How to Make a Flowmaster Muffler Louder

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Flowmaster makes a number of muffler designs, but the company is known primarily for its baffled designs. This type of muffler eschews the typical fiber-glass packing method of deadening sound for a series of metal plates, or baffles, that deflect and cancel out certain wavelengths, while interrupting exhaust flow as little as possible.


You make a baffled exhaust louder by drilling holes in it, but the key to success is where you drill the holes. The goal is to maintain the low-frequency rumble of the muffler, without allowing the high-frequency whine or drone to escape. This should be done incrementally and is a process of trial and error.

Things You'll Need

  • Floor jack
  • 2 jack stands
  • Grease pencil
  • Tape measure
  • Drill
  • 1/8" and 1/4" drill bits
  • MIG welder
  • WD-40 penetrating oil
  • Raise rear of vehicle with floor jack, using factory frame mounts.

  • Secure with jack stands.

  • Measure from the engine side of the muffler 6 inches from the edge weld.

  • Mark a line on the the bottom (wide part) of the muffler, and draw a line all the way across, parallel to the edge weld.

  • Drill a hole on this line in the center of the muffler with the 1/8-inch drill bit, using the WD-40 to cool the bit.

  • Drill a 1/4-inch hole on either side of the 1/8-inch hole, approximately 1/2-inch from its edges. WARNING: Do not drill on the engine side of this line; the main sound absorption is done in the first chamber and doing so will result in massive engine drone and high-frequency sound.

  • Start the vehicle, and observe change in sound. If it is to your liking, then proceed no further.

  • If more sound is desired, mark another line 1-inch farther back on the muffler along its center line.

  • Repeat Steps 5 and 6 and check the sound. If more sound is desired, repeat procedure again.

  • Drill no more than four sets of holes in the center of the muffler. If more sound is desired, proceed to next step.

  • Draw a line 1-inch from the edge weld on the outlet side of the muffler.

  • Starting at the center of the muffler, drill five 1/2-inch holes along the bottom of the muffler along this line, measuring 1/4-inch between each.

  • If desired, you may drill up the side of the muffler up to the apex of the curve.

Tips & Warnings

  • If you go too far and do not like the volume, repair the drilled holes you desire by filling with weld material. Do not use exhaust putty to fill holes, as it will eventually shrink and vibrate loose. Exhaust drone is the sound of the muffler's outer skin reaching a harmonic vibration point and vibrating. You can dampen these vibrations by wrapping the entire muffler tightly with two layers of exhaust header wrap.
  • Keep your tuning holes near the center line of the muffler. Most of the high-frequency sound cancellation is done along the width of the muffler case, so drilling up the sides will allow the vibrations to escape.

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