These ducts increase the efficiency of your STILLEN intercoolers by increasing cooling air flow. More cooling equals a denser air fuel charge which in turn yields more horsepower. The perfect time to install this item is when you upgrade to STILLEN's GTZ-R or GTZ-R2 fascia. This item is designed specifically to fit the STILLEN upgraded intercoolers. It may not work with other brands or the stock units. These ducts will also fit stock bumper skins.