I am looking for a post that had a lot of good info on the flow charateristics of exhaust gasses in the different types of exhaust pipes. Not looking for cat back info but information on the ability of exhaust gas to flow in crimp bends and mandrel bends as well as single pipe and dual.

I know there was a post buy a guy that had some formulas and rules and laws on the movement of gas in the pipe; How it would flow compaired to bends and size of pipe or pipes. I just can't seam to find it. ANY ONE HAVE THIS INFO?