[BMFH]

Hello All

Just something I read about.

I read about using a "J" Pipe to change the Helmholtz resonance on the Exhaust to eliminate or at least reduce the Drone.

The "J" pipe is welded to the exhaust pipe at 90 degrees and goes along with it, another pipe slides inside of it or outside of it and slides back and fourth to change the size of the resonanced chamber and also changes the frequency of the exhaust pulses.

Use a clamp on the sliding pipe to lock it in position after the quietest sound is achieved.

Just Food for Thought

Take Care,

Dusty

Some of the GTO's used a resonance pipe on the exhaust pipe by the bumber to change the exhaust resonance, some of you might have wondered what that extra pipe was for.
I think it was usually on the LH side in I remember corectly.

[rohrt]

I have read that too. I saw that at least one production car uses that method to eliminate drone.

[mbwicz1]

I have also read that some factory pontiacs used a different muffler left and right (one short and one long). I've got a 3" exhaust, 3" long-case mufflers, and 2-1/2" tails. I was thinking of switching one muffler to 2-1/2 to see if that makes a difference. Theoretically, with an X-pipe I would have resonance at different points (assuming that the mufflers are actually different inside). I'm not sure that I would want to run different length mufflers.

More food for thought...

Have fun,
Mike

[Grinch]

I did this on my 70 GTO. It has hooker headers to a pypes x pipe with dumps and pro flow mufflers(magna flow clones- straight through). The x pipe improved mid range power and also changed the tone of the exhaust. There was a very noticeable and annoying drone around 1700 rpm which was at about 35-45 mph so it was there alot of the time driving around town.

I read about the stub pipes added to the exhaust and decided to try them. You need to figure out what the frequency is of the drone you are trying to get rid of and I used a spectrum analyzer app that I put on my smart phone. This app displays the sound it is listening to by showing the different sound frequency peaks. I found the drone was around 110hz. The attached picture show how much higher the drone frequency was over the rest of the exhaust. I found a formula on line that says I need about a 16" long stub pipe. I welded up a saddle to sit on top of the pipes leading to each muffler and bought some tailpipe sections from autozone that have a gradual bend and welded the pipe stubs to those and capped them off. The stubs fit above each muffler. The pictures show it better than I can describe it.

It is 10 lbs of stuff in a 5 lb bag, but it worked out well knocking down the drone. I have electric cutouts on there too and the car runs about the same ET with the exhaust open or through the mufflers so performance wasn't hurt.

[rexs73gto]

Pontiac used a different length muffler on a bunch of duel exhaust cars. My 73 GTO came from the factory with 2 different length mufflers on it. It had one 17 inch & 1 19 inch muffler on it. The first time I was under the car was the 2nd day I had the car when we put headers on it. I looked at that & thought it was a mistake. I pulled the entire exhaust system off the car & it sat behind a friends house fro about 15 or so years until the system just got so rotted we threw it all away. The original tail pipes & extensions were on there thats why we kept it but it wasn't until about 10 years ago I found out that the 2 different size mufflers that were on there should have been there. I had looked in the parts book several times & knew it showed 2 length mufflers but always thought it was just an optional size muffler for the same car. But not until an article in the old Thunder Am talked about why they used the 2 different size mufflers. But yes there has been several on this & other forums talk about the use of 2 sizes to get rid of the drone . There is a term for it & i think it's the one used in a few posts back. But it is suppose to work.

[mbwicz1]

I wonder with an X pipe if you need to put two chambers in, or if 1 would do it? I like how you went up so that it is hidden a bit, nicely done.

Mike

[Grinch]

There is a stub resonator above each muffler. I've thought about putting 2 different lengths of stubs to see what that does to the tone of the exhaust. They are about the same length now. I also stuffed the capped end with steel wool to, in theory anyway, make the filtering of the resonator a little wider to include some of the frequencies above and below the drone frequency.