Noise

[Iainl]

Well here I go again!

Last year I tried to get my Locoblade quieter so that I could do a track day at Castle Combe and after buying a new silencer got the noise down to about 100db, but the tester did advise me I was right on the limit.

So earlier this year I repacked the siliencer with some Acousta packing, thinking it would all be fine.

Today I popped to Castle Combe to get the noise tested thinking it would all be fine, but it hit 105db @ 8000rpm  

Now need to think of another plan 

The tester did suggest removing some of the packing from the silencer to see what effect that would have, so I'll be giving that ago this weekend and keep my fingers crossed

[Pilgrim Anna]

Good luck with that!

[benchmark51]

I have heard it said that overpacking can have an opposite effect. I haven't seen it on a 7, but why not configure the exhaust to be easily altered for road or track use. You see it on the hot rods very often. All they do is remove 3 bolts and a plate (each side on V8's) and by-pass the silencer system altogether. Or just have a road can and a track one.

[SPAXIMUS]

The Acoustafil does need to be loose to start with as it only works when it has expanded with the heat. It gets quieter with use.

What I did with my Westfield was to fit a smaller internal pipe in the can, yes it will restrict top end slightly, however it slows the gas down and allows a greater area in the can to absorb the sound waves. Interpro run the stainless exhaust company behind them and they did me a new pipe that was swaged at each end to fit the can but smaller in the middle. worked a treat for me.

['The Gaffer']

As you have found out, bike engined cars are notoriously hard to get a sound pass, especially at Combe.

The exhaust setup I had in my first blade car consisted of 2 Powerflow silencers fitted in line under bonnet with a open pipe protruding to the outside.





This sounded great but was just a bit too loud for Combe, the answer came from an additional 'stick in the open exhaust hole' silencer from Merlin Motorsport. That did the trick.

[chris61]

pop up to martyn at infinity exhausts, and have a chat, he'll be able to help

[Iancider]

Some science - sorry...

Exhaust packing only deals with the high frequencies.  Lots of loose fibres, loosely packed, waggle about and waste high frequency energy  = less high frequency energy.  Packing does nothing at all for the low frequncy pulse that is the exhaust valve opening and releasing high pressure gas in a thump.  This is where baffles do their job and allow the air to compress and then release slowly - bang bang becomes phutt phutt and the high frequencies in that can be taken off with the loose wadding.  Its true that tight packing is not as good as loose packing and a straight through exhuast will always have more troublesome low-frequency content.  But there is one thing you can do and that is to de-resonate the exhaust.  It does tend to throb at a particular note so any way in which you can make asymmetrical chambers inside the silencer and make the tail pipe several different bores all helps.  This is done inside loudspeaker cabinets where clumps of sound absorber are used instead of solid masses and if you want to de-resonate a port it is several holes of different sizes.  And here is a point:  to make small speaker bassey they make it resonate at a low frequency.  It is a straight through exhaust really!

So clump your packing in non-multiple chambers and make the gauze liner tapered or different diameters. Non-resonant clumps would be say 1 to 1.3 to 1.5 in volume.  It all helps to knock the resonance out of the exhaust.  We also like round silencers and they resonate like an organ pipe so making the space inside oval and off-centre will also help.  Even a square tail pipe helps to break resonance.

Sorry - you just knew that was going to be techie... Ian

['Mendip Wurzel']

Moving the silence further away from the engine will help, but not always that easy.