How to reduce exhaust drone

Written by richard rowe
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How to reduce exhaust drone
Small displacement engines tend to put out a lot of high frequency sound. (Stephen Dunn/Getty Images Sport/Getty Images)

Exhaust drone isn't noise from the engine, it's engine noise acting upon the exhaust system. Imagine a child on a swing; if you give the child a small push, she moves a little. Wait for her to come back to you and apply the same amount of force just as she begins to swing away and she'll go a little higher. Exhaust pipes respond to high-frequency sound in the same way. Certain frequencies will keep adding energy to the vibrating tubes until they begin to hum or "drone." Reducing drone comes down to changing the tube's resonant frequency range and reducing the sound waves that cause it to vibrate in the first place.

Skill level:
Moderately Challenging

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Things you need

  • Basic hand tools
  • Reciprocating saw
  • Grinder
  • 4 exhaust clamps
  • Welder and welding supplies
  • 2 or 4 6-inch long "cherry bomb"-type mufflers
  • Large hose clamp
  • Exhaust header wrap

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Instructions

  1. 1

    Cut a two-foot section of exhaust tubing out of a straight section of the pipe. Ideally, make this cut as close to the engine as possible but no less than 12 inches away from the catalytic converter. Slide an exhaust clamp over one of your cherry bomb muffler couplings and slide the cherry bomb over the section of tube closest to the converter. Tighten the clamp. Slide the other cherry bomb over the other end of the tube, but don't tighten the clamp yet.

  2. 2

    Hold the two-foot cutout up in between the cherry bombs. Mark it so that it will slide at least 1 1/2 inches into the muffler tips when you install it in between them. If in doubt, mark it a bit long. Cut the tube and test fit it between the two cherry bombs and trim the fill-piece to fit if it's too long. Pull the piece back out, install the clamps over the cherry bombs and reinstall the tube. Tighten the tube clamps, but remove the rearmost cherry bomb clamp.

  3. 3

    Wind your header wrap around the rear cherry bomb twice and hold the wrap while you install the exhaust clamp over it. The exhaust clamp both holds the cherry bomb in place and anchors the wrap. You could use a separate hose clamp to secure the header wrap, but there's no sense in wasting a perfectly good exhaust clamp.

  4. 4

    Wrap your exhaust pipe with header wrap from the second cherry bomb all the way back to the muffler. You may need to unravel the roll completely to fit it between the top of the pipe and the chassis. Wrap the pipe in tight spirals, allowing each spiral to overlap the last by about 1/2 inch. You may wish to install a hose clamp every two feet or so to keep the wrap from unravelling as you work toward the muffler.

  5. 5

    Secure the exhaust wrap at the muffler, keeping it taut at all times. Secure the wrap using the stock muffler clamp as you did at the cherry bomb, or use a separate muffler clamp or hose clamp if your muffler is welded on.

Tips and warnings

  • This approach works in two ways. The header wrap effectively increases the pipe's resonant frequency by absorbing some of the vibration energy, working in much the same way as the harmonic balancer does on the engine. However, the cherry bombs are doing the lion's share of the work here. Not only does the muffler packing absorb some of the sound energy, but using two very short mufflers instead of one long one also targets only drone-inducing high frequency sound waves. The longer and more mellifluous low frequency waves will pass right through the mufflers losing little to none of their energy. Ultimately, you'll end up with a much smoother, lower and more pleasant-sounding exhaust note. When applied in this manner, a 2-inch by 50-foot roll of header wrap covers about 18 feet of 1-1/2-inch diameter tubing, 12 feet of 2-inch diameter, 9-1/2 feet of 2-1/2-inch tube and 8 feet of a 3-inch diameter tube.
  • Don't wrap the catalytic converter or muffler. The converter may melt and you're just wasting header wrap on the muffler.

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