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Resonance


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#1 Pickwah

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Posted 03 February 2020 - 01:16 PM

I'm having a lot of trouble with an unbareable droning at cruising speed, roughly 3500rpm. The engine is a A+ 1330 from a Metro with an airbox and a twin box stainless RC40 system.

I honestly can't put my finger on what to try next. I have applied silentcoat to every panel barring the roof, thick carpets and matts, still with no success.

It's making long journeys horrible. I do find if I press on past 4000rpm the droning goes away.

Any ideas?

#2 Quinlan minor

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Posted 03 February 2020 - 01:24 PM

I’d be inclined to check the exhaust system.
Any defective rubber mounts? Is the pipe touching the body at any point? Tap the silencers with a rubber mallet to check if there’s a loose baffle in there?

#3 Pickwah

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Posted 03 February 2020 - 01:29 PM

The rubber mounts are fine.. it's the later style that have the hanging brackets fitted to the subframe. The centre silencer is new and made a slight difference.

I can only explain the sound as like 'sitting inside a drum' the whole cabin literally fills with this awful drone.

#4 1314cc

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Posted 03 February 2020 - 05:10 PM

I had a single box stainless rc40 and it was absolutely deafening in the cabin!
Swapped to mild steel and was much easier on the ears.

#5 Moke Spider

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Posted 03 February 2020 - 06:06 PM

I found with Cams over 285 degrees, moving the 2 silencers further apart helped a lot but I found it did only hit a resonance at around the 3200 rpm mark, under and over this, wasn't an issue. It might take a few stabs at it. I just tried moving them apart first and that worked in this case. For you, they might be happier coming closer together.



#6 Pops_Guild

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Posted 03 February 2020 - 06:34 PM

If you want to audio on the outside then you may need to consider sound deadening and dampening inside to solve.

I have MLV down the exhaust tunnel and the bulkhead and closed cell on top of that. Makes a massive difference inside. Still loud at top cruising speed but you can still talk to passengers

#7 Stevie W

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Posted 03 February 2020 - 08:18 PM

I had a booming/resonance noise like yours at 4000 rpm and it appeared to be coming from the rear of the car. It was more apparent in top gear on a motorway where you tend to have the engine sit at this speed for some time.

I cured mine (as much as you can in a classic mini!) by fitting the under bonnet sound deadening kit from minispares. Admittedly it hasn't made a huge difference to the overall racket, but it did cure the booming at 4000 rpm.

My theory here was that my bonnet had a resonant frequency that was at its highest with the engine running at 4000 rpm and the sound deadening kit just moved the resonant frequency to somewhere else!

I'm not saying this is definitely your issue but it cured my issue.

Cheers, Steve.

#8 Chris1275gt

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Posted 03 February 2020 - 08:27 PM

Had exactly the same problem except mine was so deafening at anything over 1200 revs. Took the front and rear seats out lifted carpet took the dash out and rear parcel shelf. Cover under the parcel shelf behind and under rear seats entire front bulkhead Up to the top rail, arches and the small upright panels under the rear seat with a layer of this.

https://www.minispor...ni-clubman.html
Fix it with a dab of contact adhesive here and their.

Then on top of that a layer of 8mm closed cell foil backed self adhesive sound proofing on top. Seal all the joints with a roll of aluminium tape. Get something on the underside of the bonnet.Put it all back together and you would not believe how quiet the cabin will be.

Edited by Chris1275gt, 03 February 2020 - 08:29 PM.


#9 ads7

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Posted 03 February 2020 - 08:55 PM

Had the same problem, it was a combination of stainless steel exhaust and large bore engine; I now have a mild steel exhaust and a small bore engine which tend to vibrate less. Problem largely solved.

You'll be sticking with your 1330 no doubt so change the exhaust to mild steel, also consider an mg metro cast manifold in place of a tubular setup

Edited by ads7, 03 February 2020 - 08:56 PM.


#10 Pickwah

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Posted 03 February 2020 - 09:54 PM

Cheers for the input guys. As far as sound deadening goes, there's not much else I can do as I've already kitted it out with silent coat, closed cell foam and carpet sets etc.

Is there really much of a difference in a mild steel twin box setup over a stainless twin?

#11 Quinlan minor

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Posted 04 February 2020 - 06:01 PM

Yes, stainless is really not suited for use on exhausts. It lacks the elasticity and consequent noise attenuation of mild steel.

#12 1314cc

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Posted 04 February 2020 - 06:30 PM

yes I found stainless completely ridiculous!
I've a 1310cc, Kent cam etc so similar power to yours.
I switched to mild steel rc40 but with a large bore box and found it a lot more friendly to drive!




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