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. :tiphat:

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.

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.)

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! :tup:

If you dont, thats why I wrote the article.

My Berk exhaust makes it's sweetest sound right at the RPMs you specify.

Berk ftw?

This is the same thing HKS used for their exhaust.

http://i561.photobucket.com/albums/s...s-steel--1.jpg

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.

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.

Anamoly math make Dave feel stupid. Dave need pretty picture of tree branch installed to understand.

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.

I like this thread. + rep

Thanks for the reps!

nice writeup! thanks for sharing!

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 :tup: +1 rep

Different exhaust...I had straight 3 inch piping all the way back. Car sounded like a freaking truck between 2k and 3k.

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!

happy women FTW!

If there is temperature change in the exhaust due to outside ambient temps the speed of sound will change and the wave length will also be affected.

Frequency=Speed of sound/Wave Length

This may be a non-issue if the exhaust temps are not affected much by the outside temps.

my 2 cents

:gtfo2:

:eek:

http://i561.photobucket.com/albums/s...rize-fight.jpg

Play nice you two! :rofl2:

This is true for the math, but the exhaust temps should be pretty similar despite outside temperatures. Perhaps a bit cooler in the winter but still pretty close. The side branch resonators cover about 15 points of frequency completely and then begin to lose a bit of efficiency. On my car it knocked out all the drone from 2200rpms through 3000rpms. I expect in winter for the car to make a bit more noise but probably not much. Can always report back when its cold outside. :)

lol... i wonder how much power you lost by doing this mod.

Probably... none.

Awesome job man, makes sense. I guess thats why Stillen chose the muffler setup they did and why my car has 0 drone.

Yep, absolutely zero power loss guaranteed (I can attest). The side branches are capped so there is no flow through them really (they are also at 90 degrees from the direction of flow and they are right near the exhaust tips.)

Pics are attached. The black is a 2000* flat black ceramic paint. I covered the whole rear bumper and the tips and mufflers and coated the side branch pipes and connections to the exhaust tips slowly with quick spritzes of paint. Much cleaner looking and no risk of rust. In the last pic, the exhaust tip is about one inch to the right of the black pipe.

Very nice!

People also need to note that you are using Magnaflow race mufflers and NOT the FI cans. :)

Yes, these are Magnaflow 4" stainless resonators (what Magnaflow called them on the box) also the same as race mufflers really since the diameter is smaller than 6". I much prefer them to the size of the FI cans from the rear view, although FI is not really bad looking to start with.

sound clips?

One better, will have a video later tomorrow. We are making it at the TopSpeed grill out meet. :tup:

Good solid technical info, plus 1 rep. I would have purchased an FI exhaust if it weren't for the drone problem. FI should offer this in their product line. Stillen and Berk avoid drone by having a longer exhaust path.

Isnt this technology called the Hemholtz pipes?

I knew there was something wrong. That's not really a true FI exhaust. You still have stock cats?

I never said it was the FI exhaust, just the same style. I however have OEM resonators and the Magnaflow race mufflers instead of the FI setup, and yep they are stock cats. The x-pipe is actual FI back from the cats though to the x-crossover. The rest is custom.

KEVTEX is right, the extra bends and longer exhaust path move the frequency at which the exhaust resonates to eliminate drone. It is similar to a Helmholtz resonator but technically it is called a side brand resonator, running at a quarter the wavelength of the pipe resonance frequency. Similar though, much like the resonators on stock intake tubes!

Sounds good. 15 points=how much bandwidth?

Nice job on the Helmholtz solution!

Sorry points was not very tech savvy, I meant 15hz. The frequencies from 115hz to 130hz seem pretty tame now, per Audacity on my PC when I did re-recording spectrum analysis reviews.

I love my FI 18" Resonator catback but I can't agree with you more, I feel this is not the drone but vibration which literally shakes my car long with my head for a certain rpms which you mentioned, apart from that this thing is amazing.

I have to drive a distance of 90miles on motorway and 70mph cruise this drone/vibration/sounds is annoying.

I was told by someone if I can fix two flex pipes on the far most end near stock cats it can kill this thing, as our FI exhausts doesn't comes with flexpipes. Check with Tony as well and he was very cooperative as usual, but my problem comes I live quiet far so to get flex pipes shipped and get those TIG welded was an issue.

Man thanks for the pics, I am a bit nobb in these frequency and size things. Would it kill this sound if I just attached something between these two tips or the size of the pipe has really a big importance and what kind of welding did you use?

It really should not be the lack of flex pipes causing the drone, it has to do with standing wavelengths through long pipes. The length does matter a lot, what you need to know is the frequency of the drone. It should be close to what mine was.

Is the drone on your car heaviest between 2200rpms and 2800rpms? If so, you need around a 28" side branch resonator on each side, coming at a 90* angle off the area between the tips and the rear mufflers. That will kill off most of the drone. I had a local shop with a good welder make them for me for $100 and then I coated them in 2000* high temp flat black paint. They are tucked up behind the muffler so you cannot see them. Make sure to cover your rear bumper and the car undersides before you paint anything so you dont get spray paint on your paint job!

Here is the video of my setup with the side branch resonators and custom exhaust:

'10Anamoly's 2010 370Z - OEM Intakes, FI X-Pipe, OEM Resonators, Magnaflow 4" Race Mufflers, Side Branch Resonators, OEM Tips (1080P Available)

I have seen usually majority of aftermarket CBs are with flex pipes and someone said that this vibration might die with flex pipes

Anyways thanks for the reply, so 28" is the length of each pipe right and im not wrong then I should just weld these to the pipe behind the top without any hole or anything and then weld both of these pipe one behind the other right?

What should be the pipe diameter 2.5"???

For vibration I exactly havent noticed at what rpm it comes on, when you push the car hard it comes a bit early otherwise a bit late but within your mentioned range

This is exactly what Tony from Motordyne Engineering has done with the 350z. His are called Advanced Resonance Tuning pipes and they're integrated into the test pipe and DO create an HP gain.

If its a pipe vibration (not due to sound vibration) you have a different issue. If its that head thudding noise vibration in the cabin (resonance), then you want a 28 pipe coming off the pipe between your rear mufflers and tips.

1. First, use a hole saw to cut out a 2" to 2.25" opening on the pipe between your muffler and tips on each side (one hole per side of the car, hole should face towards the other pipe so the resonator you are welding on will extend into the area where the OEM muffler sat).

2. Next, have the exhaust shop weld a permanent cap (to seal one end of the pipe) on two pieces of 28" long, 2.25" pipes. Basically you want two 28" pipes, each has one open side and one capped side which reflect sound inside the pipe back at the open side.

3. Next, weld each straight piece of 28" to the area of the pipe that you cut the hole out of. You are basically t'ing off the stock exhaust with a 28" piece of pipe at a 90 degree angle, per side. This lets the sound go down that pipe and back and cancels out that annoying droning frequency.

4. End results should be that you have tee'd off of both side with a 28" capped extension pipe, each extension going up into the area where the OEM muffler sat.

Check my pics and this should all make better sense.