On 1998.5-2004 5.9L Cummins you will need to physically modify your wastegate to gain more turbo boost. Doing this without the Boost Fooler will result in check engine lights and codes set into the computer. Stock 98-04 trucks should generate 18-23psi of boost pressure stock (depending on year model). With the Boost Fooler you will be able to run 35+ psi of boost depending on your fueling enhancements.