbo
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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. 



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