Sorry Louis, sometimes I get to rambling. I am talking about the actual orifice (thats right I said orifice!) opening out of the exhaust housing of the turbo that makes up the exhaust system. Like my T60s are 2 3/4 and I am using 4 inch tubing. And actually its a little small and should be 4 1/8. Anyways its just theory and I am interested in seeing how it acts on your car. You obviously are running into room problems so it is what it is. If you look at a factory diesel truck it even follows this rule. I thought I would just throw it out there for you to ponder.