Stainless steel exhaust

[keith m 131]

Hi All,

I had a bespoke stainless exhaust fitted to my 2ltr twin cam fitted in my Morris traveller and it has vibration at 2-3000rpm which is felt through the floor and I too have a booming drone at 60mph which disappears at speeds above and below this. I have mid and a rear box fitted and I must say that the install is very neat.

I have been back twice to the place that fitted the exhaust but they say that the vibration and drone is part of an aftermarket stainless exhaust system. They say that a stainless exhaust is made of a harder and stiffer material than a standard exhaust and that a standard exhaust has some flex where the stainless doesn't. They have tried different rubber type mounts etc but no significant improvement.

I used the original manifold and downpipe and had the remainder made in stainless I have even fitted a rubber coupling between the joint of the downpipe and stainless pipe.

I have been chasing an answer to the vibration and drone in various posts on here since 2010. I have been looking at the propshaft, gearbox and other various things, and I feel that after reading a post by Thotos regarding his new stainless exhaust that the answer may lay with my exhaust.

But what do I do about it because I am so fed up with the drone when driving that I might just scrap the exhaust and start again. Since reading your post I have been looking at other forums that use aftermarket stainless exhausts and this is a general problem but nobody appears to have any solutions to reduce the vibration or drone. I have tried extra sound deadening etc. but to no avail.

Can anyone shed any light on my problem.

Regards

Keith

[Thotos]

You are obviously hitting a resonant frequency between 2000-3000 rpm. I remember as a young lad at school during O-level Physics lessons playing with a taught piano string, a paper rider (folded bit of paper perched on the string) and a tuning fork. We adjusted the tightness of the string ('tuning' it) so its resonant frequency would match that of the tuning fork and then a light tap of the tuning fork would make the string vibrate like mad and the paper rider would jump off as if by magic   

The problem is that stainless exhausts are much stiffer than mild steel so in effect they are a tighter string  . So what you need to do is to change the resonant frequency of the exhaust. Unfortunately you can't just stretch it or compress it so you'll have to try adding extra hangers, different hangers, or maybe even welding the odd extra bit of metal. But that's likely to just shift the resonant frequency so you'll end up with vibration at a higher or lower rpm. You could try 'relaxing' the tightness of the whole exhaust by adding a piece of flexible pipe as near as possible to the downpipe.



Something like the above will stop engine vibrations making the whole exhaust resonate (hopefully!  )

[keith m 131]

Hi Theo,

Thanks for the reply. I have fitted a coupling like the one you suggest to the end of the downpipe but has made no difference.

I read in your post "exhaust, Powerflow vs Longlife" that you were having the same problem with vibration and noise, how did you resolve it?

Regards

keith

[Thotos]

how did you resolve it?

I just waited a day 

The car behaved admirably with no strange vibrations at all! So I have no idea why there was the vibration yesterday, I'd like to think it was road surface related and not the diff !!!!!!  If it was the exhaust I don't understand why there was none of it today.

If you've already fitted a flexible coupling and there's no difference, are you sure it's the exhaust that causing your vibration?  I had a wheel bearing fail shortly afterwards on my car and I now think that maybe it was the wheel bearing that was causing my original vibration.

 

[keith m 131]

Its not the wheel bearings because i get the vibration when the car is standing still.

Keith

[Fiat 131 Abarth#2]

You are obviously hitting a resonant frequency between 2000-3000 rpm. I remember as a young lad at school during O-level Physics lessons playing with a taught piano string, a paper rider (folded bit of paper perched on the string) and a tuning fork. We adjusted the tightness of the string ('tuning' it) so its resonant frequency would match that of the tuning fork and then a light tap of the tuning fork would make the string vibrate like mad and the paper rider would jump off as if by magic   

The problem is that stainless exhausts are much stiffer than mild steel so in effect they are a tighter string  . So what you need to do is to change the resonant frequency of the exhaust. Unfortunately you can't just stretch it or compress it so you'll have to try adding extra hangers, different hangers, or maybe even welding the odd extra bit of metal. But that's likely to just shift the resonant frequency so you'll end up with vibration at a higher or lower rpm. You could try 'relaxing' the tightness of the whole exhaust by adding a piece of flexible pipe as near as possible to the downpipe.



Something like the above will stop engine vibrations making the whole exhaust resonate (hopefully!  )

yes Theo, very hopefully and look like this!

[Thotos]

yes Theo, very hopefully and look like this!

That's exactly what I had in mind 
That's a superbly fitted exhaust Enzo, it looks great. 

[Heini131]

I know the exhaust and the under side of this car very well 

[Thotos]

Are you showing off again Heini?   

[markwilliams131]

thotos  i missed your post on your new ss exhaust
whare did you get 

[Thotos]

thotos  i missed your post on your new ss exhaust
whare did you get 

That was a long time ago Mark. Over 2 years ago in fact. Have a look at http://www.131mirafiori.com/smf/index.php?topic=3627.msg27874#msg27874

[markwilliams131]

thanks thotos  and sorry i did read it before

[Tas131]

Would replacing the muffler possibly change the resonant frequency? Even if you can't get rid of the drone, moving it higher or lower in the rev range would make traveling at 60mph more pleasurable.

[keith m 131]

Hi Mark,

The answer is I dont know whether removing one of the mufflers would help, that is why I started the thread because i am trying to get some ideas and tips of of problems that members have had with this exhaust set up.

I want to have a good idea and understanding of what is causing the problem and then decide what to do rather than just start chopping up parts of the system only to discover it was pointless and expensive.

Regards

Keith