Exhaust size for HP
"As far as ability of an exhaust system to flow enough air for a given amount of horsepower without causing significant back pressure, this is from one of David Vizard's books:
1. For avoiding significant restriction from back pressure, the pipe should flow at least 2.2CFM per horsepower produced.
2. A straight pipe will flow ~115CFM per square inch of area (using inside diameter of the pipe)
Mufflers usually won't flow as much as a straight pipe of the same inlet/outlet diameter, so you have to factor that in, also."
http://www.hotrodders.com/forum/exhaust-diameter-formula-49188.html