How to quieten a noisy exhaust?

[deltic]

As posted elsewhere I recently had a stainless steel exhaust system fitted to the BX which unfortunately I find uncomfortably noisy. Indeed it is starting to spoil my enjoyment of the car...

I've booked it in for the garage that made it to have a look at it to see if they can do anything to make it quieter, however I'm a bit concerned I may just end up paying for the exhaust to be pulled apart with little or no benefit.

Is there any science or logic to making it quieter? Presumably bigger silencers or different baffles might help. Does anyone have any suggestions? Of the three silencers which does the most silencing and might be a good place to start?

And ideas (other than ear plugs!) would be gratefully received.

[David]

It's strange that you've posted this. I've had a stainless exhaust made for my BX, and I feel exactly the same; it's louder than I'd like.

I spoke to the garage that made it in the first instance. Before they made it they suggested that it'd only need one back box so that it wouldn't restrict the exhaust flow and resulting power, and that it SHOULD be quiet, but I told them just to put two boxes on it as I didn't want any noise.

After they'd made it, it's nosier that I’d like, but they said that the only way to eliminate any more noise would be to put another silencer in it, which frankly, I haven't got any room for under the car. Beyond that, they said that really there's little that they can do. It's a bit annoying though, especially when I've got Subaru’s and such pulling along side me revving their engines as they can hear mine!

Beyond any further modifications to the exhaust, I'm wondering whether to put some kind of soundproofing under the boot floor, so at least, that I can't hear it as much.

[deltic]

That's a shame, particularly when we both had no desire for Subaru loudness and my main requirement was for it to be quieter.

The exhaust I had replaced was particularly noisy and I wonder to what degree they copied that thus making an even noisier exhaust.

[Jaba]

Since the front pipe seems to be no longer available you could go hybrid with a frankenexhaust retaining the stainless front pipe and then using the three quieter mild steel pipe and boxes to the rear. If you still have your donor car then you could do a trial install to see what the difference would be.

I understand your frustration as I know how loud and boomy an exhaust can be on a BX. There is very little sound insulation at the back end.

[Kitch]

The cross box plays the biggest part, that I've noticed. Get rid of that at your peril!

Also, many exhaust fabricators use 'sports' silencers because they're cheaper (and most people want the noise), but in actual fact you can get OEM ones too, just they're more expensive.

[deltic]

By cross box do you mean the intermediate silencer which is an in and out silencer rather than a through silencer?

I actually took the car back to the garage yesterday and in their opinion there was little else they could do due to the limited space. I did suggest a "frankenexhaust" but again their opinion was that even just having the stainless front pipe would alter the tone of the exhaust.

I guess for now I am just going to have to live with it, my fault for rushing headlong into finding a solution. I live and learn, but at least that bit of the BX won't rust!

[Kitch]

Correct. It's known as the cross-box, I'm guessing because it sits 'across' the car.

If they've done it from the manifold back then the new piece likely won't have the divider in it, either. That plays a part in aiding gas flow, and I imagine without it, there's a chance it could sound a little more vocal, though it wouldn't be a huge difference.

Like I say, if they've made a system which just has a front and rear silencer the same kind of dims as the original ones, and forgone the cross-box, then yes that will sound pretty boomy.

[deltic]

No, it has the cross-box.

[Kitch]

No, it has the cross-box.

Ah, really? Does it have a small box in the downpipe and the rear silencer as per OEM? If so, it's probably more likely they've used sports silencers (which are the commonly-used ones in aftermarket circles).

Neil Parry has a stainless system on his gold TRS, and it sounds fine.

[Jaba]

The cross box seems to be an expansion box with the input gas having a perforated tube into the silencer, this tube may or may not have some insulation around it but it just vents into the box and finds its way out of the exit connection. I presume that removes a bit more noise than a straight through/sports silencer whose aim is to produce the minimum flow through without worrrying about higher noise levels.
The rear box is a straight through design but it is packed with fibre glass as well.



Noise levels do seem to vary between makes. The Walker front pipe and box was the quietest I have had. Some makes seem to resonate more than others and the booming can be tiresome on a long journey without a good powerful radio to drown out the exhaust.
On my current setup the exhaust noise is quite well subdued and road and wind noise are both more obtrusive.

None of this makes it easier for Deltic to solve without investing further dosh in what may be a vain attempt to improve things.

[deltic]

I rather fear that for now i have to live with it!

[deltic]

No, it has the cross-box.

Ah, really? Does it have a small box in the downpipe and the rear silencer as per OEM? If so, it's probably more likely they've used sports silencers (which are the commonly-used ones in aftermarket circles).

Neil Parry has a stainless system on his gold TRS, and it sounds fine.

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Exhaust fitted is as per the diagram.

[Tim Leech]

I guess you could revert to mil steel, I have a used cross box, hockey stick and rear box for your car albeit used your welcome too FOC (not front pipe thought)

I think yours being an auto would give it a different tone.

[deltic]

Thanks Tim, that's very kind of you to offer, but for now I think I'm going to have to persevere with the stainless one a little while longer.

[David]

I've been persevering with mine & I've realised that if I keep the engine revs down, keep the back seats up & if the radio is on, then I can't really hear it, so if I get a layer of foam under the boot carpet then the noise should be hardly noticeable. When I first drove it, the seats were down so it really echoed through the car.

I haven't really got the option of changing exhaust parts as they simply won't fit.