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PT: Exhaust Silencer Mod

Howard

Well Known and Active Website User
VOC Member
Someone recently suggested cutting a slot in the tail pipe to reduce the exhaust noise, and I can't find the comment now.

I did as suggested and the difference is amazing, not a lot quieter, but a much nicer sound that complements a run, rather than being annoying.

Whoever it was, thanks!

Howard

ps, talking of annoying noises, I might try cutting a slot in my wife's crash hat to see if it works on that as well. :D :D :D
 

vibrac

Well Known and Active Website User
VOC Member
Since women have joined the subject of silence and slots perhaps a braver man than me can pose a joke.... ;)

To bring us back on thread here is the extension on my Trigrumph all the 1/4 holes (around 3 sides) add up and equal the area of the exhaust outlet before I added the extension the end has a washer welded on with a 1/4 hole
1593184786451.png
 

Howard

Well Known and Active Website User
VOC Member
Since women have joined the subject of silence and slots perhaps a braver man than me can pose a joke.... ;)

To bring us back on thread here is the extension on my Trigrumph all the 1/4 holes (around 3 sides) add up and equal the area of the exhaust outlet before I added the extension the end has a washer welded on with a 1/4 hole
View attachment 35715
I think that would be too restrictive for Mk2s. I also think that even though the area is the same, the resistance at the multiple peripheries may be more important ................... and no, I'm not going to do the maths it's Friday night and the pub's open ................... Oh bugga it's not, where's my calculator? :D :D :D
 

Chris Launders

Well Known and Active Website User
VOC Member
Multiple small holes will always be more restrictive than one big one surely due to turbulence at the edge of each hole.
 

vibrac

Well Known and Active Website User
VOC Member
turbulence increases if the holes are punched in order to raise the holes edges inwards this technique is used in some commercial silencers
Turbulence is of course that which leaches energy from the gasses ie quietens
 

vibrac

Well Known and Active Website User
VOC Member
trying to get the Flash under the limit for track days I made a 2" absorption straight through but made the walls 1/4" thick it did the trick but weighed a fair bit
this was my post #48 and picture
 

Bill Thomas

Well Known and Active Website User
VOC Member
I was hoping you would say, It now had a Tweet !, That would be nice :D ,
I had a Goldie once, You guessed it , It didn't have a Tweet, Begger .
 

BigEd

Well Known and Active Website User
VOC Member
VOC Forum Moderator
Not sure if you can see it from this photo, it's 10 mm wide x 100 mm long in the underside of the tailpipe.


View attachment 35716
I'm not sure about the technicalities of how this slot works to quieten the exhaust but the tip has been repeated several times. How did you decide on a 10 mm width? I usually base my choice of drill on which one closest to the size I want that is still sharp. ;) Maybe I'll try it on my straight through Armours stainless to see if it takes just a little edge of it. In reply to Bill's goldie silencer comment, when I was 17 I had a BSA Gold Star and it did have a genuine silencer that twittered. Interestingly, having bought the book, by Power Into Precision by Alan Vessy, A record of d. Napier & Son, there is a quote on page 161 when talking about the Deltic 16 cylinder type 4s fitted in locomotives had a whistling exhaust note - "not too unlike a BSA DBD34 Goldstar motorcycle of the same period". (Sorry, half strayed on to another thread that I started about timing covers and Deltics. I'll give myself a good slap!;))
 

Howard

Well Known and Active Website User
VOC Member
Ed, I was being modern, I don't actually own any metric drills, so it was probably the closest sharp one to 3/8". And you're right, the Deltics had a distinctive whistling exhaust.
 

Howard

Well Known and Active Website User
VOC Member
I was hoping you would say, It now had a Tweet !, That would be nice :D ,
I had a Goldie once, You guessed it , It didn't have a Tweet, Begger .
I've got a reverse cone mega that was on the Egli a couple of years ago. It's actually a straight through silencer with absorption packing and an empty reverse cone on the end. The Egli ran really well on it, but it made an awful twittering noise on low revs. I put a piece of exhaust pipe through the cone and that stopped it.
 

Marcus Bowden

Well Known and Active Website User
VOC Member
I've been thinking of the same for many a year but situated in the tailpipe extension that takes the exhaust beyond my aluminium panniers. Fitting a sliding cup plug that is bigger than the silencer tailpipe so is touching the tailpipe when the engine is stopped then when started the cupped plug is pushed back against light spring pressure on opens up the slot depending on the amount of throttle opening, keeping a back pressure as the majority of engines require a back pressure of sorts but will also reduce the noise level.
bananaman.
 

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