Why You Won’t Get A Boring 2.4L Turbo Ascent SUV Engine In The All-New Subaru STI

The next-generation 2022 WRX STI is dropping the EJ25 2.5-liter turbocharged Boxer that has served the STI well since 2004. It’s time for the next-generation models to get an upgrade, but why is Subaru using an engine from an SUV? Subaru introduced the new FA24 2.4-liter turbocharged engine in the Ascent family hauler. Here’s why the STI’s FA24 won’t resemble the 3-Row SUV’s engine.

Subaru needs to replace the EJ25 with an engine that will bring more power, and be more fuel efficient. Can they do both? Yes, Subaru engineers succeeded in a big way and it’s why it will be the brand’s STI engine of the future. We want to give a shout out to Donut Media, where the technical information for this article came from.

The EJ25 hasn’t been without its problems. Subaru has had multiple lawsuits, right or wrong, over the Boxer engine’s reliability. Customers want a performance engine that's going to be reliable and not be in the shop for repairs. How will Japanese automaker squeeze up to 400 horsepower out of the smaller engine without blowing it up?

Donut Media suggests Subaru brought out the FA24 in the Ascent first to make sure it would hold up without any problems before introducing a performance version in the STI. It’s given the Japanese automaker time to study the new engine and best see how to get even more power from the all-new Boxer powerplant.

One of the biggest improvements in the new FA24 2.4-liter turbocharged engine over the EJ25 powerplant is the addition of direct fuel injection as opposed to the older version Port Injection in the 2.5-liter. Port injection places the fuel injector in the intake manifold upstream from the intake valve. The air and fuel mix together before entering the combustion chamber and it’s less efficient.

Direct injection shoots the fuel directly into the cylinder delivering a much more precise air/fuel mixture and improves efficiency. It increases the pressure and thus horsepower, and also lowers the temperature in the cylinder, lowering the chance of pre-ignition.

Subaru also improved the cylinder head giving the air a straighter shot into the cylinder by taking out the bends. This takes out turbulence and improves the air flow at higher revs. More air plus more fuel from the direct injection equals more power.

But Subaru had to beef up the engine block and internals to handle the increase in horsepower. In a turbocharged engine it’s even more critical because of the extra boost from the turbocharger. Subaru increased the thickness of the cylinder walls, and added larger connecting rods, which are twice as thick as the EJ25 turbocharged engine in the WRX STI.

Subaru also needed to upgrade the valve springs. The FA24 comes with new ”Beehive” valve springs that are smaller at the top, wider in the middle, and smaller on the bottom making them stronger and will handle more pressure and higher revs (+500-700) than a normal spring.

Subaru and Subaru Tecnica International will upgrade the internals even more in the new FA24 that will enabled Subaru to drop the Ascent-sourced 2.4-liter turbocharged engine in the 2022 WRX STI, and deliver more power and torque than the outgoing EJ25. It’s going to be more fuel efficient, possibly produce up to 400 horsepower, and it should be a more reliable performance engine for enthusiasts.

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Denis Flierl has invested over 30 years in the automotive industry in a consulting role working with every major car brand. He is an accredited member of the Rocky Mountain Automotive Press and the founder of Subaru Report where he covers all of the Japanese automaker's models. More stories can be found on the Torque News Subaru page. Follow Denis on Facebook, Twitter, and Instagram.

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Photo credit: Subaru Global