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

'10Anamoly

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.

Attached Images

a.JPG (68.1 KB, 1191 views)

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2013 Ford Mustang GT

374rwhp, 360rwtq SAE / GT500 Axle-Backs / Brembo Pack / Track Pack / Recaros / 6spd

6MT

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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Andrew

2009 370Z Sport/Touring 6MT sold
2013 Honda Civic Si coupe D/D

'10Anamoly

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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2013 Ford Mustang GT

374rwhp, 360rwtq SAE / GT500 Axle-Backs / Brembo Pack / Track Pack / Recaros / 6spd

370Zsteve

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

Berk ftw?

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SOLD MY Z MARCH 2018 - another Core OG moves on - new ride 2019 Z Corvette Grand Sport - no mods necessary but already have eyes on HFC and intakes LOL IT NEVER ENDS.

Trips

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Holliday

Play nice you two!

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Holliday

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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'10Anamoly

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.

Attached Images

1.jpg (220.3 KB, 847 views)

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2013 Ford Mustang GT

374rwhp, 360rwtq SAE / GT500 Axle-Backs / Brembo Pack / Track Pack / Recaros / 6spd

lllll210lllll

WOT burns? black on ur bumper??????

'10Anamoly

Yes, you have to use a hole saw to open up a 2" to 2.25" hole on the FI exhaust. That is where the sound waves travel through, down the resonator pipe, and back to cancel out the droning frequencies.

It's a trade off, keep the drone and dont touch the pretty exhaust or mod the exhaust and enjoy the silence (lack of drone, depeche mode style).

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2013 Ford Mustang GT

374rwhp, 360rwtq SAE / GT500 Axle-Backs / Brembo Pack / Track Pack / Recaros / 6spd

docaam

Im big Depeche mode fan .... just trying to prepare myself to get this done as need it badly, lets see if someone with FI exhausts wants to do before me

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Old 370Z SOLD but not forgotten
370Z 40th, G37s, S30 RBZ, Hakosuka Skyline

docaam

Man when you posted this thread car was in workshop and I have been calculating this drone on rpm range...the vibration starts at 2100 and is there till 2800 depends on the gear and speed as sometime it dies on 2700. Its more if you stay in single gear and accelerate slowly all the way till 3000, but if you push the pedal hard then its a bit less.

So can you throw some light according to your calculations as I have no means to measure those frequencies.

For the pipes does those be in SS or normal exhaust pipe will do the trick. I am about to pull the trigger by making hole in FI exhausts

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Old 370Z SOLD but not forgotten
370Z 40th, G37s, S30 RBZ, Hakosuka Skyline

'10Anamoly

Since the FI setup is the same pretty much as mine and the drone in is the same range, I would use 28-30" pipes, one per side. Any shorter and it will tune out drone above 3000rpms where you dont have the problem. Regular steel is fine but I would use some hi-temp paint to keep it from rusting ever.

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2013 Ford Mustang GT

374rwhp, 360rwtq SAE / GT500 Axle-Backs / Brembo Pack / Track Pack / Recaros / 6spd

ProfessorDave

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

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Rain, snow, sleet, hail: 2009 Mazda 3 sGT HB
Everything else: 2009 370z, PW Touring 6MT

'10Anamoly

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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2013 Ford Mustang GT

374rwhp, 360rwtq SAE / GT500 Axle-Backs / Brembo Pack / Track Pack / Recaros / 6spd