The stock black rubber tube from the CCV fitting to the stock air box is re-used. Insert the 4" diam exhaust pipe or other 4" OD pipe into the stock tube and secure with the stock hose clamp. Fit the new filter with new hose clamp on the end of the 4" pipe and you're done. I wouldn't bother replacing it with flex pipe, unless you are building a whole new intake from the filter to the turbo, and removing the CCV fitting and doing the CCV mod.
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