While there are ways to increase the performance of your Volkswagen Jetta, if you have a turbo installed you're going to want an Intercooler kit.  Intercoolers are designed to help lower heat damage to the engine and turbo as well as increase performance.  The intercooler does this by cooling intake and exhaust gases to the Jetta by either an air-air or air-liquid cooling system.

As air heats up, it expands, lowering the amount of molecules able to occupy a single space.  This is important because as a turbo spins the friction produces heat which is then transferred to the intake air and with less air entering the cylinders it leads to a less efficient fuel/air burn.  An intercooler kit is designed to help lower this problem.  As the turbo compresses the Jetta intake air it is then sent to the intercooler to lower it's temperature and then routed back to the Jetta cylinders, allowing more air to enter the cylinders and providing a more efficient fuel/air burn (greater performance).  However, an intercooler does yet another operation to help lower the temperature of the Jetta intake air.  As the Jetta engine expels the exhaust, it is routed through the intercooler before it reaches the turbo (which is what turns the turbo).  With the intercooler cooling the Jetta exhaust gases, far less heat is able to be transferred to the turbo.  With the intercooler keeping the turbo and Jetta engine cooler by cooling the exhaust and intake gases, it helps prevent overheating of the Jetta engine which could lead to cylinder head cracking or detonation.

Now as for the cooling systems, like stated previously, there are two types; air-air cooling and air-liquid cooling.  Now both types of cooling do the same thing, cool the intake and exhaust gases, but each do it differently and have some other slight differences.  The air-air intercooler usually weighs less then an air-liquid (due to it not requiring the extra components, including water, needed to operate) but isn't able to cool the Jetta intake and exhaust gases as well as an air-liquid.  The air-liquid intercooler does weigh more and usually uses plain water to cool the intake and exhaust gases, however, the air-liquid has a couple other features the air-air doesn't.  An air-liquid intercooler usually requires less piping for the system, which allows the Jetta intake and exhaust gases to reach the engine and turbo faster (lowering turbo lag).  Some air-liquid intercooler systems even have the ability to have ice stored to cool the Jetta gases even further, but would require constant replacement.

• An Intercooler Kit will increase the performance of your Jetta
• Intercooler Kits help protect your Jetta from heat damage
• Intercooler Kits come in two type of cooling for your Jetta;  air-air and air-liquid