When to go bigger?????

Basic flow calc for exhaust, feel free to double check my numbers for cfm/sq inch, this is off the top of my head while killing time in a worthless meeting at work and it's been awhile since I've needed to do this

~ 2.2 cfm per HP needed

HP*2.2 = total CFM

Divide by two if running dual exhaust

Assume 115 cfm /sq inch of exhaust so...

3.14*r^2 for pipe sq inch * 115 to get potential flow per pipe and see if it meets your needs

example:
650HP --> 325HP per side
325*2.2 = 715 CFM per side

3" exhaust = (3.14*1.5^2)*115 = 812CFM per side <--good to go with headroom