A downpipe connects the turbo exit to the catalytic converter. By installing a performance downpipe alone you can look at an increase of 10-20 HP. Not only are they made from a higher quality material which features mandrel bending & they are usually larger in diameter which allows more exhaust flow. Since your turbo is essentially a pump its important to have the pipes entering and exiting it free from restriction. In our inventory you will find we carry two different styles of downpipes, off-road downpipes and catted downpipes. Catted downpipes work with your catalytic converter so you wont have to worry about emissions. Off-road downpipes work without the catalytic converter. Sport Compact Warehouse is your premier source for performance downpipes. Contact one of our representatives with any questions you have and we will help you make a more informative decision for your 1990 GMC S15.