"J hook" refers to the linkage connecting the wastegate actuator to the valve's lever arm. An aftermarket J hook keeps the wastegate closed longer, allowing the turbocharger to develop greater boost. The wastegate itself is a safety device (of sorts) that controls boost and keeps the turbocharger from over-speeding. Once a maximum allowable boost pressure has been achieved, the wastegate opens and allows exhaust gases to bypass the turbocharger turbine. These bypassed gases do not contribute to the function of the turbocharger and therefore, by continuously metering exhaust flow through the turbine, the boost pressure can be held at a maximum safe level. However, the Cummins is well suited to handle additional airflow, and increasing the max pressure achieved by the turbocharger increases overall performance and reduces exhaust gas temperatures under load.
J Hook Mod Pros & Cons
| Pros | Cons |
| Noticeable performance gains | May place additional strain on the turbocharger |
| Lower exhaust gas temperatures | |
| Relatively inexpensive |
Most aftermarket J hook upgrades are in the $50 to $60 range and will gain you up to an additional 10 psi. However, as with any modification, the actual realized gains will depend considerably on the application and any supporting upgrades. Many owners also opt to fabricate their own adjustable wastegate linkage to replace the J hook using standard parts available at most hardware stores. This modification is limited to the 2001 and 2002 model year 5.9L 24v Cummins equipped with an automatic transmission.