This is how Subaru made 341 HP from WRX STI’s Engine

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The key to unlocking the BMW M4 GTS is water injection. However, it’s done in a slightly different manner.

The 2019 Detroit Auto Show was a highlight. It featured the WRX STI S209, the first S-series Subaru that entered the U.S. market. Although it came with an unofficial 341 horsepower rating, the engine should actually increase output by 10% over the regular WRX STI. This makes the EJ25 Boxer the most powerful Subie ever.This is how Subaru made 341 HP from WRX STI's Engine

The STI S209 has new parts. It now has a HKS turbo with a bigger turbine and compressor, more boost at 18.2 psi instead, forged pistons, connecting rods, as well as a new intercooler water jet. How do all these parts work together to create a higher power output?

Engineering Explained returned to the whiteboard to explain how the entire system works to power the new WRX STI S209.

It is important to note that while the water-spray injection of the WRX STI S209 shares many similarities with the Bosche powered water injection in BMW M4 GTS the former differs in many ways.

First, the setup, which was only possible because of the low and horizontally-positioned Boxer engine, allowed Subaru to position the intercooler on top of the engine. Engineering Explained claims that this reduces the engine’s travel time and helps to reduce turbo lag. Subaru was able to take advantage of the ambient air cooling via the STI S209’s hood ventilations.

This is how Subaru made 341 HP from WRX STI's Engine

The water-injection spray can be controlled by the paddles at the steering wheel and allows the driver to manually turn the sprayers onto intercoolers. The water injection cools the air from intercooler by 10 degrees Celsius (18 Fahrenheit) in high temperatures. As we all know, cooler and denser air allows for more fuel to be injected into the combustion chamber.

Subaru has taken precautions regarding the new water-injection device. The water tank is located in the trunk of the WRX STI S209. It should be only filled with pure water. To allow for expansion (a.k.a. freezing), it should be kept half-full or half-full in the WRX STI S209’s trunk.