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OGB's noise was close to the limit at Castle Combe last year judging by much tutting and muttering about noise reducers etc by the guy with the noise meter. For reference, the system is a 4 branch extractor manifold into a 2 box straight through Revington system.

 

I've been looking at options to reduce the noise a bit and the motorcycle world seems abundant with devices to achieve this. Has anyone tried something like this? At the very least it looks a bit more elegant than the usual muffler bolted on for the occasion and might do as a more more permanent item? Other blogs talk about unwelcome attention from the boys in blue for excessive noise (you know the ones, Corsa's with 6" exhaust pipes) and one train of thought is that any non standard system is not permitted. I suspect that we may be getting away with these systems as drivers of classic cars and as, generally, we don't race round terrorising the locals or racing round industrial estates.

 

$%28KGrHqZHJFUE91-s7eVEBPfIr0kZTg~~60_12

 

In short, a baffle stuffed up the exhaust pipe, (and bolted) but no doubt at the expense of increased back pressure and possible reduction in power.

 

There are a number of designs out there, anyone any experience of using them?

 

Phil

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Phil

I have the same problem in spades. One straight-through silencer needs an insert like that in your photo to excite the packing in the 'silencer' bit.

Got a link?

Peter

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  • 2 weeks later...

Years ago I thought up an insert to better throw sounds waves into the glass wool in a straight-through 'absorbtion' silencer. As sound is longitudinal waves of compression and rarefaction passing along the axis of the pipe, the placing of absorbtive filling of fibre 90deg off-axis did not seem to me to be optimal. Compression waves around the circumference of the gas flow might be damped but down the middle the full compression - and noise - could persist to the outside world. So the idea was an insert a spiral flange ( strictly helical as it does not taper) at 45deg to the long axis. That reflects the soundwaves by 90deg direclty into the wool. While the spacing between each turn of the spiral can be kept to give the same cross-section area as the exhaust pipe. So gas flow should not be restricted much.

I see that a USA company makes spiral inserts, at a price: (see half way down)

http://www.cartechbooks.com/techtips/exhaust-systems-mufflers-and-catalytic-converter-guide/

Anyone tried this on sound levels? **

 

Peter

 

Spiral baffle inside perforated absorbtion straight through 115 to 98dB , post 12 here:

http://www.clubcobra.com/forums/australian-cobra-club/116358-quietening-sidepipes.html#post1200426

Edited by Peter Cobbold
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Peter

 

Any number of these on Fleabay

 

# 121457128589

 

# 181635196698

 

# 321491954464

 

etc

 

Seem aimed at motor cycles, I guess for MOT purposes, and typically £15 - £20

 

Not having any fabrication facilities, an off the shelf item has its attraction, but of which style. I take from your spiral examples, these are placed within the overall dimensions of the box, to deflect the sound waves directly into the wool, but short of opening up and re-welding the box, would a baffle stuffed up the exhaust pipe have a similar effect and if so, I guess the longer ones would be more effective??

 

Motorcycle exhausts tend to have a very short tail pipe so these longer baffles would seem to be getting to where they might be most effective.

 

Would be very interesting to do some tests on a dyno as well to measure the effect on performance.

 

On that note, I caught a programme a couple of weeks ago where they were playing with the effect of denting the downpipes on an American V8. Apparently, the steering column/box often gets in the way of a large engine in small engine bay and the accepted mod is to bash the downpipe to give the clearance. Popular opinion was that this would noticealy affect the power, which is a bad thing, so they ran some tests on a dyno.

 

It was a bit un-scientific but it was not until all eight downpipes were reduced in cross sectional area to 50% of the original that there was any noteable reduction in power on the dyno. Now it was an open system and almost certainly wasn't a tuned length to start with but it was certainly a surprise and counter intuitive.

 

Anyway, back to our humble stright 4s or 6s, does anyone have any experience of using the bolt on baffles typical available at track days and whether there was any affect on power?

 

Phil

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Phil,

I hadnt seen the bike silencer inserts, thank you. The power losses at top end on othe engines from the Supertrapp type can be considerable judging by other forums**, but I haven seen any TR data. I tried a crude tubular insert but the effect on noise was not detectable by ear.

I'm pressing ahead with the helical reflector idea.

There are data on pressure loss from baffles set at different angles across a pipe.

http://prntscr.com/9xuac5

source was: http://www.ijsr.net/archive/v2i7/MDIwMTMxNDA=.pdf

We see that baffles set near right angles to the gas flow create a lot of back pressure. However once angled at about 15 to 20 deg or more off the axis the pressure is much less.

I have a straight-through 2.5 inch bore dissipative silencer with I think glass wadding. So I have ordered a 60mm earth auger to chop to length and bolt into the bore:

file:///C:/Users/user/AppData/Local/Temp/V2I6_IJERTV2IS60874.pdf

The helix is as at about 45deg so the pressure loss should be tiny. I'm hoping the helix will reflect sound waves into the wadding and improve sound absorbtion. If the silencing effect is audible then I'll fit a repackable box with heavier wadding.

I cant find any data on those USA spiral baffles being used inside a absorbtive silencer. My experiment may not work - we'll see.

Peter

 

**post 9:

http://www.rmsolo.org/forums/showthread.php?t=4321

Edited by Peter Cobbold
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A few more thoughts on this issue

 

Fleabay item # 281503147949 has a bit more explanation as to how these baffles are supposed to work and it seems to be for the reasons Peter sets out ie, to deflect the pressure waves into the absorbant material in the silencer box.

 

So, putting one of these in the tail pipe isn't going to do the job they are designed for. All that is going to do is constrict the flow of exhaust gases. This would be fine for a motorcycle silencer with a very short tail pipe but on a TR with 30cm odd of tail pipe with a bend in it as well, the baffles aren't going to sit within the box.

 

However, there might be an alternative and that is to insert a baffle from the front of the box where it will deflect into the absorbant material (making sure the baffles are pointing the right way)

 

This one comes from Customised Choppers £11,99 12" long and 1.75" diameter but others lengths and diameters available (they have a range of types as well)

Custom800.jpg

 

It's probably the wrong diameter but it has a larger diameter both ends which would encourage the baffle to fit along the longitudinal axis of the box in the middle of the straight through pipe. It might rattle a bit as it would only be fixed at one end but with a bit of judicious fettling the rear end (inside the box) could be made to be a tight fit.

 

I feel a crawl under the back end of OGB coming on . .

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I would not like to be on track if a quickly bolted on baffle came off the car in front.

 

Make sure anything is fitted very securely.

 

At Silversone I once drove through the remnants of an exploding F5000 suspension on the Hangar straight and it isn't nice :wacko:

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Which is a very good reason for not stuffing a baffle up the tail pipe - where these clearly won't work as intended - or bolting on the type available at Castle Combe. At the very least I was looking for something a bit more elegant than those and a bit more permanent to spare the ears of the neighbours or the attention of marshalls

 

What I was looking at was, take the rear box off, put in a baffle from the front/engine end, then contect back up.

 

If the baffle comes out then. it could only be as a massive backfire which straightened out the 10 deg bend in the tail pipe as the baffle was spat out - unlikely I think

 

Phil

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Some time ago in the dim and distant past I had an exhaust on my Vitesse that tended to run a little close to the noise max for road rallies. For emergency "get through the noise test to avoid exclusion" purposes, I made up what was, in effect, a mini-silencer (not Mini silencer!).

 

The outer diameter was exactly the same as the tail-pipe's inner diameter (let's say 2"). It was about a foot long (maybe less, it was a long time ago...) and had a perforated inner pipe with a diameter of maybe 1.5" and was stuffed with wire wool between inner and outer. It was held in place with two brake-pad locating pins with r-clips holding them in place - these went through both the tail pipe and the "silencer"at a shallow angle (they sat vertically with the pinhead at the top, and the r-clip at the bottom). Nice and safe.

 

It was probably total bollocks with regard to the otherwise free-flowing exhaust because it added a restriction right at the end of the system - but it was worth it for the odd occasion where I would otherwise have been excluded from the event.

 

I hope you can imagine what this looked like - I don't have any photos, I'm afraid.

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I've used a these Merlin https://www.merlinmotorsport.co.uk/p/merlin-mild-steel-racing-muffler-m-ex-muff-4 ones in a few different sizes on my cars. My cars generally all ok for 105db, but struggle for any of these 100 -102db or lower noise number (at 4-4.5k revs). Combe are the hardest to get car through on noise. I have 2 boxes on the back of my 6, one big one and one smaller one which can be used with a straight through pipe (fine for most places with 105db) and a small long cylindrical silence for places like Combe and Zandvoort. Does the job perfectly (it is bonkers with just the one box on it at 7k :D ).

 

They do a roaring trade in those pipes at Merlin for track days............. :blink::o:angry:

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  • 2 months later...

Allotted spanner time was spent in replacing a vertical link and trunnion so no time to experiment with the above.

 

However a bit of 'net research brought up the concept of deflecting the exhaust gases downwards into the ground in order to absorb some of the acoustic energy.

 

I invested £10 in one of these, a drop down tail pipe trim

 

http://www.ebay.co.uk/itm/201327196034?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

 

No issues at Castle Combe this year and smiles from the tester. The noise reading was 97dB so assuming the meter was correctly and consistently calibrated compared to last year, I'll take that as a worthwhile investment. It was very marginal last year in getting through the sound test (the limit at CC is 100dB). And absolutely no discernable drop in performance.

 

Looks a bit odd at the back though...

 

Phil

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