Killing Drone With 1/4-wave Tubes

MFE92

10 Year Member
Aug 25, 2010
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Phoenix
Some of you may be familiar with this really long thread on the corral and others like it, which detail the science between adding capped resonator pipes branched off the main exhaust that are exactly 1/4 of the length of the soundwaves of the "drone zone". The idea being the sound wave enters the capped pipe and bounces back so that when it gets back out, it's exactly 1/2 a wave out of phase, so it meets the next pulse in the system head-on, totally cancelling it out. I can't do this on my mustang because the aftermarket suspension components leave me no room to add teh extra pipes. I've added Dynomax VT mufflers to it, which work, but I just bought a Pontiac G8 GT with a Magnaflow exhaust that droned like crazy at 1900 RPM, 125 Hz, but otherwise sounded great. I didn't want to change it, I just wanted to kill the drone.

The G8 folks are hip to the phenomenon, and a company called SOLO makes a catback that incorporates adjustable-length capped J-shaped pipes that mount back by the mufflers. It's important to note that the phenomenon doesn't care where the pipes are mounted, or how they're shaped, just how long they are. It just so happens that there's a perfect spot for them on the G8's platform back by the mufflers near the bumper. These ones are adjustable kind of like a trombone, but they come pre-assembled at the perfect length. I had them installed yesterday, and the difference is incredible. And measurable ;)

As seen by this screen shot from my handy-dandy sound meter app on my iphone, the big hump in the 125hz range is the drone:

Oldnopipesjpg.jpg




I don't have any sound files, but the meter tells the story: Same RPM, same stretch of road, same time of day. Look how much smaller that hump is, and look how many fewer decibels it's reading. That 10-decibel difference is huge. The way the decibel system works, every 10 decibels higher means twice as loud. That means the old setup was twice as loud as the new one, or put another way, the new setup is half as loud as before. :

Newpipesjpg.jpg


The drone has been virtually eliminated, and I'm a happy camper. This :poo: works, and it works on any car with enough clearance to add pipes of the appropriate length.
 
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My 05 G. Marquis has goofy looking factory growths on the intake tube. Thanks for explaining what they have to be for. Now if I could do the same to my Mustang's exhaust!

I remember stories of flat heads getting exhaust cutouts with a removeable gas cap end plug for track use (actually made from gas tank filler necks). Combine both the drone tube and EZ open cutout on the end and you'd be set for street/track use. The new electric opening cut outs would make it even easier.
 
Here's a chart showing the length of pipe for various drone RPM's, but this only applies to 8-cylinder engines. The math is different for other configurations.

pipe
length @
1075
ft/sec
RPM HZ (8-cyl) 113 deg
1500 100 32
1600 107 30
1700 113 28
1800 120 27
1900 127 25
2000 133 24
2100 140 23
2200 147 22
2300 153 21
2400 160 20
2500 167 19
2600 173 19
2700 180 18
2800 187 17
2900 193 17
3000 200 16
 
Or just get Magnaflows and not have drone to start with :nice:

Cool idea though. If this eliminated the drone from Flows i *might* consider thinking about maybe contemplating entertaining the idea of potentially someday running Flows again. Maybe.
 
I'm not even running flows... I run what is supposed to be (and was before a major motor upgrade) a quiet exhaust... UNTIL... That magic portion of RPM where the wife has to put her fingers into her ears. :O_o:

So this may be just what I need.
 
From a PM conversation Jerry Beach & I had on July 30, 2006. Sorry no pictures, but if you read carefully, you'll get the idea. Maybe he will show up with some pictures. jerry beach

jerry beach said:
I will get the car up off the ground tomorrow or the next day (the tranny cooler is still fighting with me) and take some better pics of it.

When I get it up for the rear spring change, I will send you some pics Joe. This is my friends car, he adapted this setup from diesel generators that ran indoors and had to be quiet. The length of the pipes matter, they need to be close to 1/2 the distance from the muffler to the tailpipe ends. It cost $60 at Modern muffler in Ocala, once he convinced them to do it. They just criss cross behind the rearend and in front of the gas tank. The ends are sealed. I will take some good pics so you can see exactly what was done.

Jerry.
 
Working at a dealer for 17 yrs I got to talk to a lot of engineers and read a lot of tech bulletins. We hung exhaust weights on stuff that they would design and changed PCM calibrations all the time to bump the RPM's one way or the other up or down to get rid of drones, converter shudder, etc. Pretty neat the stuff you learn. We applied the same stuff to drive line vibrations, manual transmission gear rattle, etc.