spiral louvre silencers?

[steve briance]

My 7A CQ20V has a collection of joints, bends, rust and leaks on the standard centre section and cat replacement pipe. Time is running out for it and I have just started looking into my options...

I think a good quality fabricator is the way to go as someone with a sense of pride in their work will be able to take it all off and sort it in one nice smooth piece. I am looking to retain a middle box and have the new joint mate up with the old back box which is still healthy and has lots of life left in it. One of the issues for the fabricator when picking a middle box is space - off the shelf silencers of any reasonable size don't tend to fit as they have pipe in one end and out the other where as the factory design has both pipes in one end.

I went to a place locally who were very helpful and took the time to discuss what I was after, answered all my questions and demonstrated mandrel bending a piece of scrap tube to 180 degrees. (amazing to watch!) They fabricate in 304 grade SS and offer a lifetime warranty. Overall quite impressed apart from one thing which left a nagging feeling...

Their silencers features this spiral louvre core. (see pic)

I can't make my mind up and research online seems to suggest this is intended to and does successfully reduce sound levels BUT at the expense of flow. With the louvres facing the flow i can imagine all sorts of horrible turbulence and with the louvres facing away it would be slightly better but still its not a smooth path. Some of the marketing suggests the spiral layout helps create a vortex to accelerate the gas out of the system, this just sounds like a plain and simple lie to me. I know exhausts have been discussed to death on here but i couldn't see anyone mentioning this style of silencer so am wondering if it is a new thing? The silencers seem to be marketed widely in the US as Magnaflow where huge V8 engines need quietening down so in that application maybe they suit quite well.

Thoughts or experience?

[Greg_S]

I really dont know enough about the options on the silencers. Will be interested to hear yours and others experiences

[steve briance]

Well, I have just had a very interesting phone conversation with the bloke who took his time to speak to me when i dropped in there.

The company apparently developed their boxes in association with MIRA and as they are just down the road, it is entirely feasible. He mentioned that they worked with MIRA on gas flow using manometers and found that the spiral flow silencers had no significant effect on flow. When they build a system, they mate a 2.5 inch pipe with a louvered core of 2 3/4 inch so the gas runs uninterrupted through the clear 2.5 inch space BETWEEN the louvers then to carry on to the tailpipe through 2.5in tube. The sound pressure escapes sideways into the absorbent material and the gas pressure continues on through. The bloke certainly sounded knowledgeable and explained that when exhaust gas flows down a pipe the noise is always trying to escape out sideways and the gas pressure out backwards. This sideways acting 'sound' pressure explains why stainless steel exhaust tube sounds different to mild steel tube - due to the different metallurgical make up and often thinner section being less able to dampen the sound.

The company has 40 years experience and a strong following in the motorsport, kit and classic market and i don't know if this would be the case if their products were rubbish. who knows.

There is definitely food for thought, hopefully someone with a grasp on fluid dynamics will step in and clear it all up soon!