Originally Posted by Senna-F1

Here is the science behind the velocity stack.

http://www.profblairandassociates.co...mouth_Sept.pdf

I was going to model my own custom stack based on this article. Still may, but I already had the 6” to 4” stack sitting in my MR2 so I used that one instead.

This issue I see with doing anything larger than about 3” is the rectangular section just doesn’t allow for a large round shape. At this point my 3D part through that section is right at the equivalent of a 3.9” dia pipe.

Here is a graph showing how small increases in pipe diameter have a large effect on square area. So every little bit helps. Also the model I created.