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Discussion Starter · #1 ·
My magnaflow had the usual 2000 rpm drone, so I tried something a little different (I think) to get rid of it. The drone is now not just better, it is completely gone - the exhaust is downright quiet cruising at 2000 rpm. I read an article on the internet about "tailpipe resonance", and decided that this has to be the problem with aftermarket catbacks. They probably all have the same 6ft tailpipe mine has, which if the exhaust is at about 500 degrees, will resonate at 134 Hertz, which is the engine firing rate at 2000RPM. So I added a 134 Hz, 1/4 wave resonator to both tailpipes. Its just a section of pipe "T"ed into the tailpipes right in front of the gas tank. They are about 26" long each, which maybe a little shorter than ideal, but that's what fits in the available space. 26" must be long enough, because it sure as heck works. One end of each pipe is welded shut, and the other is T'ed into the tailpipes. If anyone is interested, I can try and post a picture.
 

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Discussion Starter · #2 ·
I have the Bassani baffles; they helped a little, but I still hated driving the car on the highway. The resonators are far more effective than the baffles. At 75mph, the exhaust is almost silent now.

I don't think there is a loss of power, since the resonators are at right angles to the air flow. I just cut a hole in the tailpipe and attached the resonators at a 90 deg angle to the tailpipe. No air flows through them, just sound.

I'll try and post some pictures later on today. No laughing at my welding though - it was my first attempt with a MIG.
 

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Did it change the sound outside the car any?
 

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Discussion Starter · #5 ·
Driving around with the top down, the car is definitely quieter overall. Same basic magnaflow sound though. Its much quieter around 2k rpm, but less so at higher rpms. I think you could probably adjust how effective (quieter) the resonators make the exhaust by using smaller diameter tubes and a smaller hole in your tailpipe. The resonators work by diverting part of the sound from the tailpipe into the resonator, where it travels down the resonator, hits the end of the pipe, reflects back down the pipe, and re-enters the tailpipe. If the sound is around 134 Hz (2k rpm), then it re-enters the tailpipe 180 degrees out of phase and partially cancels the 134 Hz sound in the tailpipe. If you make the hole and resonator smaller in diameter, then a smaller amount of acoustic energy diverts into the resonator, and a smaller amount of cancellation should occur. Mine is so effective, that you could probably use a smaller hole and smaller pipe and get a smaller, but still useful result. I used 2.5" pipe. You need to keep the length of the pipe the same though. I also stuffed a handful of fiberglass into the pipe for some dampening. It would be interesting to hear how this worked with other mufflers and pipe sizes.
 

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Interesting. Thanks for the pic.

My car has stock exhaust with magnaflows welded in, so I wonder if the 26" would be right.

I suppose with a little creativity one could make the resonator length adjustable... not sure if moving it one way or another a few inches would change the sound that much to make it worth while.
 

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Discussion Starter · #7 ·
If the drone is caused by tailpipe resonance, then the ideal length is 1/2 the tailpipe length, if the temperatures are the same. But I think a little shorter is closer, because its probably cooler in the resonator than the tailpipe. The speed of sound is lower in the cooler resonator, which makes the resonator look longer. Actually, I think it can probably be off by several inches and not make that much difference. Measure your tailpipe from the exhaust tip to the muffler - mine is about 71".
 

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Discussion Starter · #8 ·
If anyone else tries this, be sure and leave room for the sway bar to move up and down. I found this out the hard way. There is plenty of room in there though, especially if you use pipe smaller than the 2.5" that I used.
 

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So all you did was cut some holes in the tailpipes and weld up some 26" long closed pipe to the holes? Would 2 1/4" pipe work as good and what size hole needs to be cut into the tailpipe?
 

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Discussion Starter · #10 ·
Yes, that's all there is to it. I also grabbed a handful of pink fiberglass from my ceiling insulation and stuffed it into the pipe. It worked so well, that I didn't try any other combinations - I like mine just like it is on the first try. I would think that the bigger the pipe and hole you cut, the quieter it would make your exhaust. 2 1/4" would probably work good. Maybe a lot smaller would work too, but we'll never know until someone tries it. I cut my hole the same size as the pipe - 2 1/2 inches. My exhaust may be too quiet now for some people, but its perfect for me.
 

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Graham,
Thanks for the information. I have a set of Magnaflows that are sitting in storage right now because the Borla venturis nor the Bassani resonators worked to get rid of the drone.

Does the orientation of the resonators matter, i.e. one coming out the side and another coming out with an elbow to avoid them hitting each other. I was concerned about fatiguing the welds.
I'd try to keep the phase lengths the same.
Marc
 

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Discussion Starter · #12 · (Edited)
It seems to me that as long as the sound can get down the pipe, bounce off the end and get back, then bends shouldn't matter too much, but I'm no real expert on these things. We just need to try some things. I need to weld mine in, as I get an exhaust rattle over big bumps.

After driving around for a few days, I have to say that the exhaust is significantly quieter overall, maybe too quiet for some people. The growl that the magnaflow had is replaced with a smoother, more mellow sound. I really like how it sounds, its much better for everyday driving, but its less aggressive. I'm guessing that smaller pipe may still get rid of the drone, but keep a more aggressive sound, but you guys need to experiment around to see what happens. I've got what I want so I'm probably done - a straight through exhaust with minimal restriction, but not too loud. I guess its always possible that the acoustic reflections could somehow effect horsepower? (edit - I really don't think this is much of a possibility. Acoustic reflections occur already at the muffler, cats, etc). My car still feels plenty fast.

I'll try and post a sound clip, but someone needs to tell me how.
 

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Discussion Starter · #13 ·
you could go as far as makin em removeable
That is what I'll probably do. Since I can't weld stainless steel, I'm planning on having a muffler shop weld in some stainless 2 1/2 inches stubs off the tailpipe, then I'll clamp the mild steel resonators onto the stubs. The whole thing needs to be really solid to keep from rattling around over bumps. We may need to be able to remove them to work on the differential, or maybe something else back there.
 

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kudos to you graham! strap her on the dyno and see if any HP/TQ was lost, i dont think there would be much at if, if any. if you do loose some hp over like 5hp or so, i'd try connecting them , them being the tailpipes, like an H but keep them packed with fiberglass and see what happens, just another idea to toss into the mix.
 

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spydermandb said:
kudos to you graham! strap her on the dyno and see if any HP/TQ was lost, i dont think there would be much at if, if any. if you do loose some hp over like 5hp or so, i'd try connecting them , them being the tailpipes, like an H but keep them packed with fiberglass and see what happens, just another idea to toss into the mix.
I wonder if you connect the tailpipes right after the mufflers (before the bend up and over the axle) if it will have the same affect.....
 

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Discussion Starter · #16 ·
To cancel the drone with a connection between the tailpipes, I think it would take a section of pipe about 5 ft long. That length would add the right amount of delay to the sound so that the right and left sides would combine 180 degrees out of phase, and cancel each other, at 2k rpm. I thought about doing that, but decided that two 2.5 ft resonators (which do essentially the same thing, but uses the sound from the same side for cancellation), would be easier to make fit. A shorter connection may still do something useful though, you need to try it and see. You should be able to reduce drone without cancellation (add something to the tailpipe to dampen out the resonance, or change the way the sound leaves the muffler so it doesn't resonate in the tailpipe the same way). I'm sure with some experimenting around, we could come up with several ways of reducing the drone.

This stuff is nothing new: its common practice in electronics to make filters with transmission lines (coax, etc), by combining a signal with itself, but out of phase, with lengths of line. An exhaust pipe is a transmission line, so the same techniques work. The cable company used to control who got HBO this way. They would "T" off your cable with a section of special cable outside your house (making a 1/4 wave resonator similar to my exhaust resonators), which filtered out the channel HBO was on. We just cut the resonator off with a pair of wire cutters, and HBO would come in loud and clear! This was quite a few years ago, so hopefully they won't come after me now.
 

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Discussion Starter · #17 · (Edited)

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Discussion Starter · #19 ·
I called Bassani directly and ordered them. They were not made for the Magnaflow catback, but they happen to fit. They slide into the 3" exhaust tips and attach with a screw. They help a little.
 

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GRAHAM - just to let you know, I installed the side branch resonators on my '02 GT. WOW, what a difference. The drone is gone.:joy: This fix works as advertised.

Only problem I'm working on right now is some banging when the suspension travels to far. I think it's the sway bar hitting the lower resonator, but I'm hopeful I can correct it.

Oh yeah, installed them with flanges so I can get'em out of the way if I need to work on the rear end.

....Jakes
 
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