What are you using for valves?
I run the summit brand cutoffs on my 944 and have had pretty good success.
the problem is the valves are going in 4'' and 5'' pipe. I have looked around and it seems a valve for 3'' pipe is $300, and so on. even if I plumbed them in the 3'' part of the system I still need four of them so it would still be very expensive.
I plan to make my own and operate them by hand. I was thinking of a design similar to a throttle body. they wont be used all the time, only when I have something on the bed I want to keep clean.