Recent developments in Fibre Chemistry have brought about changes in the way I pack my performance silencers.

Going back a few years;

When I first laid out the Straight-Back Performance System for the Elise/Exige, it was obvious that noise suppression was going to be crucial to its success. The existing lay-out, a long transverse silencer was not an option at that time. Observing dyno runs from the rear of the car, a fiery orange glow was appearing inside our silencer. If it wasn’t burning fuel, what was happening?

A clue was supplied by another customer complaining about the lack of a decent, quiet(ish) silencer for his 111S. He had noticed shards of packing from his recent exhaust purchase appearing on the dash of his car BUT not the glassfibre – the Stainless Wire wrapping!

 Now this silencer ended up incredibly noisy but all of the packing was still present (we had thought it empty). The long-strand glassfibre immediately adjacent to the wire wrap had melted into an impenetrable glass sheath, preventing the sound waves dissipating into the rest of the box. The wire itself, as you can see, had mostly disappeared!

Glassfibre is well up to the temperatures present in an exhaust and I and many others have used the Stainless wrap and long-strand fill procedure for many years. What caused the fibres to melt then? With the short run to the silencer on the new layout, the heat from the manifold caused the fine filaments of Stainless to overheat and break down, in turn melting the glass strands next to it.

A mid-engined layout, (and Caterham?) would be very susceptible to this phenomenon. The 111S owner had found the evidence on his dash that although Stainless Steel is well up to Hi-Temp exposure, in a fine strand packing material, even on his transverse layout, it wasn’t good enough!

I now use Acoustafil

I’m using the highest-spec material, S2 G, in all my Silencers and Cat Pipes. Besides noise suppression, the actual EXHAUST NOTE is a very important reason for a silencer purchase.

Recently, I’ve had several cars putting in some miles on it, road and track. the image to the right is a post-mortem on a 111R silencer's internals. All the packing is still there (a VERY important property!) AND it’s in amazing condition!

Conclusion?

Wrapping the perforated tube in Stainless wire wool always seemed to me a bizarre solution. The perforations are there to allow the hi-frequency, hi-energy sound-waves to expand out and into the ‘E’ Glass where they bounce around and to some extent, are dissipated (given a large enough silencer). Blocking, or at least shielding the perforations with a metal mesh certainly hindered that whole expansion/dissipation process.

When the stitching of the Acoustafil S2 G melts, the expansion of the fibres creates a low-density area immediately around the perforated tube. Silencing efficiency is vastly improved with the hi-frequency waves passing effortlessly through to the main body of the silencer for efficient absorption.

S2 G certainly costs more than the old Stainless wrap and Long-strand ‘E’ Glass but I will be using it (to my own formula) from now on and with every confidence that my silencers are the answer to today’s stringent noise regulations, yet they can still make a Lotus sound like a Sports Car should.