Stop the Drone - Helmholtz Style

[SlingShot]

Originally Posted by brandotron

This is going to be the next big exhaust mod on this site.


That's the L99 muffler, right?

Correct ... The LS3 is straight through

[Roger Ramjet]

I didn't invent it but neither did GM - credit Mr. Helmholtz. I just applied it to keep from trying more muffler combinations. And, my A6 sounds like a muscle car!

[Roger Ramjet]

Originally Posted by toehead93

You're kidding, right? Why waste the time, effort, and money when you can pick up stock L99 mufflers for next to nothing. IMO, the better, cheaper, and more efficient way is to simply add an $85 mufflered x-pipe to replace the H-pipe. It will help reduce drone when coupled with rear mufflers and you won't lose flow like you will with the u-bend.

There is no H-pipe on L99/A6 cars - just the huge center common resonator box. Ever since I removed that center resonator my car has had the drone. A tru-X muffler in the center made the drone worse. Separating into 2 center mufflers with a H-pipe helped. Now with the J-pipe the drone is gone.

[brandotron]

Originally Posted by toehead93

You're kidding, right? Why waste the time, effort, and money when you can pick up stock L99 mufflers for next to nothing. IMO, the better, cheaper, and more efficient way is to simply add an $85 mufflered x-pipe to replace the H-pipe. It will help reduce drone when coupled with rear mufflers and you won't lose flow like you will with the u-bend.

Congrats to the OP on the craftsmanship, it looks like you did a good job.

Pick up a stock, quiet L99 muffler? The silent, water collecting trash can? Kinda defeats the point of adding the bullets, doesn't it? OP's way gives you the tone you want minus the drone--AND it saves weight.

[Rodrunner]

Mr. Helmholtz would be proud...if he were still around. Must have taken some pretty intense calculations to come up with the length and diameter of the J-pipe. Nice going, Roger. I'd expect no less.

[KeepItGreasey]

Originally Posted by Roger Ramjet

There is no H-pipe on L99/A6 cars - just the huge center common resonator box. Ever since I removed that center resonator my car has had the drone. A tru-X muffler in the center made the drone worse. Separating into 2 center mufflers with a H-pipe helped. Now with the J-pipe the drone is gone.

maybe i'm a newb, but i don't know what drone is.

i have an L99, and i wish my exhaust was louder.

by "drone is gone", do you mean that your exhaust is silent in the cabin when you're cruising at 1800-2000 RPM now?

[Roger Ramjet]

Originally Posted by KeepItGreasey

.............. i don't know what drone is ..........by "drone is gone", do you mean that your exhaust is silent in the cabin when you're cruising at 1800-2000 RPM now?

Exactly! What seems like a nice sounding exhaust note at lower speeds gets very intense and irritating at highway cruising speeds.

[KeepItGreasey]

ahhhh cool. yeah, i used to go half deaf on long drives in my truck haha other than that, it never really bothered me. but i'm glad i know now, and it's cool that there is a science behind eliminating it.

[Scott@Bjorn3D]

Originally Posted by Roger Ramjet

There is no H-pipe on L99/A6 cars - just the huge center common resonator box. Ever since I removed that center resonator my car has had the drone. A tru-X muffler in the center made the drone worse. Separating into 2 center mufflers with a H-pipe helped. Now with the J-pipe the drone is gone.

Actually if you cut the resonator open you will see it has the H-Pipe affect in it the way it is made. My car does not drone to bad because the GMPP axle backs I have are the L99 version.

[BigRed585Lbs]

For the techies:
http://www.rexresearch.com/helmholtz...resonators.htm

[Roger Ramjet]

Keep in mind this fix can be done for about $150.

[toehead93]

Originally Posted by Roger Ramjet

There is no H-pipe on L99/A6 cars - just the huge center common resonator box. Ever since I removed that center resonator my car has had the drone. A tru-X muffler in the center made the drone worse. Separating into 2 center mufflers with a H-pipe helped. Now with the J-pipe the drone is gone.

I was calling the L99 an H-pipe because it does connect both sides of the exhaust, but yes it is just a big heavy resonator. I replaced mine with a Magnaflow mufflered x-pipe when I installed my LT headers and kept the stock L99 mufflers. I don't have any drone whatsoever but I deactivated AFM. Did you have drone only when AFM kicks or at anytime? Do you have a video of the exhaust, I'm really curious to hear it. What type of mufflers do you have on there now?

[mikey8]

Originally Posted by Hicompression

I believe GM does too.

This is what I was referring to.

[BigRed585Lbs]

Simply put, the formula for determining the length of a Helmholtz chamber used to knock out a drone frequency is:

1100 feet/second (roughly the speed of sound) divided by the target frequency you're trying to eliminate (in cycles/second) gives a full wavelength in feet per cycle. Divide this number by 4 to get a quarter wavelength (a much more convenient length to work with). Finally, multiply this wavelength by 12 to get the chamber length in inches.

Tap this length of pipe into the exhaust tube, making sure the other end of the pipe is capped off, and you have a Helmholtz chamber that takes the annoying frequency and bounces it back into the flow 180 degrees out of phase, knocking it out. The results: no more drone.