I have also heard that FM's dont flow that well even though they sound great. Have a super 40 on my hemi and havent ran into any problems.

These mufflers have to be a little better than stock dont they?

 

Not much:

Stock on my 08 flows about 350cfm
40 series FM flows 352cfm
My Magnaflow flows 579cfm

 

lxman1

Any thoughts on Magnaflow muffler # 12579? It is a large muffler at 5x11x22 with a dual 2.5" core instead of a single 3" core. It should flow well and do a good job with sound and drone control.

My Magnaflow # 12279 5x8x24 sounded good but I didn't like the drone. Running a stock 09 muffler now.

 

Mine is the muffler that comes in the 16700 kit. I think it 24", not sure of the other dimensions. It doesn't drone while cruising.