Watch The 2019 Subaru Sti S209 Intercooler Sprayer Work
The 2019 Subaru STI S209 gets a lot of features that distinguish it from your average WRX STI including wide body work, a giant wing, additional chassis braces and an extra 31 horsepower. It also brings back a performance gizmo that hasn't been featured in a U.S. market Subaru since 2007: the intercooler sprayer. As the name implies, it sprays cool water onto the air-to-air intercooler when pressing a button. In the case of the S209, that button is one of the paddles on the back of the steering wheel. That triggers the water nozzles hidden in the hood scoop to spray water. You can see what happens in the video below. Note that when the car is in motion, air would carry the spray farther back covering more of the intercooler.
Now why would you want an intercooler sprayer? Well, the S209, like all WRX and STI models, uses an air-to-air intercooler to cool off hot air coming from the turbocharger on its way to the intake manifold. The cooler your air, the denser it is, meaning more oxygen to combine with fuel to turn into more power. The effectiveness of the intercooler depends partly on the temperature of the outside air flowing over the intercooler. If it's really hot out, it's not as effective. So spraying water onto the intercooler can counter this to ensure that the car is making maximum horsepower even in toasty conditions.
This also means that although this may feel like some special secret performance-boosting weapon, it's really only effective in specific circumstances, and will just get you back to normal power, rather than make the car noticeably faster on the street. Long track days in the summer and after waiting in line at the drag strip or autocross are the kinds of situations where you could see power diminish due to hot air and a lack of moving air. Normal driving won't necessitate its use nor will driving in cool temperatures. In fact, the S209's owner's manual specifically says not to use the sprayer in temperatures below 41 degrees Fahrenheit because of the risk of frozen water in the tank.
That brings us to another aspect of the intercooler sprayer: maintenance. In fairness, there isn't much to maintain, but if you want to use the system, you do need to keep the tank filled. It's located in the trunk where the spare tire would normally be. Inside of course is a water pump that connects to hoses that run water to the front. There is a warning light that will illuminate on the instrument panel if it runs low. You are only to use distilled water in the system, likely to prevent buildup of anything that could block up the system. And in freezing temperatures, you'll need to keep it filled no more than halfway to ensure the tank doesn't crack.
Considering the small benefits and extra care that come with an intercooler sprayer, you can see why Subaru only featured it in the U.S. market STI from 2004 to 2007. Still, we love nifty performance gadgets, so we're glad to see it make a return in the S209. And considering the car's performance and target audience, there's a real possibility that some owners will see benefits from it.
