Exhaust drone.

[chevycrew]

Yep I got it.

Started stock, quiet... Cut off the resonator, slightly louder, not bad.

Had to remove the stock muffler for room for other things... I now have a small 3" straight through ebay stainless oval case muffler with 3" pipe from the muffler back. Drone was there, but not bad, and bearable.

Now, I have a hollow cat, (was robbing power due to the guts were melted) and the drone is horrible!


I have read that simply adding a glasspack after the muffler will kill, or at least tone down the drone.


Opinions? I really dont want to spend much to tone it down, and putting back in a huge muffler isnt an option.


Thanks!

[MrSmithsTB]

I'd try adding a glasspack in place of the resonator . Should quiet things down some.

[chevycrew]

Thats my plan, I have a 3' straight section behind the rear axle that would be easy to slip in a glasspack.

Here is a pic of my cat guts too:



(note: pic taken on the bumper of the wifes truck...)

[chevycrew]

And my exhaust. The case on that muffler is only 14" long.

[chevycrew]

Link to the muffler I am running as well:

http://cgi.ebay.com/ebaymotors/CENTER-O ... 53e8dd79b7

[fishsticks]

I have a little drone at cruise, and I'm getting tired of the interior noise when I get on the gas. I've considered putting a glasspack on as well to help. I run a 2 chamber welded muffler...

Has your CEL lit for Cat efficiency yet? I melted a 2" hole clean through my last cat element and my CEL never came on.

[chevycrew]

I cheated, I had a code with the bad cat, but now, my rear o2 sensor is tuned off, no more codes.

[chevycrew]

Also look up side branch resonator. Seems to be working good to cancel the resonance. I need to figure how long I need to make one.

[chevycrew]

The formula that I have found, is:

rpm of resonance / 120 x # of cylinders = hertz of resonance.

Then speed of sound at the temperature of your exhaust where you are putting the resonator / the hertz of resonance = wave length of drone

Wave length of drone / 4 = 1/4 wave length, or side branch length.




I still need to fine tune my numbers, but, roughly looking at 3000 rpm drone, and a 26" long side branch pipe.

example of 6 cylinder 3000rpm drone.

3000/120x6=150 1250/150=8.33 8.33/4=2.08ft

Exact rpm, and exact temperature are variables.


I may be wrong, I'm no physics major, but I will try per the formula and see what happens.

[v7guy]

branch resonator... wierd. I just looked it up, seems relatively common too. My stock resonator was really irritating me when I was tossing on the shocks, I wonder how well it works.

[chevycrew]

I thought I should add, addition of the tailpipe is what created the drone, that's why I'm working on that for a fix.

[Trail X]

branch resonator... wierd. I just looked it up, seems relatively common too. My stock resonator was really irritating me when I was tossing on the shocks, I wonder how well it works.

What do you think that air box on top of the engine is?

[chevycrew]

James is correct, in general todays cars and trucks a quiet air intake due to having branch resonators that help cancel out the noise.



UPDATE: Huge difference at idle, smooth, mellow, almost like an LS1.

I have not driven it yet, but I'll drive it to work tonight and report back.


I cut a 3" hole in the current exhaust, then welded in a 18" long 3" extension. Then an additional 9" chunk was slid into the first part 2" for an overall of 25". There is an additional hanger welded to frame crossmember and to the exhaust clamp making everything removable. The 9" part being clamped in allows me to build various ends to fine tune length and drone without welding on the truck.

[v7guy]

branch resonator... wierd. I just looked it up, seems relatively common too. My stock resonator was really irritating me when I was tossing on the shocks, I wonder how well it works.

What do you think that air box on top of the engine is?

I understood it was to tune the sound of the engine, like the rear coffee can. Just didn't realize it could be done with a straight section of pipe with a given volume/rpm. I always guessed it was a way more complicated process.

[navigator]

Steve, not sure if your solution is temporary or not but won't that new pipe pump 1/2 your exhaust under the body and accelerate rust while also possibly allowing dangerous fumes into the cabin while wheeling?

[Trail X]

It's capped.

[chevycrew]

The side branch is capped, its just a tube to bounce sound waves back into the exhaust at the 1/2 wave length interval.


Drove it to work last night, 3000rpms is much more mellow, but, now I have a small drone at 1500 rpms. I will still do some tuning to see what it likes.


I finally sealed up where I cut the rear boxes off behind the rear tire... This may have been a large contributing factor to the drone as well. More test drives once all the foam has dried.

[navigator]

It's capped.

oh, I missed that!

[DmccartneyFF2TB]

Hows this working out for ya???

[chevycrew]

Great, logged a couple thousand miles so far, and wife and 7 month old daughter dont have any complaints about the noise.


It dont sound stock, but is pretty mellow for as open as it is.