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Dual valve springs and titanium retainers were optional. Valve spring swap altered seat pressure: from 70 pounds to 121 pounds. |
Displacement On Demand system deactivates four of the engine’s cylinders when its is in steady-state cruise mode. DOD is unaffected by the rocker swap. |
Briefly, here’s the scenario: The advantage of the Crane rockers is that they continuously change the ratio, meaning that the arcs of the valve-open and valve-closed position have two varying radius points, which result in changes of the rocker arm ratio as it travels through its cycles. Depending on the shape of the rocker arm and the location of the pushrod seat, the changes can be substantial. A stock 1.7 rocker arm pushes the valve off the seat at a ratio of 1.54:1, but as the pushrod seat rises on the valve-opening cycle, the effective rock arm ratio eventually rises to 1.69:1. The Quick-Lift 1.8:1 rocker pushes the valve off the seat at a ratio of 1.89:1 and by 0.175-inch lobe lift they cycle to a 1.82:1 ratio that is maintained throughout the lifter travel. This design allows a greater amount of air into the cylinders by creating a larger lift at every point of the curve earlier in the lift cycle and then maintaining the lift longer throughout the cycle, all the while maintaining the same set-to-seat timing.
While the flywheel output of the LS4 is rated at 303hp and 323lb-ft, baseline dyno runs (1.7:1 rocker ratio) yielded 255.5hp and 358lb-ft of torque (torque rating enhanced by converter multiplication in Second and Third gear and/or factory rating is on the very low side). As applied to the Impala SS, the Crane 1.8:1 Gold Series rockers increased the output an additional 18.1hp and 16lb-ft of torque at the front wheels. Hey, it’s a start.
