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Fancy Tech: How to Get Rid of Exhaust Drone, works great!

So I had the FI style exhaust and I agree its the deepest exhaust I've heard on a 370Z and sounds AMAZING, especially above 3000rpms. However, the downside is the

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Old 07-29-2010, 09:45 AM   #1 (permalink)
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Default Fancy Tech: How to Get Rid of Exhaust Drone, works great!

So I had the FI style exhaust and I agree its the deepest exhaust I've heard on a 370Z and sounds AMAZING, especially above 3000rpms. However, the downside is the drone between 2200-2800rpms was bad, worse for others depending on your tolerance, and was prevalent at cruising RPMS.

Being a mathy person, I figured I could fix the problem by hitting it at the source, what causes the drone. Drone is caused by the pipe resonance so to combat this, you need to use the exhaust wavelengths against themselves to cancel them out.

One way to do this is to put on a chambered muffler which breaks up the sound waves and avoids hitting the natural pipe resonance frequency. This also kills some performance because it hurts flow. This is how Corsa mufflers work as they are tuned per car to break up the frequency at which the exhaust system naturally resonates without affecting the flow. However they are very expensive and they dont make one for our car.

Standard resonators (Magnaflow, OEM, etc) are excellent at destroying high frequency resonance in the upper RPM ranges (3k plus) but dont do much for low frequency resonance (100khz-140khz). Changes in pipe size and tailpipe length can push the natural resonance frequency for the exhaust out of the normal RPM range but unfortunately with the FI style and most straight through exhausts I've seen on 370Z's, resonance happens from 2krps to 3krpms frequently and can be very annoying.

SO, finally the fix:

To get rid of drone (pipe resonance), you need to cancel out those specific pipe resonant frequencies. To do this without using a chambered muffler you can use whats called a Side Branch Resonator.

The side branch resonator is a pipe, around the same size as the exhaust pipe, which T's off the OEM pipe between the rearmost mufflers and the exhaust tips. For the FI style, it comes off at 90 degrees from that pipe and should go out straight across to the other side (in the area where the OEM hunker muffler was). One end is welded to the opening on the OEM pipe and the other end is welded shut with a cap, no filling in the pipe at all. The cap reflects the sound back into the OEM pipe. The length of the side branch pipes matters a LOT! To cancel out the frequencies you need to have them reflect back at a quarter wavelength from the side pipes that T off the main pipe. The simple answer is for the FI setup, if you have heavy resonance at 2500rpms (ish) then you need 28" side branch resonation pipes, one per side.

I did this on my car, first with 22" side branches and had no luck. After re-doing my math and using some Audacity frequency analysis, I figured out that they had to be 27-28" to work at the RPM I wanted. Longer pipes tune out frequencies lower in the RPM range. Shorter pipes (increasingly less effective) hit the higher rpms. I had the mechanic fix the pipes to 28" and VOILA, absolutely zero drone from 2200rpms to 2800rpms. At 2000rpms there is a slight rumble in the cabin, but no drone. Same at 3000rpms which changes into an aggressive purr. Absolutely nothing changed about the exhaust sound or performance in any way besides the reduction (went from heavy drone that was very uncomfortable to NO DRONE) in drone. Plus, its all tucked up behind the bumper where the muffler used to be so you cant tell.

I hope this helps for those who hate the droning but want to keep the performance, sound and looks! I guarantee that it works amazingly, others on other car forums have had the same results when you use the right length of side branch resonators. If you dont mind the drone or dont want to cut into your gorgeous exhaust, enjoy as is.

Diagram is below and more info for those who want it, will have pics soon! Red and black are two pipes, each about 28" long total, capped at the end furthest from the connection to the OEM pipe. They meet the OEM pipes at 90 degrees and do not connect, they just pass each other on a side and are welded together for stability.
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Old 07-29-2010, 09:48 AM   #2 (permalink)
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Why would you want to get rid of this so-called "drone"? I love that sound!! That's the best thing about my Stillen set-up! (Except for WOT burns, of course.)
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Old 07-29-2010, 10:04 AM   #3 (permalink)
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Drone is the head pulsing vibration inside the cabin that gives people nasty headaches and hurts your ears if its too loud. If you like that, more power to you!

If you dont, thats why I wrote the article.
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Old 07-29-2010, 10:08 AM   #4 (permalink)
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My Berk exhaust makes it's sweetest sound right at the RPMs you specify.

Berk ftw?
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Old 07-29-2010, 10:17 AM   #5 (permalink)
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This is the same thing HKS used for their exhaust.



I can't stand drone either! +1 rep for all the engineering involved. I wish FI would have made an exhaust that did not drone so bad.
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Old 07-29-2010, 10:34 AM   #6 (permalink)
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The HKS canisters are different. Those are resonator chambers (notice how they are big on one end like a resonator and arent just straight piping capped off at the end) with perferated holes in them and packing, probably a more expensive version tuned to do the same thing in a smaller space. This article is mainly for the FI guys who have drone and want to fix it but that is the idea.

The Saclam exhaust however has side branch resonators, its the side pipes people were calling backpressure tubes. Check out the layout, they just put theirs further up the exhaust system. It's probably why their exhaust sounds like a race car up top and doesn't kill your eardrums cruising around inside the cabin. The pipes in the middle that parallel the main exhaust pipes are side branch resonators, the length clearly tuned by Saclam to get the note they wanted.
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Old 07-29-2010, 10:44 AM   #7 (permalink)
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Anamoly math make Dave feel stupid. Dave need pretty picture of tree branch installed to understand.
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Old 07-29-2010, 10:47 AM   #8 (permalink)
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ProfessorDave, I will have pics later today for you. In the meantime, the pic of the Saclam exhaust shows the side branch resonators, just in a different position than I have mine.
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Old 07-29-2010, 10:53 AM   #9 (permalink)
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I like this thread. + rep
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Old 07-29-2010, 11:18 AM   #10 (permalink)
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Thanks for the reps!
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Old 07-29-2010, 11:36 AM   #11 (permalink)
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nice writeup! thanks for sharing!
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Old 07-29-2010, 11:56 AM   #12 (permalink)
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repped! I was in the process of doing the same thing in my S when I sold it...that drone was freaking killing me...
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Old 07-29-2010, 12:00 PM   #13 (permalink)
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Quote:
Originally Posted by DanGizinski View Post
repped! I was in the process of doing the same thing in my S when I sold it...that drone was freaking killing me...
The drone is really that bad? Damn.......in for updates and hopefully videos when done +1 rep
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Old 07-29-2010, 12:15 PM   #14 (permalink)
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Quote:
Originally Posted by 370Zsteve View Post
The drone is really that bad? Damn.......in for updates and hopefully videos when done +1 rep
Different exhaust...I had straight 3 inch piping all the way back. Car sounded like a freaking truck between 2k and 3k.
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Old 07-29-2010, 12:28 PM   #15 (permalink)
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Thanks again for the support, I'm glad this thread is going over well. Drone is a bit hard to capture on video, (especially since its all gone now) but I can definately post pics of the setup and back it up with a personal guarantee that it cut out 99.999% of the noise around 2500rpms, and I'm not joking that the drone was BAD. My lady was riding in the car and loved the exhaust until we were cruising at 70mph on the highway and our heads were being microwaved by the drone and vibration. Cant wait to show her its gone when I get home!
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