The 8100 series shocks are a 7400 aluminum shock with a remote reservoir. The reservoir improves the performance of the Penske with an increase in nitrogen volume and the addition of a floating piston that separates the gas from the fluid. A "separator" piston greatly reduces aeration of the fluid. In addition the reservoir provides for compression adjustment with six settings to fine tune the compression. Shocks with rebound adjusters allow you to make small adjustments externally by turning a small nut located behind the shaft end eye.

The 7500 series shock features an internal nitrogen chamber, eliminating the external reservoir. Unlike the 7400 or 7100 series this shock has a piston separating the fluid from the gas much like a reservoir shock. This meets rules that prohibit external reservoirs while still have the advantage. These shocks do require a little extra clearance in the shock mounts.