These are probably the best flowing mufflers available,and are what I will use on my next car. I have 40 series flowmasters,and love the sound, but tests show they don't flow as well as ultra flows.
The Ultraflo's are the best sounding mufflers I have ever had. They sound deep and clean with no sewer pipe crackle. The Thrush "Weled" sound excellant as well and are half the money of Flowmasters, altho I heard they are not making them any more....