Subaru WRX STI Engine | 2.5L Boxer Turbo Specs

For the full breakdown, see our best WRX STI Replacement Oil Pump guide.

The Subaru WRX STI engine is a 2.5-liter turbocharged SUBARU BOXER four-cylinder that powered the highest-performance U.S. WRX variant through the 2021 model year. Known for its flat-four layout and all-wheel-drive integration, this engine pairs a die-cast aluminum block with an 8.1:1 compression ratio and Subaru’s SI-DRIVE management to deliver 310 horsepower. Here is what the STI’s powertrain offers and what owners should know about fuel, fitment, and keeping it running strong.

The WRX STI Engine: What Powers It

The STI uses a 2,457 cc turbocharged and intercooled SUBARU BOXER engine with a semi-closed deck cylinder design and five main bearings. The bore and stroke measure 99.6 x 79.0 mm, and the block is die-cast aluminum with integral cast iron cylinder liners — a construction that saves weight while handling sustained high-boost operation. The 8.1:1 compression ratio is low enough to let the turbocharger run meaningful boost without detonation on premium fuel.

Subaru’s SI-DRIVE system adjusts throttle mapping and engine response through selectable modes, giving the engine different personalities from daily commuting to spirited driving. The only transmission on U.S. STI trims is a 6-speed manual, paired with DCCD all-wheel drive that distributes torque between front and rear axles based on conditions. Subaru’s 2021 WRX brochure confirms these specs for the final U.S. model year of the STI.

The engine architecture traces to Subaru’s EJ series, with the U.S. STI using the EJ257 variant — a 2.5-liter design distinct from the 2.0-liter EJ207 in Japanese domestic models. Subaru Tecnica International produced the standalone STI between 2014 and 2021 for the United States, and no STI trim has been offered on later WRX generations. The semi-closed deck stiffens cylinder walls under boost, and the five-main-bearing crank assembly supports forged internals rated for the engine’s 290 lb-ft torque output.

How Much Power Does the STI Engine Make?

The STI’s 2.5-liter turbo engine produces 310 horsepower at 6,000 rpm and 290 lb-ft of torque from 4,000 to 5,200 rpm. That torque plateau — sustained over a 1,200-rpm band — gives the STI strong mid-range pull rather than a narrow peak. The engine requires premium fuel exclusively: Subaru specifies 91 AKI as the minimum and recommends 93 AKI for maximum performance. Using fuel below 91 AKI risks knock under boost, which the management system counters by pulling timing and reducing output. EPA fuel economy for the 6-speed manual STI is 16 mpg city, 22 mpg highway, and 18 mpg combined.

STI Engine Platform and Fitment Details

The 2021 STI rides on a 104.3-inch wheelbase with an overall length of 180.9 inches, a width of 70.7 inches, and a height of 58.1 inches. The car is manual-only — no automatic or CVT was offered on U.S. STI trims — and uses Subaru’s DCCD all-wheel-drive system. Factory tires are 245/35R19 summer performance rubber on both Standard and Limited trims, meaning the car is not equipped for winter conditions from the factory; Subaru specifies these tires for warm-weather use only, so owners in cold climates need dedicated winter tires.

The STI engine differs substantially from the standard WRX’s 2.0-liter DIT unit, which makes 268 hp and 258 lb-ft. The STI’s 2.5-liter uses an older EJ-based architecture with larger displacement, different block construction, its own turbocharger, and distinct engine management. For owners maintaining or upgrading their STI’s 2.5-liter engine, the oil pump is a known reliability consideration on the turbo Boxer platform. Our roundup of the best WRX STI replacement oil pump covers compatible options and what to prioritize in an upgrade.

Subaru’s official 2021 spec sheet lists 91 AKI minimum fuel and 93 AKI recommended, with the 16/22/18 mpg EPA figures applying exclusively to the 6-speed manual. The STI’s curb weight is 3,450 lb for the base trim and 3,514 lb for the Limited, with a 15.9-gallon fuel tank. The 2021 model year was the last to carry the STI name on a WRX variant in the United States, making this 2.5-liter turbo Boxer the final U.S. iteration of Subaru’s performance-oriented EJ257.

FAQs

What fuel does the Subaru WRX STI require?

The STI requires premium fuel with a minimum octane rating of 91 AKI. Subaru recommends 93 AKI for maximum engine performance. Using lower-octane fuel can cause engine knock under boost and may reduce power output as the management system adjusts timing.

Is the WRX STI engine the same as the standard WRX?

No. The STI uses a 2.5-liter turbocharged SUBARU BOXER engine making 310 hp and 290 lb-ft, while the standard WRX uses a 2.0-liter DIT engine with 268 hp and 258 lb-ft. The STI also has a different block design, turbocharger, and a 6-speed manual transmission only.

What year did Subaru stop making the WRX STI in the U.S.?

Subaru produced the WRX STI for the U.S. market through the 2021 model year. No STI trim has been offered on subsequent WRX generations in the United States, making the 2021 model the final year for this engine and variant.