I would think for an 80-in motor the fat cat would be better. Scott at Hillside cycles even once told me the Borzilla is really only for 124 cube inches and above..

 

Here's my understanding. If we flow the same volume of air (or fluid) through a large pipe and a small pipe the velocity will be higher through the small pipe. There's a sweet spot between a small pipe being too restrictive and a large pipe slowing the velocity too much. There's just as much water flowing through the narrow part of the river as the wide part.

 

nd eHinting at the "Why" ... custom...one off pipes, are the Best for Highest performance..
BTW the Wider the RPM band being Used...the more Complicated the Exhaust!! There is more than one reason that the crossover Headpipes on a "warmed" 80 incher work so damn good....accross the RPM Range.
Yes, Velocity (at the Port) is Where it's At!!!! For Fill.