Originally Posted by Grubrunner
Muffler and tips is the difference.
I do believe pipe diameter is the same throughout the path, although there may be a slight route change in comparison to the OEM as I think someone here once mentioned they noticed.
See my sig for the Magnaflow complete catback.
Pipe diameter out of the muffler is different. Magnaflow cat back is 2.5" tails, OEM is 2.25". I doubt on a stock or mild truck you would ever notice a difference. 2.25" pipe flows about 356 cfm x 2 = 712...so:
CID (cu. in. displacement) = 345
RPM (revs per minute) = 5500
VE (volumetric efficiency) = 95%
Your motor flows about 521CFM based on CFM = [CID*RPM*VE]/3456.
Accounting for losses in efficiency, you're looking at ~439 CFM max (assuming 5500 RPM and 80% VE) and less than that if you figure 80% VE and 4000 RPM (327CFM).
Basically, dual 2.25 with a free flowing muffler should be more than sufficient.