bigdraft said:first pic is both air filter setups for comparison.
second pic is open air filter installed.
In the first pic, both systems are shown from the turbo connection to the filter. But, the only difference is from the breather element housing to the filter.
On the old body style (OBS) trucks, I dont see why you would have to run a open filter to run a 4 inch downpipe. Only issue I can see is the firewall clearance for the 4 incher.