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Post by harry on Jun 11, 2021 9:29:10 GMT
I was wondering if someone has cured this with a branch resonator? If so, what position and the dimensions of resonator please? I have a 200 cell cat, standard centre section, with single exit Racing beat back box.
The drone is between 3k & 4K revs, the exhaust only registers 86 dbs on a recent track day, so overall noise not an issue. Tired of wearing earplugs!
Mark
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Post by wannabe on Jun 11, 2021 12:52:09 GMT
IIRC the MX5Parts exhausts are known for being a bit drone-y but I thought the RB exhausts were supposed to be alright! How far out does your exhaust come from the bumper cutout? When the tip fell off my old rear box  and the exit was under the boot itself, the drone in the cabin was unbearable because the noise was exiting directly into the boot and resonating into the cabin, lol. It's not vibrating against anything, is it? Just the noise output being the issue? |
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Post by howardb66 on Jun 11, 2021 14:15:51 GMT
That’s odd because mine didn’t drone at all when it was SC’d. Cobalt back box, IL Motorsport centre section.
You’ve checked it’s all done up tight? Mounts in good condition?
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Post by batou on Jun 11, 2021 14:27:08 GMT
IIRC the MX5Parts exhausts are known for being a bit drone-y but I thought the RB exhausts were supposed to be alright! How far out does your exhaust come from the bumper cutout? When the tip fell off my old rear box and the exit was under the boot itself, the drone in the cabin was unbearable because the noise was exiting directly into the boot and resonating into the cabin, lol. It's not vibrating against anything, is it? Just the noise output being the issue? My MX5Parts rear box is not droney at all... I think its either people talking Canoe Spheres or people who've never fitted an aftermarket exahuast before. Yeh its louder... duh |
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Post by wannabe on Jun 11, 2021 14:31:04 GMT
Easy solution: Drive faster so you are exceeding 4k rpm.  |
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Post by wannabe on Jun 11, 2021 14:32:17 GMT
IIRC the MX5Parts exhausts are known for being a bit drone-y but I thought the RB exhausts were supposed to be alright! How far out does your exhaust come from the bumper cutout? When the tip fell off my old rear box and the exit was under the boot itself, the drone in the cabin was unbearable because the noise was exiting directly into the boot and resonating into the cabin, lol. It's not vibrating against anything, is it? Just the noise output being the issue? My MX5Parts rear box is not droney at all... I think its either people talking Canoe Spheres or people who've never fitted an aftermarket exahuast before. Yeh its louder... duh lol I think people on nutz mentioned it (IIRC) so not just MX5OC people who start threads looking for tyres that aren't noisy on a roofless sports car  |
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Post by howardb66 on Jun 11, 2021 15:42:23 GMT
My MX5Parts rear box is not droney at all... I think its either people talking Canoe Spheres or people who've never fitted an aftermarket exahuast before. Yeh its louder... duh lol I think people on nutz mentioned it (IIRC) so not just MX5OC people who start threads looking for tyres that aren't noisy on a roofless sports car Think harry knows a little bit about noisy cars- his other car is a 4.5 Cerbera & Mrs h has a S2K. |
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Post by batou on Jun 11, 2021 16:07:55 GMT
My MX5Parts rear box is not droney at all... I think its either people talking Canoe Spheres or people who've never fitted an aftermarket exahuast before. Yeh its louder... duh lol I think people on nutz mentioned it (IIRC) so not just MX5OC people who start threads looking for tyres that aren't noisy on a roofless sports car Maybe with an unresonated decat possibly, all I know quieter than tmye Subaru (resonated 2nd decat centre pipe & axle back) and the Colbalt on a 1.6 eunos with a cat. |
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Post by warpspeed on Jun 11, 2021 23:42:37 GMT
The reality is, sound is created by pressure pulsations. And your exhaust is also going to have a natural resonance at some particular rpm. Now the only way to get rid of the lower (drone) frequencies is to add some slight back pressure. The "obstruction" smooths out the pulsations into a more steady constant gas flow, reducing the low frequency pulsing and the noise level. The problem, is that a free flowing sports type exhaust under light throttle cruise conditions will have almost zero back pressure, and its going to drone. Adding more restrictive muffling, especially right at the back will fix it, but then flat out the back pressure will become unacceptable. Best fix for droning is a Dynomax VT muffler. This has a spring loaded flap inside the muffler that adds a very slight back pressure at small throttle openings. This closes down the flow area and holds back the noise and droning under light throttle cruise. Flat out the flap opens up fully, and the overall back pressure is minimal. This works best placed right at the back of a large bore exhaust system, where the pulsing can be contained within a large pipe volume. I have no connection to Dynomax, not trying to sell you anything, just telling you what I and my petrol head buddies know works really well. |
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Post by dadbif on Jun 12, 2021 7:04:38 GMT
Made something similar to that 30 odd years ago, flap on a spindle with a balance weight that held it shut vs the spring they use. Used it on an viva, used to work well…
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Post by warpspeed on Jun 12, 2021 7:47:21 GMT
I made one myself too, a very very long time ago.
The problem with it was the pivot of the throttle plate was pretty loose, and it caused a really annoying metallic rattle at engine idle, but it sure killed the drone.
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Post by lowender on Jun 12, 2021 8:43:35 GMT
I have a 200 cell cat, standard centre section, with single exit Racing beat back box. I would suggest a Racing Beat resonated centre section as you have the RB back box. No drone or unpleasantness on mine with this setup. |
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Post by harry on Jun 12, 2021 18:24:17 GMT
Thanks for all the suggestions, I’ve spoken to a very helpful guy at Racing beat Europe. He suggested the cat maybe broken up inside, if the cat proves to be ok, I’ll try a racing beat or IL motorsport centre pipe with resonator as suggested above. As Howard 666 said, I’m conditioned to a bit of noise with the Cerbera 😜
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and the exit was under the boot itself, the drone in the cabin was unbearable because the noise was exiting directly into the boot and resonating into the cabin, lol.
le sigh