I used 1 1/2" all the way but i used a coupler coming out of the box. I got a 1 1/2" coupler in the rubber grommet and it fit likle a glove. I also used 1 1/2" all the way, but i came out where the factory one does. Right there behind the gast tank. I just drilled a hole in that black peice and ran it straight up. It was easy and its also easy to remove incase of warranty work.