How Subaru Squeezed More Power From The WRX STI S209 Engine

How Subaru Squeezed 341 HP From The WRX STI S209 Engine

The Subaru WRX STI S209 is the most powerful STI they’ve ever made. Here’s how they do it.

Finally, North America has a version of S-series of Subarus that can compete with their Japanese counterparts. It’s still got the same EJ25 engine that we’ve had for years, but it’s been tuned to at least produce as much power the 2.0-L Japanese version.

But how did Subaru perform this incredible feat of engineering? We’ve got Engineering Explained to provide us the detailed answer. Using whiteboards, as is his wont.

First, let’s start with the published power figures. Subaru says the S209’s 2.5-L turbocharged boxer 4-cylinder engine will produce 341 horsepower, which is up from 310 horsepower on a regular STI. We don’t have published torque figures, but it’s expected to get 315 lb-ft compared to the regular STI’s 290 lb-ft.

To get that roughly 10% boost in power required quite a few changes under the hood. The turbocharger has a turbine that’s 6% larger, the compressor is 8% bigger, and the exhaust improves airflow by 17%. The turbocharger also has more boost, up to 18 psi from 16.2. Forged pistons and connecting rods are both lighter and stronger than the stock engine, making it more efficient and thus more powerful.

But the biggest changes to the S209 have to do with the intercooler. Or rather, the addition of a water injection cooling system for the intercooler.


On the steering wheel of the S209 are a pair of paddles. They’re not paddle shifters--instead, they activate a system that sprays the intercooler with water. This causes evaporative cooling which cools the air more than if the intercooler were just exchanging heat with whatever the hood scoop sucked in.

How much cooler? Subaru says it’ll go about 10 degrees lower than ambient air, which is a lot.

Water is stored in a 3.4-L bottle in the trunk and then injected on to the intercooler via some rubber tubes. Subaru recommends using pure water and not filling the tank past the halfway point if you live anywhere cold enough to freeze water if left outside.

Engineering Explained expects this water injection system to provide 10 horses of the 341 the S209 comes with.


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