FlowMaster are a chambered muffler that bounces sound vs MagnaFlo which is a "straight through" type (slight S curve perforated pipe wrapped in fiberglass)
Flow Master 40's are the old school muscle car sound but are on the loud side.
MagnaFlo has more a low end tone but not as loud
a straight through aka "Turbo" muffler vs chambered muffler will produce more HP
I had FM 40's on my 67 Camaro. The loud interior noise on Fwy was more than, at my age now, I want. I got the new FlowMaster FX muffler which is a "turbo" type, not chambered. Maybe FlowMaster wants more of the Turbo muffler market also as it has not been out that long. It has more note than the MagnaFo or DynoMax Turbo mufflers and when you hammer it, you know its there. SS, welded for around $44 each. I got mine on Amazon but the usual places sell them
These are all the FlowMaster muffler types
https://www.holley.com/brands/flowma...flowfx_series/