Acoustics and "BS" noise police

Fitz

Fitz

Senior member
Location
Cheshire UK
Hello all, do we have any exhaust design engineers on board

Following my success in the UK BEARS F3 racing this year (another Laverda winner in the class) I was warned by one the scrutineers at Cadwell about the noise level of my monty, it has passed every meeting in the past two years however he took it upon himself to tell me that it will not pass next season. Don't know if he is just bullshitting me, I doubt that, he is good lad, ex racer type

I run a genuine works Barca exhaust system that I re-packed before the start of the season, it has regularly measured 102dB < 105dB and has raised some eyebrows on occasion, in my opinion it's not noisy and the noise it does make is excellent, I have had loads of people telling me how nice it sounds flat out

I have one that I had made when I first built the bike but it has those boring cylindrical tubes that just look awful, not too bad from the side but from the rear its just but ugly (not unlike myself)

I have tried to read up on "muffler" design but there is more BS stated about this subject by "experts"

Some "informed" opinion suggests that for relatively small engine power and from the period the engine was designed, changes to back pressure and pressure waves will not affect performance too much, a little restriction in the system is unlikely to be a problem as valve overlap (which seems to be key) was not as crucial a design feature as it is with today's modern engines with precise fuel injection exhaust design and valve overlap to match

The opinions vary as people are simply trying to use their interpretation of the science to sell their products

I think that there may be a possibility to introduce some kind of noise deflector into a short extended section between the end of megaphone section and the reverse cone that is riveted on. That way I could retain a good look but reduce the noise

If we have any real smart Alecs I could consider re-designing the baffle tube which is a pretty basic affair, just a perforated tube with two end caps to locate it centrally in the megaphone section

I have put an image of the Barca and version I had made - any ideas 
 

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Hi Dave

Exhaust system design is hellish complicated. One of those 'the more you know, the more you realise you don't know' subjects. I have worked in-and-around exhausts for 20 years or so (4 wheel automotive).

Your Barca exhaust [basic external] shape is pretty crap for noise reduction, though pretty good for flow. One of the reasons why the plain cylinder works is the dramatic change in cross-section as you enter the muffler. This enables (depending on the internal pipe layout) the gas to expand rapidly and thus energy disipates. Anyway, I digress... the Barca noise performance depands greatly on the internal layout.

So if, you want to stick with the Barca shape (I would - looks nice!), please make a sketch of the internals and we can critique it. Maybe scribble it onto paper and take a photo?

Paul H
 
That nice bloke did a great job messing with you. Didnt Mike Waugh make a quieter exhaust for them. Whats the muffler made in the USA that you add discs too to add to the sound level, forget there name. Snap Supertrap.You can dial in the sound level that passes the test. All that bouncing wave stuff only workes in a narrow powerband from what I have read.
 
Hello Fitz,

I am far away from being an expert, but what typically happens on an absorbtion-type muffler like yours is that the padding burns away over time. You can reduce or even avoid it if you a) use steelwool first around the inner tube and then add the padding  and b) make the outer diameter a bit larger, so you can use more padding.

adding to that, you could use a dB-eater type of end-cap which combines the absorbtion type with a small reflection part. If done right, it does not cost you any power but reduces the noice dramatically...

I have such a system on my 1000 SFC. You can see what I mean from this picture (regarding the padding...:)

auspuffsr-racingjpg.jpg


HTH
 
Don't know anything useful about exhaust design but I do know what happened when I tried steel wool packing in a standard Monty exhaust.
Had used it previously in a spark arrestor on a marginally legal 4 stroke trail bike and it did the job come MOT time.

On the Monty it ended up as very hot sparkly confetti.
Not sure if it was as a result of the overrun fuel which tended to burn in the exhaust or just the shock waves that smashed it up.

Have you tried the fill that Malcolm Cox supplies with his Monty systems?
 
Hi Fitz
If your cam timing relies on the reverse cone to function fully, then it's tricky trying to reduce noise without affecting that in some way. I run a 5C SFC cam in my motor and running it with the factory 2/1 it was designed to work with makes the motor sing. I reckon it was louder on the overrun than on full throttle under power!!

But a) I can't ride it on the street like that (99% of my riding) or b) take it to any track day and have it pass noise restrictions (well, it failed miserably at Phillip island ...depends on the track).

For the street I retained the reverse cone, but fitted a perforated baffle inside that attempted to follow the taper of the megaphone section. It was loud, but usable on the street (as long as I didn't give it an enormous handful of throttle and revs with Mt Plod in the next lane  :o  ). I could still feel the cam work, but just not quite as crisp. I later added a straight large dia perforated canister to it, and refitted the reverse cone at the end of that (so the baffle tapers our then flows directly into the larger dia parallel baffle). Loads quieter and still breathed OK.

As a comparison, fitting a pair of std Contis stifled it half to death. It's pretty clear that noise restrictions are going to become more and more severe at race tracks, so getting a handle on how to retain power and lose noise is essential - it;s certainly something that REALLY interests me. My hotrod will hopefully have two big Yoshi cans hanging out the back ... no idea whether they'll fit or how they'll work, but I would like a road bike that flies but doesn't immediately attract the attention of evryone within a 150db radius!

Good luck with it, and do keep us posted.
 
sonicboom500 said:
Don't know anything useful about exhaust design but I do know what happened when I tried steel wool packing in a standard Monty exhaust.
Had used it previously in a spark arrestor on a marginally legal 4 stroke trail bike and it did the job come MOT time.

On the Monty it ended up as very hot sparkly confetti.
Not sure if it was as a result of the overrun fuel which tended to burn in the exhaust or just the shock waves that smashed it up.

Have you tried the fill that Malcolm Cox supplies with his Monty systems?
Click to expand...
You can use stainless mesh around the perforated tube then the baffle material, which helps prevent the burn and compression of the fibre stuffing.
 
That's spooky, I'd edited the stainless mesh bit out of my post as I knew some of the Moto-X guys did it but hadn't tried it myself
so couldn't say if it worked or not.
 
The fibre fill, on a strand level, is quite britle, despite it feeling soft when woven when in a big clump. Add temp and this becomes exacerbated. This is why a boundary SS mesh is used, to allow a hot pressure pulse (coming through a perf hole) to disperse (pressure-load-area and all that) before it gets to the fibres. Stainless must be used, of you want it to last. Too fine a grade SS wool may also fail (like fibre) because of localised temperature increase > oxidation > loss of stuctural integrity.

You can see in Lothar's pic below that SR-Racing are using steel mesh at the front only, this is because the pressure pulse there have higher energy there than further back. Best not to use SS mesh unless it is required - if the fibres survive, that's better.

Dave - are you getting much gas rush noise? This can tip the dB into the 'red' zone - maybe cheap and easy to clean up. Looking forward to your sketch of the internals.



 
Years ago I remember reading about Cook Nielson trying to get his " Old Blue" past the noise police at Laguna Seca . It was a hopeless cause until he fitted a tach that read about 1/3 higher than it should. The noise test was taken at about 1/2 redline if I remember correctly.
 
You need clarification on why this person told you you would not be allowed to race,because currently the noise level is 105db and I've heard nothing to suggest that next year will be any different.

Saying that I would hazard a guess that a Marshall(s)  have report your bike has been too noisy out on the track - ride past level.

I'm amazed you not had any problems with noise before!
I stop using Monty type exhaust system years ago due to noise levels, and built a 2 into 1. The main thing in keeping the noise down is in the use of a modern muffler, the longer the length the quieter the system. Your original system is short and is tapered and acts like a megaphone .The very short reverse cone on the end in theory helps the mid range!

Possibily two ways forward  a system with a modern end can or a sort of replica system with a longer end piece where part of it is straight with no taper that doesn't allow the noise waves to expand.

Andy

 
I too would go for the dodgy tacho - it worked for me for many years - and that was in the days of 115dB limits! You could also try running an electric motor....
If I come up with any more useful suggestions Fitz I will let you know... :LOL:
 
I've been building race exhausts for many years. Locally, one circuit has a 105db limit at the property boundary and another has a 92db limit measured about 30m off the track edge. Then I've built systems for bike engined speedway cars which have a 95db limit.
The common denominator in getting them to work is can volume. Straight through style, perf tube inner with non metallic packing works best for me. Bigger can volume the better. Packing I usually use a woven mat type sold for kart mufflers, usually with stainless wire binding to the center tube to hold it in place.
Aluminium can over steel too - for some reason alloy doesn't resonate as much. Has to be well mounted to hold together though...
Megaphone shaped cans are not the best. The pressure wave is at it's strongest as it enters the muffler so should see the biggest volume of packing immediately - not a thin tapered layer....

For some methanol applications I've built resonant chamber mufflers with internal tubular bypass and chambers and no packing. The only reason for this is that the packing tends to accumulate methanol - and catch fire, unpopular with officials...

Four valve heads are less sensitive to pipe lengths and tapers than two valve as they usually have less overlap and are less reliant on pipe effects for power.
 
Thanks all, very helpful as usual, I have found a local performance exhaust manufacturer who I am going to take the bike to at weekend for an appraisal, I will then whip one off the bike and show pics with dimensions for the technically minded

As I said, I had re-packed the system before the start of the season with "performance" synthetic material

I breaks my heart at race meetings when I see things like 7R's G50's Manx's and many other beautiful classic and vintage race bike with those f#####g ugly shiny sections of drain pipe stuck on them

The little Honda twins seem to look OK with them in the main but they were never a classic race bike anyway - a Manx Norton, god give me strength

I had looked at mounting two stubby cans below the oil catch tray but I don't think I have enough ground clearance, the Ogier bike just had two stubbies underneath but god know what noise level that made

I had also thought about having a high level system made but after experiencing being sat in the assembly area while the marshals clean up the carnage left by others in the event before mine, I would also need to come up with an asbestos arse cover for my leathers
 
Screw the baffles, take them out and rattle every window in town when you leave ;o)

Rental Hog Rider's get away with it - Why can't we - HIC!
 
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