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Where the 341-HP 2019 Subaru WRX STI S209’s Power/Torque Boost Comes From

The 2019 Subaru WRX STI Type RA’s engine is already highly strung, so squeezing an extra 31 hp and 40 lb-ft out of it took considerable doing. One upgrade STI investigated long and hard was forged pistons, but in the end the Type RA’s cast pistons ended up proving more durable. The following revisions contribute about 20 of the roughly 90 pounds the S209 gains over the Type RA.

Read our review right here.

• No more power could be coaxed from the RA’s turbo, so STI worked with HKS to develop a larger unit. The turbine wheel diameter increases from 53 to 56mm, and the compressor wheel grows from 60 to 65mm. Boost jumps from 16.2 psi to 18.9 psi (and if our test car’s boost gauge is to be believed, it registered just over 20 psi for a brief moment).
• To improve the performance of the intercooler lurking just beneath that hood scoop, two sprayers in the scoop deliver a two-second blast of distilled water at the press of a repurposed steering wheel shift paddle. A 3.7-liter tank resides in the trunk.
• Fuel injector volume and fuel pump capacity are both increased by 20 percent.
• A conical air filter and straighter, larger-diameter intake ducting reduce pressure loss by 25 percent while improving induction noise.
• The mufflers reduce back pressure by 17 percent relative to the Type RA (which is already 50 percent less restrictive than the basic WRX STI) and deliver a sportier sound. The shiny exhaust tips are also larger in diameter and polished by craftsmen in Japan’s Tsubame-Sanjo region, known for producing cutlery and polished iPhone cases.
• Sodium-filled exhaust valves with stronger springs carried over from the S208 (as are the flywheel and clutch, though the latter boasts a different disc material and a 10 percent stronger clutch spring).
• All rotating parts are carefully mass-balanced, ensuring the pistons in each engine weigh the same to prevent vibration.
• The engine control unit is remapped for more power.
• Motul synthetic fluids are borrowed from the racing program (300V 5W-40 in the engine).

The post Where the 341-HP 2019 Subaru WRX STI S209’s Power/Torque Boost Comes From appeared first on MotorTrend.

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