Read this from Ron Nichols - it's down in the thread a little on the first page:
Howe Racing Mufflers
Personally, I don't think many Cobra owners would install mufflers with a 4.5" body diameter. According to Dynomax & their flow criteria, a straight-through Ultra Flow muffler with a 3" core will flow over 2200 CFM, which translates into over 1800 loss-free horsepower with a dual exhaust. Here's their formula:
How to determine necessary exhaust flow:
* Engine HP x 2.2 = loss free airflow capacity needed for a muffler used in a single system.
* For a dual system divide the single system number by 2 to determine the loss free airflow capacity needed for each muffler.
EXAMPLE:
* 300 HP x 2.2 = 660 SCFM single or 330 SCFM dual.