Just like in the BMW M4 GTS, the key is water injection, but in a different way.
One of the highlights in the 2019 Detroit Auto Show is the arrival of the WRX STI S209 – the first S-series Subaru to enter into the U.S market. It came with an unofficial 341-horsepower rating, but ultimately, it should increase the total output by 10% from the regular WRX STI, making the overhauled EJ25 Boxer the most powerful Subie engine yet.
We all know that the STI S209 got new parts to be able to achieve those numbers, such as a new HKS turbo with a larger turbine and compressor, more boost at 18 psi instead of the previous 16.2 psi, forged pistons and connecting rods, and a new intercooler water spray. But how do these things work together in order to produce a much higher power output?
Luckily, Engineering Explained went back to the whiteboard (again) to enlighten us on how the whole system works to power up the new WRX STI S209.
Take note that although the water-spray injection in the WRX STI S209 has several similarities with the Bosche-powered water injection in the BMW M4 GTS, the former differs in numerous ways as well.
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. This, according to Engineering Explained, reduced the travel time of the cooled air to the engine and aids in reducing turbo lag. It also made Subaru take advantage of ambient air cooling through the STI S209's hood vents.
On the other hand, the water-injection spray, which can be toggled through paddles behind the steering wheel, allows the driver to manually engage the sprayers onto the intercooler. In instances where the outside temperatures are high, the water injection helps in cooling down the air from the intercooler further by 10 degrees Celsius (18 degrees Fahrenheit) making use of evaporative cooling. And as we know, denser and cooler air entering the intake manifolds allows more fuel into the combustion chamber, thus, more power.
Subaru has precautions with this new water-injection system, though. The water tank, which is located in the WRX STI S209's trunk, should only be filled with pure water. It should also be half-empty (or half-filled), as well, to allow expansion (a.k.a freezing) just in case outside temperatures drop.
Source: Engineering Explained via Youtube