Dunstall Replicas - 1971 OIF T120R

[NM Bonny]

These are original silencers (71-4159) that came stock on my '78 Bonny. These came stock on many year models. They are often referred to as "cigar" silencers which are awfully quiet, and visually way too long, but have a very nice tone. I believe they are baffled because the tone has never changed.

Here is how I arrived with these "megaphone" mufflers: I cut the tapered end off the stock muffler right behind the weld (because one of them became irreparably damaged) and in doing so I discovered those four exit holes/short tubes that look like they were designed to look that way, and, nothing fell out of or became loose from the muffler as a result of the cut, so I grudgingly did the same to the other one. It was a daring operation. I did this way back in the mid eighties when spare parts were practically none existent and there was no internet to find replacements. The operation caused the mufflers to have a bit more robust sound but not loud at all (unlike peashooters or Dunstalls), yet the tone and performance was slightly improved over stock originals so I have kept them on the bike all these years. I have yet to see any others like them and (in my opinion) they look cool as all get out. I wonder if the import versions are true to the originals in construction because they are much less expensive than English made ones if one were to take a gamble and cut on them. I have since added English-made peashooters for Triumph and another set of stock originals to my collection so I have several nice options for tone and looks.

Wow, that's a wild idea, very creative. I just stuck my finger into mine and there are two holes in the final baffle plate instead of your four--and they're off to one side of the muffler. You lucked out with four, nicely spaced holes. It does look cool!

Well, my OEM mufflers are actually not in terrible shape; a few small dings but no rust or deep scratches. I think it's going to be the sound that'll make the decision for me. I was listening to the Lemon Drizzle Gang's video of a '70s Bonny with the Dunstall replicas, albeit repacked with upgraded material, and comparing that with The Mighty Garage 'shakedown cruise' video of a restored '71 Bonny with OEM mufflers... and the latter sounded like a sewing machine in comparison.

 

[NM Bonny]

My old pre-unit Tbird has original Dunstalls fitted but the bike has not been on the road for many a year. However I can confirm that they did originally have said perforated discs but a bit of judicious prodding had those out without any collateral damage.

With regard to the silencers being a tad OTT regarding decibels when out and about, you are obviously not riding fast enough LOL.

The cans on my Ducati SS work on the same principle, perforated tube wrapped in a fine SS mesh around which the wadding is wrapped. With these big twins the wadding takes such a battering that it’s not long before the exhaust gasses find their way to the carbon fibre sleeve which then discolours. On my 3rd repack which also entailed wet & dry sanding of the sleeve to remove the discolouration I’d had enough of this ritual so I purchased a length of 60mm ID SS tube to slide over the perforated tube and it’s SS wrap, profiled the ends to mate with the caps at each end of the cans and sealed the join with silicone mastic, some of you will call it tub caulk. The sound is amazing, louder than my son’s Panigale 899 with a Termi system on.

What is it they say about loud pipes? Mine, together with the open clutch may even resurrect my ancestors LOL.

Tsk, tsk... time to get that T-Bird back on the road! Ah, good to know about the inner perforated discs. I had suspected they were just tack-welded in place.

Nice work on the high-end muffler upgrades. I'd love to hear that exhaust note some day if you find yourself with nothing better to do...

Yes, loud pipes do help avoid people sideswiping you; the so-called 'mufflers' on my '51 FL Pan-Shovel bark a bit, and I've yet to have someone drift over on me. I guess my goal with this bike would be to have an exhaust note that's sexy but not so raucous that I can't tolerate it for an extended cruise out into Indian Country. I do want to explore NM's two-lane curvy backroads next Spring.

 

[solomon]

Lemon Drizzle Gang's video of a '70s Bonny with the Dunstall replicas, albeit repacked with upgraded material, and comparing that with The Mighty Garage 'shakedown cruise' video of a restored '71 Bonny with OEM mufflers... and the latter sounded like a sewing machine in comparison

I had a T14OE, now deceased by fire, that was mint, including Meriden cigar silencrs. So, I can make a good comparison.

First off, they are huge. Just don't look good. Never could really love the bike aesthetically.

Yes, they are quieter. But, afterall, that's their purpose, and the engineers knew what they were doing. I found it a very comfortable ride. I never once wished for more volume. Because....

No, they sounded nothing like a sewing machine. Here's a little anecdote to give an impression of their presence...

I do my maintenance in the garden, or house. My garden has a high brick wall, the other side is a pedestrian walkway, fairly well used. Think I was fiddling with carbs. Anyway, needed to start in the garden, rather than wheeling it to the road. I don't want complaints, and perhaps unusually for a motorcyclist, don't want others to have to put up with excessive (and to them unpleasant) noise.

I started the engine. I heard a little boy coming, could hear him talking. He said he could hear drumming. I killed the engine. Once I could hear he'd gone along a fair way, I restarted the bike. I heard him exclaim, "I can hear drumming again!"

That little boy was right. They sounded like drums. Deep, a sort of powerful menace. A rumble in the jungle.

To my ears, they sounded better. Volume in no way equates to powerful presence. Quite the opposite, it's just irritating. There are car drivers that mod their exhausts. I always think, "What a plonker," when they drive past. Same with loud bikes, actually. Fairly confident people will think the same of a poorly silenced bike. Mean biker, or plonker?

Knocking out the perforated disc in the Dunstalls will probably make it louder. It might well mean losing lower end torque. Fact is, people will argue both ways. Often a bike will seem more powerful to a rider just because it's louder. Think I'd go with the exhaust engineers that designed it, no man with hammer and a long screwdriver.

YouTube videos taken on a mobile phone, played backon a mobile phone , may well not record that low, low tone of the Meriden cigars. Will more likely record the rasp of a badly silenced bike. So difficult to make a comparison. You'd need to play back through hifi with a subwoofer, and hope the recording is quite good.

The recording I posted here did use a (cheap) external mic. Played back through the hifi via Bluetooth, it's not too bad a representation. Still lost the rumble a bit though.

So, you might be wondering, with my Dunstall replicas, am I a mean biker, or a plonker? Probably both. It's distinctive, has a precise thump, rather than a scream, so indicates large cylinder capacity. It shakes the ground at idle. Literally. I left it running, went in a house, was surprised how the house was (very slightly of course) shaking. But, it's all too loud. Most people will put me in the second category. I should wear earplugs in longer rides, otherwise permanent hearing damage is a cumulative risk. There's plenty on this forum with hearing loss.

 

[NM Bonny]

I had a T14OE, now deceased by fire, that was mint, including Meriden cigar silencrs. So, I can make a good comparison.

First off, they are huge. Just don't look good. Never could really love the bike aesthetically.

Yes, they are quieter. But, afterall, that's their purpose, and the engineers knew what they were doing. I found it a very comfortable ride. I never once wished for more volume. Because....

No, they sounded nothing like a sewing machine. Here's a little anecdote to give an impression of their presence...

I do my maintenance in the garden, or house. My garden has a high brick wall, the other side is a pedestrian walkway, fairly well used. Think I was fiddling with carbs. Anyway, needed to start in the garden, rather than wheeling it to the road. I don't want complaints, and perhaps unusually for a motorcyclist, don't want others to have to put up with excessive (and to them unpleasant) noise.

I started the engine. I heard a little boy coming, could hear him talking. He said he could hear drumming. I killed the engine. Once I could hear he'd gone along a fair way, I restarted the bike. I heard him exclaim, "I can hear drumming again!"

That little boy was right. They sounded like drums. Deep, a sort of powerful menace. A rumble in the jungle.

To my ears, they sounded better. Volume in no way equates to powerful presence. Quite the opposite, it's just irritating. There are car drivers that mod their exhausts. I always think, "What a plonker," when they drive past. Same with loud bikes, actually. Fairly confident people will think the same of a poorly silenced bike. Mean biker, or plonker?

Knocking out the perforated disc in the Dunstalls will probably make it louder. It might well mean losing lower end torque. Fact is, people will argue both ways. Often a bike will seem more powerful to a rider just because it's louder. Think I'd go with the exhaust engineers that designed it, no man with hammer and a long screwdriver.

YouTube videos taken on a mobile phone, played backon a mobile phone , may well not record that low, low tone of the Meriden cigars. Will more likely record the rasp of a badly silenced bike. So difficult to make a comparison. You'd need to play back through hifi with a subwoofer, and hope the recording is quite good.

The recording I posted here did use a (cheap) external mic. Played back through the hifi via Bluetooth, it's not too bad a representation. Still lost the rumble a bit though.

So, you might be wondering, with my Dunstall replicas, am I a mean biker, or a plonker? Probably both. It's distinctive, has a precise thump, rather than a scream, so indicates large cylinder capacity. It shakes the ground at idle. Literally. I left it running, went in a house, was surprised how the house was (very slightly of course) shaking. But, it's all too loud. Most people will put me in the second category. I should wear earplugs in longer rides, otherwise permanent hearing damage is a cumulative risk. There's plenty on this forum with hearing loss.

Thanks for your thoughtful input.

It may well be the tinny speakers in the laptop reducing the Mighty Garage's video soundtrack to a comparative sewing machine sound. It is, indeed, the quality of the sound I find so attractive on the Lemon Drizzle Gang's video, not the loudness. But for sure what I need to do is ride the bike down my most oft-traveled routes--down the blacktop from the foothills of the mountain to the Rio Grande, then either northwest out Hwy 550 toward El Cabezon and the Jemez Mountains (1st pic), or south along the river on Hwy 313 (2nd pic).

As for deferring to the engineers who designed [x] muffler, I do understand the principles at work. I just don't always defer to others' designs but prefer to experiment and arrive at a configuration I like. (My side business is designing CNC-machined parts to hot rod the A-Series engine, including the world's first blow-through, intercooled supercharger conversion for the Midget and Sprite.)

As I mentioned, the vintage Sportster mufflers on my '51 Pan-Shovel (Panhead with '66 Shovelheads installed - 3rd pic) came from the factory with a solid disc tack-welded halfway along the perforated steel pipe. They were designed by Harley to force 100% of the gasses out through the holes forward of the disc, along the chamber encircling the core, back through the holes aft of the disc, and thence out the exit pipe--which is 1-3/4" like the inlet. No glass or other material along the core, as that is unnecessary and indeed contrary to their proper function.

Gasbox, which makes these repros, informed me they drill the disc as you can see in the 4th pic to allow more flow--and create more noise, unavoidably. But, as I said earlier, the way this style of muffler operates is different from a glass-pack straight-through like Dunstall replicas--or as I've assumed the latter are designed, anyway.

The reasons that Bloodknot (who can correct me if he disagrees) and I would prefer to knock out the perforated disc midway along the perforated pipe are: (i) it impedes the flow more than we desire; and (ii) it likely forces some of the gasses outward into and through the glass encircling the perforated core--which in a glass-pack muffler is ideally intended to absorb sound, not to manage exhaust flow per se. Sure, some gasses occupy this space, but ideally you don't want a large portion of flow moving through it.

In order to fine tune the flow, I would think one would adjust the size of the exist holes; as for the sound, one would install more and better quality insulation.

All that said, if the original Dunstall mufflers had the perforated disc, presumably they just envisioned people having to periodically replace burned out sound-deadening material. That's no huge task with the removable core. But I think it'd be fun to experiment with and optimize the design to achieve just the right loudness and lifetime of the sound-deadening material.

 

[sikatri]

Interesting. Look forward to seeing where this project goes.

 

[NM Bonny]

Interesting. Look forward to seeing where this project goes.

Thanks, I'm curious myself.

One thing I might try initially is inserting a solid disc to sit pressed against the existing perforated one midway along the core pipe in a pair of replicas--and at the same time remove all of the glass wrap (which I understand is cheap and minimal as they come out of the box on these replicas, so would be replaced in any case). The temporary solid disc could be made out of sheet metal with little tabs around its circumference to hold it in place, then just tapped down into the core from the inlet end.

That would transform them into a strictly baffled-type muffler similar in design to the vintage Sportster jobs I have on the Pan-Shovel. I have tried uploading a brief video of how those sound, albeit with a 1200cc V-Twin, and although they are... well, not quiet, neither are they 'anti-social loud' like straight pipes (which I cannot imagine riding with for any distance at all), but no dice. Of course, these bikes are just over half the displacement of the Pan-Shovel, and the cans are longer and more voluminous than the teensy late '60s Sporty cans you see above. So, I'm really curious to see how they sound transformed into a baffled design.

If they were too restrictive and quiet, which seems unlikely, I could either: (i) knock out the existing perforated disc--and with it, the add-on solid disc--and insert a series of temporary solid discs with varying sizes of central holes, similar to the Gasbox method in pic no. 4 above; or (ii) drill out the holes in the core slightly larger. A temp disc probably wouldn't stay in place over a bumpy road, but there's a little paved, smooth section before I hit the awful dirt and gravel spur off the highway that would allow me to experiment.

That, together with experimenting with different-sized exit holes in the end plate, would produce an array of sound and flow combinations.

And of course if none of that results in the right mix, I still have two routes, both of which involve wrapping the core with better material and returning to a traditional 'glass pack' design:

(1) see how things work with the straight-through core and better material combined with the various end caps I'd have made up for the above baffle-only tests;

(2) tack weld the original perforated plate--or a new one of identical dimensions--back into place, and nothing lost to the fun experimentation.

Of course, I'd post a video of each successive iteration for comparison and discussion purposes.

Solomon: can you tell me what file extension you used to upload your brief videos? I have been unable to upload either .mov or mp4s to show what the vintage Sporty cans sound like. Neither is very long, under 2 minutes, so size doesn't seem to be the issue...

 

[solomon]

These are original silencers (71-4159) that came stock on my '78 Bonny. These came stock on many year models. They are often referred to as "cigar" silencers which are awfully quiet, and visually way too long, but have a very nice tone. I believe they are baffled because the tone has never changed.

Here is how I arrived with these "megaphone" mufflers: I cut the tapered end off the stock muffler right behind the weld (because one of them became irreparably damaged) and in doing so I discovered those four exit holes/short tubes that look like they were designed to look that way, and, nothing fell out of or became loose from the muffler as a result of the cut, so I grudgingly did the same to the other one. It was a daring operation. I did this way back in the mid eighties when spare parts were practically none existent and there was no internet to find replacements. The operation caused the mufflers to have a bit more robust sound but not loud at all (unlike peashooters or Dunstalls), yet the tone and performance was slightly improved over stock originals so I have kept them on the bike all these years. I have yet to see any others like them and (in my opinion) they look cool as all get out. I wonder if the import versions are true to the originals in construction because they are much less expensive than English made ones if one were to take a gamble and cut on them. I have since added English-made peashooters for Triumph and another set of stock originals to my collection so I have several nice options for tone and looks.

I've got a pair of near-mint original Meriden cigar silencers. Saving those for a not-quite concours build. I'm going to look out for a battered pair, and try cutting the ends off.
Thanks.

 

[solomon]

Solomon: can you tell me what file extension you used to upload your brief videos

Save them to YouTube, then post the link. Forum then automatically shows the thumbnail.

 

[solomon]

One other thing, NM. When I switched to the Dunstalls, I also switched to non-balanced header pipes.
This probably, don't t truly know because did at the same time, did two things.
Made the exhaust system louder.
And made each 'beat' more distinct. Even at high revs I can now hear the pulsing engine, not just an unearthly scream. I e., noticeably a big twin, not a multicylinder ghoul

 

[Bloodknot]

The original Dunstall had a very distinctive sound on the overrun. I suspect the replicas don’t, looking at the tail end of these pipes.

 

[NM Bonny]

One other thing, NM. When I switched to the Dunstalls, I also switched to non-balanced header pipes.
This probably, don't t truly know because did at the same time, did two things.
Made the exhaust system louder.
And made each 'beat' more distinct. Even at high revs I can now hear the pulsing engine, not just an unearthly scream. I e., noticeably a big twin, not a multicylinder ghoul

That's may well be, as I've read that part of the reason for the balance pipe was to reduce decibel level, since each pulse can flow into two mufflers. Your second point is very interesting, as acoustically each combustion would travel a slightly different path to reach the respective muffler's outlet. So, indeed, the Lemon Drizzle Gang's video of the 650 (actually, it received a 750 kit) with the repacked Dunstall replicas isn't quite the same as your bike, as it has a balance pipe...

 

[NM Bonny]

The original Dunstall had a very distinctive sound on the overrun. I suspect the replicas don’t, looking at the tail end of these pipes.

Any chance you could post a photo of the outlet on the original Dunstalls on your T-Bird? I'm curious to see the difference, as I may make up my own to play with the flow and sound qualities of a pair of replicas.

 

[sikatri]

Any chance you could post a photo of the outlet on the original Dunstalls on your T-Bird? I'm curious to see the difference, as I may make up my own to play with the flow and sound qualities of a pair of replicas.

I had the same thought, would love to see photos of the original and the replica ones. Interesting discussion.

 

[NM Bonny]

I note that the EMGO replicas are listed as 27-1/4" long, whereas the OEM baffle-type mufflers on my '71 are 25-1/2" long. The O.D. of the big end is about 4" before the final radius begins. The baffle chamber on the OEM mufflers measures 20" seam to seam; it's unclear from the exterior where the baffle terminates at the forward end in the EMGOs, as there is no seam to indicate the beginning of the baffle chamber, as with the OEM mufflers.

Just for comparison, the baffle chamber on the vintage reproduction Sporty mufflers pictured earlier measures 9"(!).

Solomon: what is the O.D. of the Dunstall replicas, if you don't mind?

Since I'm proposing to experiment with converting Dunstall replicas to baffle-type mufflers, I have attached a couple of videos of the '51 Pan-Shovel running the Gasbox modified Sporty mufflers. The first is in the garage, so it's an echo chamber and louder than out in the driveway (I had just fired the bike for the first time after rebuilding the top end, so was running through a few heat cycles and bedding in the rings and valve seats I had to recut, due to S&S screwing up the setup in the brand new, 'ready to run' heads - the exhaust guides were reamed too small as well...).

The second is via GoPro strapped to my chest, so the software messes with the sound the moment the wind kicks up, but with these two videos at least you get the idea of the unique sound quality of this rudimentary style of baffled can. (The speedo reads 12% low due to a larger front final drive chain sprocket, BTW.)

Anyway, if the Dunstall replicas were converted to baffle-type using the method discussed earlier, we'd be talking a baffle chamber twice as long and more voluminous than the teensy, cigar-shaped Sporty repros. Plus two more differences: the Pan-Shovel does not have a balance pipe, and its cylinders are nearly twice as large as the Bonny.

Initially, of course, the add-on disc midway down the perforated core would be solid, so as to redirect 100% of the gasses out through the holes, around the disc, back through the holes, and then through the multi-hole outlet. But that would leave three variables to fiddle with: R&Ring the disc with versions featuring progressively larger center hole, like Gasbox does with its repro Sporty mufflers; enlarging the holes in the perforated pipe, which is now actively controlling flow as well as managing sound; and varying the number and size of the holes in the end cap.


View: https://youtu.be/kc8taUX7rMg



View: https://youtu.be/lUk8CQuLf-o

 

[Bloodknot]

I will post photos of the Dunstalls as soon as I can. In the meantime here’s a photo of the Tbird before I stopped riding it. Res is not too special but please bear in mind that this photo is 55 years old.

The last 30mm or so of the outer shell of the silencer reduces in diameter. This is what I know as a reverse cone mega. Looking directly at the outlet you will see this reverse cone, maybe reducing by no more than 25mm in diameter. At the largest diameter there is a circular plate through which the internal perforated tube just protrudes for support. There are none of these fancy tube clusters that the replicas seem to have. You must bear in mind that these were made in an era when exhaust noise levels and the restrictions that go with them were decades in the future.

 

[NM Bonny]

I will post photos of the Dunstalls as soon as I can. In the meantime here’s a photo of the Tbird before I stopped riding it. Res is not too special but please bear in mind that this photo is 55 years old.

View attachment 55876

The last 30mm or so of the outer shell of the silencer reduces in diameter. This is what I know as a reverse cone mega. Looking directly at the outlet you will see this reverse cone, maybe reducing by no more than 25mm in diameter. At the largest diameter there is a circular plate through which the internal perforated tube just protrudes for support. There are none of these fancy tube clusters that the replicas seem to have. You must bear in mind that these were made in an era when exhaust noise levels and the restrictions that go with them were decades in the future.

Ah, so you've got something closer to these?

Dunstall megaton repros

I see that British Cycle Supply sells the above-linked muffler in two inlet sizes, but also sells this 'economy' version:

economy megaton

The differences include adapters for different inlet pipes on the economy version, and also a welded reverse cone, versus a riveted or screwed-on cone on the higher price model, presumably so one can repack them...? Also, the higher-priced one is shorter with a longer reverse cone section (2") and is 26-1/2" overall, whereas the economy version has a shorter cone (1-3/4") and is 28" overall.

 

[Bloodknot]

Ah, so you've got something closer to these?

Dunstall megaton repros

I see that British Cycle Supply sells the above-linked muffler in two inlet sizes, but also sells this 'economy' version:

economy megaton

The differences include adapters for different inlet pipes on the economy version, and also a welded reverse cone, versus a riveted or screwed-on cone on the higher price model, presumably so one can repack them...? Also, the higher-priced one is shorter with a longer reverse cone section (2") and is 26-1/2" overall, whereas the economy version has a shorter cone (1-3/4") and is 28" overall.

The economy one is nearer the mark but still too long and too acute. The reverse cone on the originals is shorter, less aggressive on diameter reduction with no visible join to the main cone. It looks as if it has been formed from, not joined onto the main cone.

This may give you a better idea. This was taken between house moves some 15 years after it was on the road so please excuse the state of it.

 

[Rudie]

Dunstall megaton repros
economy megaton

Just an aside but afaik Paul Dunstall never made anything shaped like those. The top one I knew as a "Daytona megga" (as in short for "megaphone") in my formative biking years, although the rear cones were always cast aluminium alloy, not chromed steel. Came in "long" and "short" lengths, linked are about the right length for "long", "short" were about half that. My memory says most old (not "classic" then) British bikes had one length or the other.

 

[NM Bonny]

The economy one is nearer the mark but still too long and too acute. The reverse cone on the originals is shorter, less aggressive on diameter reduction with no visible join to the main cone. It looks as if it has been formed from, not joined onto the main cone.

This may give you a better idea. This was taken between house moves some 15 years after it was on the road so please excuse the state of it.

View attachment 55879

Nothing to excuse, re: the bike: motorcycles were intended to be ridden IMHO...

Interesting... so, removable baffles on those old cans?

P.S. - the photo from the earlier post, presumably in its current state, is just gorgeous. I like the flat bar and may fit one to this bike (see "Restoring & modifying 1971 OIF T12R" in this forum).

 

[Bloodknot]

Nothing to excuse, re: the bike: motorcycles were intended to be ridden IMHO...

Interesting... so, removable baffles on those old cans?

P.S. - the photo from the earlier post, presumably in its current state, is just gorgeous. I like the flat bar and may fit one to this bike (see "Restoring & modifying 1971 OIF T12R" in this forum).

Removable baffles , never heard of such a thing in those days.