While the regular Subaru Impreza and other members of the Subie lineup switched to the new Subaru Global Platform four years ago, the higher-performance WRX and WRX STI versions of the Impreza have yet to follow suit. They're still based on the previous-generation Impreza models. If a new report from Forbes is accurate, however, that might finally be changing, thanks to a new engine being developed for the next STI capable of delivering 400 horsepower and 361 lb-ft of torque. New engine in hand, Subaru can now stuff it into a new WRX.

Citing sources close to Subaru, Forbes says the next WRX STI will arrive in 2021 and sports an "all-new" turbocharged 2.4-liter boxer four-cylinder. Subaru already has a 2.4 turbo engine, the FA24, in the Ascent three-row SUV and XT variants of the Legacy and Outback, so it's more likely that the STI will get a highly upgraded version of that mill. In its current form, the FA24 produces 260 hp and 277 lb-ft, so some changes would need to be made to reach that lofty 400-hp goal.

Today's WRX STI uses the venerable EJ25 turbocharged 2.5-liter and makes 310 hp and 290 lb-ft in standard trim. The limited-production S209 variant cranks that up to 341 hp and 330 lb-ft, which is no small feat but also likely the most that Subaru is able to reliably extract from the aging engine architecture, which was first introduced in the mid-'90s, without things going "kaboom" or impacting streetable engine response. It isn't yet clear what will power the new base WRX, which today uses a newer-than-the-EJ25 turbocharged 2.0-liter FA-series engine with 268 horsepower.

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The new STI-spec FA24 will reportedly get a reinforced crankshaft, beefier pistons, and, naturally, more boost pressure. Subaru engineers are said to be benchmarking the 2.0-liter turbo found in the Mercedes-AMG A 45, CLA 45, and new GLA 45, which makes an impressive 415 horsepower and 368 lb-ft of torque in "S" guise (and "only" 382 ponies and 354 lb-ft in regular spec). In addition, the next-gen STI will receive a version of Subaru's full-time symmetrical all-wheel drive system and enhanced braking capability.

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