Dunstall-type 2 into 1 into 2 exhaust

When you fit a 2 into 1 in place of two separate pipes, perhaps you need to change the cam timing ? Kadency effects also operate on four-strokes. With a Commando, the trick is to make it pull harder rather than rev higher. But when the tail pipe is too small, that seems to stifle the motor right across the whole rev range. - Most noticeable at the top. Years ago I went through the exercise of cutting back the collector on a 2 into 1 pipe and welding on progressively bigger tail pipes until I regained decent performance. I got decent midrange but lost a bit off the top, which did not matter. If you get there quick enough but run out of steam a bit, towards the ends of the straights, it is usually OK.

When you think about it, the 2 into 1 into 2 Dunstall/Blair system is really a 2 into 1. The two end pipes (silencer side) receive the exact same signal being delivered to the one piece collector in the middle. The bump in the mid range being reported by many above in this thread is similar to what you get from the Steve Maney 2 into 1 though the Steve Maney does not seem to diminish the top end that much, if at all.

With the Dunstall/Blair system you have each 1-3/8" dia. header feeding one 1-3/8" dia. collector which then feeds into two 1-3/8" dia. exhausts. I have seen a few of these systems and from recollection I am stating they are all 1-3/8" dia. so I might be off but for discussion purposes the system boils down to:

You have each header of 1 unit of area feeding a collector of 1 unit of area which then feeds into an exhaust of 2 unit areas.

Most 2 into 1 systems I have seen are comprised of two header pipes which lead into a collector piece of some length and usually larger in diameter than the header pipes which then may transition into a megphone exhaust.

The Dunstall 2-1-2 system was not a constant size throughout. The header pipes are 1 3/8”, but they feed into a 1 ½” collector then 1 ½ “ single pipe which feeds into a 1 5/8” collector and back into two 1 5/8” pipes to the mufflers.
The Viking exhaust system differs from the originals with the angle the header pipes come out of the head. Possibly 5 years ago I had Overlander make a new stainless system for me from my originals. As far as I know Overlander kept a pattern. He is much cheaper than Viking.
http://dropbears.com/overlander/
I cannot comment on the performance of the Dunstall system verses a 2 pipe system as I have never run a 2 pipe system. I have never had the Dunstall system come apart as others have claimed and I can say the original system including mufflers lasted over 40 years, and the chrome on the mufflers was still good (except for where the bike had been dropped)
ando

I concur with ando that the pipe system increases by 1/8" at these junctions as said.
RennieK has previously also stated this as fact prior to my purchase of a 2-1-2 system in 2011.

Fast Eddie, PM sent...........the answer is yes.

Thanks ando for the actual pipe diameters.

So further to my post above, the Dunstall/Blair system is comprised of the following:

• two head pipes at 1.48 Sq inches each
• one collector at 1.77 Sq inch
• one dialation to a megphone of 2.07 Sq inches equivalent (the two post collector pipes and muffler/megaphone act as one).

I ran a Bub two into one on a Combat in a Featherbed racer and again, from my recollection, the head pipes were 1-3/8" diameter which went into a long collector transition to a megaphone. The things worked very well - pulled like a tractor.

Dances with Shrapnel said:

Thanks ando for the actual pipe diameters.

So further to my post above, the Dunstall/Blair system is comprised of the following:

• two head pipes at 1.48 Sq inches each
• one collector at 1.77 Sq inch
• one dialation to a megphone of 2.07 Sq inches equivalent (the two post collector pipes and muffler/megaphone act as one).

I ran a Bub two into one on a Combat in a Featherbed racer and again, from my recollection, the head pipes were 1-3/8" diameter which went into a long collector transition to a megaphone. The things worked very well - pulled like a tractor.

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If I'm on the right track with such things, me thinks the Combat had the right kinda cam to capitalise on the effects of the 2:1 system. And the stock cam has less benefit like-for-like with a 2:1.

Reggie said:

I concur with ando that the pipe system increases by 1/8" at these junctions as said.
RennieK has previously also stated this as fact prior to my purchase of a 2-1-2 system in 2011.

Fast Eddie, PM sent...........the answer is yes.

Click to expand...

Top man, thanks Martin. There's no rush BTW, my next dyno'ing won't be till I've got the 920 motor fitted. I'll PM you closer to the time (ie, next year).

When you have a 2 into 1 pipe on a motor revving at 6000 rpm, the two header pipes each cycle at 3000 cycles per minute and the tail pipe cycles at 6000 cycles per minute. So the theoretically the length of the tail pipe should be half the length of the header pipes. Mine is nothing like that. The tail pipe is about half as long again as the header pipes. I've often wondered what size an expansion chamber would be if we applied the software we used to calculate chambers for two -strokes, to a Commando. With two strokes in the 70s, there was a mod to the rear cones of the expansion chambers which made the motor drop dead at high revs but gave better performance overall . My 2 into 1 goes straight through the top limit of the rev range.

Heck, I guess that I did it all wrong. I put a Dunstall 2-1-2 on my Combat because I liked the way it looked. When the Dunstall silencers rusted out I replaced them with a set of Conti mufflers that I also liked the looks and sound of. I incorrectly assumed that I only needed to please myself. I'm still committing those same type of sins on my 850. Oh well, at least I'm happy.

eskasteve said:

Heck, I guess that I did it all wrong. I put a Dunstall 2-1-2 on my Combat because I liked the way it looked. When the Dunstall silencers rusted out I replaced them with a set of Conti mufflers that I also liked the looks and sound of. I incorrectly assumed that I only needed to please myself. I'm still committing those same type of sins on my 850. Oh well, at least I'm happy.

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Not sure who you think is trying to limit your personal choice here? Certainly not me.

The fact that I think it looks wierd is in no way a criticism of the fact that you like the way it looks.

I asked the OP some questions. He didn't answer, but others have. We are all sharing our personal opinions and prejudices here :wink:

Have fun, please yourself, I try but most of the time what I have falls short of what I would like it to be.

MexicoMike said:

I almost bought a set of these back in '06 but decided I didn't want to get rid of the center stand. Now I'm thinking about it again. The one I almost bought was a NOS off Ebay but I don't see any now. Anybody here know if they are being made anywhere?

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Here's the original post. Mike wants to know where he can buy a 2-1-2 system. The only reason that I posted what I did was to make light of how we all tend to stray from the original queries, me included. As for where Mike can get the system that he's looking for, I too would go with Viking. I have a Viking stock 750 style system on my 850 and the fit and finish are above reproach so I'm sure that their 2-1-2 would be equally dandy. Time to shut off this computer and go for a putt. By all!

THANKS GUYS!!!!

Yep, I'm looking for where to OBTAIN the 2-1-2 Dunstall (or copy), not to debate it. I already know I want a set (and have for years) so I'll converse with Viking.

Reggie said:

In about 2011, I asked Paul Bryant (Viking exhausts) to built a Dunstall 2-1-2, but with a few subtle differences.
I asked for the down pipes to tuck in more closely to the down tubes and for the single pipe underneath to run fairly close to the crossover frame tube. Also I asked for the silencers which were in this case peashooters made by Paul, to run from a higher start point than the standard Dunstall and to more or less follow the standard Commando silencer lines/angles.

The pipes definitely changes the power characteristics a bit.
1. Tick-over was very quiet. (Not a power characteristic...I know).
2. At low revs there was a bit less torque.
3. At (IIRC) about 3,500 / 4,000 rpm the exhaust note changed and the bike pulled noticeably much stronger from there to 7,000 rpm and felt as if it wanted to keep revving which had not previously been the case, but I had to show self restraint and change gear.

As I said, this is using Peashooters which were 1"5/8 diameter, the exhaust tubing enlarging 1/8" at each of the Y junctions, and starting from 1" 3/8 on the down pipes.

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Martin,
What was the reason you stopped using the 2-1-2 exhaust system ?
sam

Trident Sam wrote;

Martin,
What was the reason you stopped using the 2-1-2 exhaust system ?
sam

Click to expand...

There are a few reasons why I took the 2-1-2 off the bike, but main reason was because I was going to tour ( and do most years) and wanted the centre stand fitted. I don't like using the side stand very often when the bike has panniers and a tank bag fitted as I think the extra weight may possibly strain the frame, although it may just be me being cautious.
The other reason is because I upped the gearbox sprocket from 21 to 23 teeth for touring, and as I had asked Paul Bryant to raise the height of the final Y piece compared to the standard Dunstall system so that the silencers were a lot nearer the original angle, this caused the chain with this sized sprocket fitted occasionally to rub on the Y section where the chain crosses it. Paul B probably used a 21/22t sprocket when he was designing the system?

I have attempted to post pictures for MexicoMike to look at as I think he said he was going to "converse with Viking" and that's who made mine.

Fast Eddie wrote;

There's no rush BTW, my next dyno'ing won't be till I've got the 920 motor fitted. I'll PM you closer to the time (ie, next year).

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Ok, I'll wait to hear from you Nigel. It'll be interesting to see the results.

Thanks for posting the pics, Sam! Now I am once again worried about ground clearance...which I have been all along but somewhat suppressed that concern recently. But looking at the pics and knowing the numerous speed bumps (Topes) that are part of every small Mexican town - and sometimes out in the seemingly middle of nowhere - the 2-1-2 definitely looks like a crushed exhaust system waiting to happen.

Possibly the centre pipe appears lower than it actually is in the pictures....that grass is quite long :lol: :lol: . I personally haven't had any problems with the centre pipe hitting speed bumps possibly as most speed bumps around here are reasonably gentle or are a centre island with two outer islands so I ride through the gap, but I don't live in Mexico and don't know how aggressive Mexican speed bumps are. Even so, don't forget that the front wheel will have risen/ may be still coming down from the ramp, or the rear wheel may have started to ascend the bump which increases ground clearance.
I specifically asked for the centre pipe to run more closely to the underside of the bike when compared to a "standard" looking 2-1-2 made by Paul Dunstall when I requested the build for mine. Because of this design request, now that I've gone up to a 23T gear/box sprocket the chain will just catch the left side of the rear "Y" piece which is a bit of a ball ache.

p.s. I'm Reggie not Sam, but I don't mind

Sorry Reggie!

The Mexico Topes are generally not like US speed bumps in most places. They are commonly rather abrupt - almost like if you placed a piece of pipe on the road as opposed to a somewhat gradual "ramp." They are less than a foot wide and 5-6 (or more) inches tall. Some small towns will have 6-8 of them in less than 1/4 mile of road. Or there can be a single Topes on the main road approaching a point where a small dirt road joins the main road. My Norton will often scrape slightly unless the tires are inflated a bit above the factory spec pressure.

FWIW, when I say "main road," above, I'm talking about a numbered road which can be anything from a four lane to a narrow two lane. The "Cuota" is the equivalent of the Interstate, which is pretty much the same thing as the US - nobody enjoys riding on it unless the goal is to get somewhere as fast as possible. Mexican speed limits are in KM but everyone treats them like MPH. IOW, a typical Cuota speed limit might be posted at 110KMH but any vehicle capable of it will be doing 100+ MPH. Of course, they are passing the pickup truck going 35MPH with four kids in the bed as well as goods stacked up 6 feet above the cab. And you frequently see 4 people on a 150cc motorcycle - dad driving, mom behind him holding a kid and , a kid on the tank; no helmets on anybody! That being said, Mexican drivers are VERY aware of motorcycles and the typical US accident where a car turns in front of a bike is virtually unheard of. In the 10+ years we've been here, I know of ONE car/bike crash.

Sorry...I've badly hijacked my own thread!

Not hijacking at all and completely relevant. Did a fair amount of touring in the mountains around Lake Chapala, south of Guadalajara, MX and the topes are as you describe them; large, abrupt and periodically unpredictable.
Based on the pictures of the Commando above I doubt the exhaust would have a chance with the topes.

I have a question. A couple of recent posts mentioned different gearing to be used with the improved (?) exhaust systems. How do you know if your bike is pulling harder, if you don't raise the overall gearing and find yourself going faster ? What I have found is that with the heavy crank, the motor always tends to spin up at the same rate and is not as much affected by the gearing, as it is in bikes which have a light crank. When I race, I tend to keep the revs between 5,500 and 7000 RPM and race-change the close ratios. So I don't depend so much on the motor spinning-up quickly in response to the throttle.