We recently received a set of Penskes from a customer that was looking to replicate the dimensions and valving in our new coil over line.
The Penske's we tested were VERY stiff. They may have been ordered that way, but after speaking with the customer we determined the best course of action was to soften the valving of the shock and increase the spring rate. As they were you basically had no adjustment range with the Penske as it was so stiff to start out with.
Here's the graph compared to our single adjustable unit:
