Exhaust drone - 3inch K24a3 CL9

Hi Guys,

I've recently upgraded to a 3 inch cat back single exit exhaust and am currently chasing to remove a very loud drone between 2400 and 3500 rpm.
It's especially bad when you stop accelerating, making around town driving anoying.

Current exhaust setup is:
Littlebo custom 4-2-1 headers
High flow 2.5inch cat
Reducer to 3 inch
2 foot straight through hot dog resonator
Then 3 inch all the way to 2.5 inch baffled muffler.

The resonator and muffler are both redback exhaust branded.

I have tried to the fix this drone by adding a Helmholtz resonator that is just before the muffler and angled towards the other exit.
The length is roughly 370mm.

It's helped to remove a lot of the deep bass notes, but it's still far from perfect. I had a friend on my car recently and they said it sounds like a truck, especially when you are on deaccleration.

Any suggestions on what to do?

I am planning on modifying the Helmholtz resonator to make it slightly longer at 430mm to see if this helps, using the below forumla.
((343)÷(3000×4×(1÷60)))÷4.
The 3000 is my Rpm


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You need a proper muffler in the middle not a hotdog. something like this
http://www.garage88.com.au/images/legalisrs15.jpg

also what kind of hiflo cat? 100cel? the biggest issue with my exhaust noise was the 100cel cat,it made the car unbearable compared to the oem cat with same catback setup

I don't actually know the cel. I believe it's 100.
What's funny though is that I've had the headers and the high flow cat installed with the stock catback for a couple months without any annoyance at all. It honestly sounded stock.

I do have a 3 inch s bend muffler that I could probably try and put into the exhausts, but it's off centred so it's a littles annoying to install.

Aren't the hot dog resenators meant to remove drone.

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in my experience hotdogs remove raspiness not bass

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Originally Posted by EKVTIR-T

in my experience hotdogs remove raspiness not bass

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I'll definitely keep this in mind. Might be a bit of a weekend job

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I extended the length of the Helmholtz resonator by 120mm and it seems to be quieter. Not by a lot, but a bit. I also added a hanger on the end of it as well now that it's longer which has dramatically improved the stability of the exhaust. Before on idle you could even hold it to stop it shaking. Now it site still just like the factory exhaust.

So still worth modifying it, but still have drone. One day I'll have to get another resonator, bit for now it's driveable.

See my Instagram post for some video clips.

https://www.instagram.com/p/BeSdGIkBDJJ/

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What is the size/diameter of the Helmoholtz resonator?

i believe the "offsetting" effect of the Helmoholtz is reflected by the diameter itself compared to the pipe size.

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Originally Posted by Fredoops

What is the size/diameter of the Helmoholtz resonator?

i believe the "offsetting" effect of the Helmoholtz is reflected by the diameter itself compared to the pipe size.

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It's 3 inch. Same as the rest of the pipe

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Fix of last resort, put a twinloop on it

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Originally Posted by Fredoops

Fix of last resort, put a twinloop on it

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It's basically just before the muffler anyway. Probably should have posted this picture before. Showing where the Helmholtz resonator is located. https://uploads.tapatalk-cdn.com/201...080e807648.jpg

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You sure that location being almost at the T junction is not causing extra reverberation?

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Originally Posted by Fredoops

You sure that location being almost at the T junction is not causing extra reverberation?

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No idea to be honest.
But I really don't have anywhere else to place it, as the current location moves into the passenger side of the car utilising one of the hangers from the second muffler.

It's just ease of placement


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I dont think your setup is ever going to be considered to be quiet/drone free.

Its 3in, its not mean't to be quiet, its sized for people chasing power, and sound is the by product. (usually speaking)

Its a full setup starting from headers. Im my experience, catback alone are usually fine for sound needs. Once you add headers to the list it actually gets louder, like neighbors dobbing me in to EPA louder.

I dont even know what Im looking at in the pic you posted. You say its a single exit but there's an T intersection and im pretty sure in saying this, but that T intersection would not be an ideal way desgin an exhaust, its not flowing straight out.

If crusing around town in peace is goal, then stock car should be goals.

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Originally Posted by amant02

I dont think your setup is ever going to be considered to be quiet/drone free.

Its 3in, its not mean't to be quiet, its sized for people chasing power, and sound is the by product. (usually speaking)

Its a full setup starting from headers. Im my experience, catback alone are usually fine for sound needs. Once you add headers to the list it actually gets louder, like neighbors dobbing me in to EPA louder.

I dont even know what Im looking at in the pic you posted. You say its a single exit but there's an T intersection and im pretty sure in saying this, but that T intersection would not be an ideal way desgin an exhaust, its not flowing straight out.

If crusing around town in peace is goal, then stock car should be goals.

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Hi amant02,

The exhaust is a single exit.
That t intersection in the picture is a Helmholtz resonator which is welded shut at the other end.
This is meant to bounce back sound waves at a particular resonance which can kill drone at certain rpms.

What I found interesting was that with the factory catback and the headers and highflow cat, the exhaust sounded the same. You wouldn't even know it had headers.

I do understand what you are saying in that if I wanted a quiet ride I should have left it stock.

But I do want the best of both worlds.
At the moment it's definitely liveable with and haven't cause me any issues.

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Quote:

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Originally Posted by AgroVeg

But I do want the best of both worlds.
At the moment it's definitely liveable with and haven't cause me any issues.

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Looking at the size of the rear muffler, it isnt really big anyhow so i dont think it would do a lot of dampening.

the factory exhaust is quiet because of the rear muffler, which was a turbo design (ie: restrictive).

As i said before, worse case scenario put a twinloop muffler on it, and it should kill the noise (at least at lower rpms) I reflashed someone from SA with PLM healders, high flow cat into a straight thru 2.5 inch (no resonators) into a twinloop, under 3k rpm it's very quiet.

Quote:

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Originally Posted by AgroVeg

Hi amant02,

The exhaust is a single exit.
That t intersection in the picture is a Helmholtz resonator which is welded shut at the other end.
This is meant to bounce back sound waves at a particular resonance which can kill drone at certain rpms.

What I found interesting was that with the factory catback and the headers and highflow cat, the exhaust sounded the same. You wouldn't even know it had headers.

I do understand what you are saying in that if I wanted a quiet ride I should have left it stock.

But I do want the best of both worlds.
At the moment it's definitely liveable with and haven't cause me any issues.

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I dont think your getting the best of any world and just stuck with the problems of both.

I would scrap this exhaust if your not happy with the sound. Cause your not getting the sound you want and your 100% loosing hp over the stock setup.

This was the best sounding honda I ever owned.

https://www.youtube.com/watch?v=QU8-OmjfBHw

Injen CAI
PLM Headers 4-2-1
RM10A - Cat back.
Straight pipe no cat (hi flow cat fail emission testing anyway)

All 2.5in. No drone, no raspyness.

Only go 3in in a Honda if your going all out power and chasing that few extra kws.