From what I have read it is FLOW not area or cross sectional area that you are concerned with. If you have a garden hose that is 3/4 inch I.D. have you ever noticed how it flows about twice as much water as a 1/2 inch I.D. hose? The formula for flow is .25 X Radius Squared (.25xr2) so in our example of the garden hose that is .5 inch in dia. .25x.25x.25=.015625 and for the hose that is .75 inch in dia. .25x.375 x .375=.0351562 That is why the 3/4 inch hose seems to be flowing twice as much water. Granted I am not William Mulholland, but I did learn at a very early age that the hose that is a little bit bigger does flow twice as much water. The mathmatical number that you come up with when measuring the inside diameter of pipe is "a factor" and of course is of value in comparing different size exhaust pipe,however in the real world, you would need to compare the compressed diameter of the exhaust pipe,unless you are using mandrel bent tubing. Hope this helps.