I got a 1998 4.6 Nascar with the Roush exhaust...at idle it has a quiet rumble and sounds good, if it were a little louder itll sound so much better...i dont want to make it loud i want it to perform better so magnaflow or flowmaster?

 

Defintely go with the magnaflows. I have a 12158 si/do and love the sound(not too loud under full pedal). I also noticed an improvement in torque out of the hole.

 

I was planning on getting the single cat and running it with the factory pipes and the roush tips...thanks for the reply