I have the Flowtech Warlock mufflers (on my 84) and they're pretty quiet even with an offroad H-pipe, i hear my headers louder than the exhaust and there's only a slight drone at 2,000rpm. These mufflers are normal "turbo" type mufflers. There's probably quieter mufflers out there, but the warlocks are alot quieter than i ever expected (they're about as loud as a stock new mustang GT -'99+ sytle), but have a nice tone but is not loud.
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previous name: 84stangLX
'84 Mustang 5.0 LX
my LX
'89 Mustang GT
my GT