Mufflers: Other than noise reduction, get a high enough flowing muffler to handle the HP. Don't use a muffler that flows less than the correctly sized secondary piping. For each peak HP, have the total muffler flow be 2.2cfm. He gives a graph of percentage of maximum power retained with a muffler on versus cfm per open pipe horse power to show that at 1.5cfm/hp, you lose about 17% of your peak hp, 5% loss at 1.75cfm/hp, and 2% loss at 2.00cfm/hp. At 2.25cfm/hp and beyond, the loss is less than 1%! On the same graph is a curve for backpressure versus cfm per open pipe horsepower:

1.50cfm/hp gives 2.0psi backpressure

1.75cfm/hp gives 1.1psi

2.00cfm/hp gives 0.3psi

2.25cfm/hp gives 0.1psi

In reading at the provided link I found the thing about vissard and flow ............... I hate when they do not list test pressure ......... but if I assume a 28 " test then you get 146 cfm per square inch in plan pipe..........so duel 2.5 inch pipe would mean that you would have 4.908 sq in per pipe or 9.818 per system or 1433.355 cfm for the system which means it would support 637 hp ......... I am not sure I agree with that ......... I have seen sbc engines that are high hp that do better with 3 inch ex over 2.5 ......... also is this just testing the mufffler on the end of a headder on a dyno or the whole ex system to the bumper .......... becouse I think that the longer the pipe the bigger it should be ...........