While you may have a turbo installed on your Subaru Impreza WRX/STI, it is possible you may not have an intercooler kit.  Intercoolers help to provide your Impreza WRX/STI with even greater performance and even protection from heat damage to your engine.

An intercooler kit is designed to cool the intake air after it passes through the turbo and before it enters the cylinders.  With the intercooler cooling the intake air of your Impreza WRX/STI, more air can enter the cylinders at one time (as air heats the molecules expand and vice versa) allowing for a more efficient burn of the fuel/air gases.  This more efficient burn provides engine of your Impreza WRX/STI with another boost to performance.  The intercooler does more then just increase the performance of your Impreza WRX/STI though, with a turbo spinning it creates heat and the exhaust gases used to turn the turbo are themselves very hot.  The intercooler will cool the exhaust gases of your Impreza WRX/STI before they reach the turbo, which will help keep the turbo cooler and as a result the intake air will be cooler as well.  With the turbo producing far less heat and the air going into the engine of your Impreza WRX/STI cooler, this helps keep the engine temperature in limits and protects against cylinder head cracking and detonation.

As you're deciding on what intercooler kit to buy for your Impreza WRX/STI, another thing you need to consider is what type of cooling you want.  An intercooler can come with two types of cooling systems; air-air cooling and air-liquid cooling.  Each cooling system has its advantages and disadvantages for your Impreza WRX/STI.  With an air-air cooling its generally cheaper and weighs less, although doesn't provide as great of performance increase as an air-liquid.  The air-liquid system can cool the gases better then an air-air but usually weighs more, it also requires less piping for the gases, which can decrease the turbo lag (quicker response time from your turbo).  Some air-liquid intercooler cooling can even store ice, allowing it to cool the gases even further, but would require you to constantly replace it.

• An Intercooler on your Impreza WRX/STI will increase performance
• With an Intercooler in your Impreza WRX/STI engine it will help protect against heat damage
• An Intercooler can come in two types of cooling for your Impreza WRX/STI; air-air intercooler and air-liquid intercooler
• Air-air intercooler boosts performance and weigh less in your Impreza WRX/STI
• Air-liquid intercooler boost performance, weighs more, and can even lower turbo lag on your Impreza WRX/STI