Dunstall 2-1-2. Good or bad?

Is anybody aware of, or had any experience of any performance gains/ defecits that occur by putting a 2-1-2 Paul Dunstall type exhaust system as opposed to the standard system?

The reason I ask is because a friend of mine had an 810cc Commando in the mid seventies, that as far as I recall, was converted by the Dunstall factory to 810cc, and had one of these systems on. I would have thought that they would be power enhancing somewhere in the rev range, as it was built to be a fast bike of its time, but when you look at racing Commandos, they all appear to wear 2-2 systems?

So were they a fashion statement, or a performance enhancer? Paul Bryant is now making them, and I quite like the look of them, and am curious.

I think I've found the answer to my own question. I've managed to find my "Norton tuning by Paul Dunstall" notes, and in it he says,

"By far the best system for fast road work is seperate exhausts, joined by a balance pipe close up to the cylinders".......[

Click to expand...

It still begs the question, were they purely for fashion?

Hi Reggie,

I remember seeing road tests at the time which claimed better acceleration/torque from the 2-1-2 system (supposedly designed by Gordon Blair). Most people who race Commando's nowadays use 2-1 systems, although I've got both for mine....the 2-2 gives slightly more top end, and fractionally less mid-range than the 2-1

many years ago I bought one along with a whole lot more of the dunstall stuff. I always felt that my highly calibrated seat of the pants dyno that it had better mid range power and a nice mellow tone.

I had one a stock 850 in the 70's
I agree with Bill.
By my unscientific butt dyno, it lost a little on the bottom but had a stronger mid range with better run out on top.
It took a larger main. That says something.

"By far the best system for fast road work is seperate exhausts, joined by a balance pipe close up to the cylinders".......[

I believe this statement is made before the Blair, 2-1-2 system was available.

Living with the 2-1-2 system on the street is a pain. With all the connections, clamps, the weight of the silencers, the rubber engine bouncing around, it's forever rattling loose.
Be better off having a welded system back to the silencers.
You still have the center pipe scraping speed bumps, losing the center stand.

Dunstall claimed the system was tuneable by varying the length connections. This is true. I was just happy trying to keep the thing tight.

However it does work. Having doubts??
What do all the big time twin racers run? Ducati, Aprilia, RC 51???

Another note. The original Dunstall silencers worked better than the pea shooters on the stock balanced head pipes. They produced a better idle with a more crisp, smoother acceleration. Stock jetting.

However, the pea shooters are very Norton and just downright cool.

HTH

I ran the Dunstall 2 into 1 into 2 system and although I did a lot of other things to the bike all at once it seems to work very well. Paul Dunstall (or at least word on the street in the 70's was) you gained about 6 HP just from the system. Paul Dunstall removed the baffles for racing and ran open megaphones.

The whole theory behind it is when the exhaust valve opens a shock wave similar to an explosion travels down the exhaust pipe followed by the gases at a slower rate. At the 1st restriction (baffles) this shock wave travels back at almost the same intensity followed by some gasses. The pipe needs to be long enough to hold all the spent gasses before it gets back to the exhaust valve when it's still open. A 2 into 1 scavenges this backward flow and actually creates a vacuum in the opposite cylinder exhaust port waiting for that exhaust valve to open. When it does there is no dead air to force out but instead a vacuum to aid in removal of spent gasses. This process leap frogs itself from side to side. An added benefit occurs on the opposite cylinder in the beginning of it's intake stroke, when the exhaust valve is still closing, the dead gasses are still being drawn out while the intake valve is opening which helps air flow into the cylinder from the carb and ensures more fresh fuel/air mix is brought in instead of a bit of the old burned gasses getting sucked back in. The whole engine breathes more efficiently.

The only complaints I've had is if you were pushing the bike off a curb or along uneven surfaces the 4.5" clearance will bottom out. This has never happened on the road though and I got to like not having the pipes along the sides. The other issue is you lose your centre stand although there is a Dunstall one you could get. I'll never switch back on my cafe. When I put saddle bags on for road trips for which I had to lower the muffler angle I tried only one muffler and it worked as good as two mufflers with no loss of performance noticed. The system gets bigger and bigger at every join (the diameter of the pipes increase in size). I've never had issue with vibrating or coming loose and did lots of hi-way miles.

I'm happy to see Viking making these:

http://www.vikingexhaust.com/getPart.asp?id=85

Thanks for all the informative replies.

I had never considered the lack of ground clearance that having the pipe under the engine would cause, or the potential of the joints coming loose, also that the design of the 2-1-2 was after the Dunstall tuning notes were published.

I know that the Dunstall silencers were longer than the stock pea-shooter, any thoughts about whether the stock silencers would be as effective on the 2-1-2 sytem, or nearly as effective as the Dunstalls? Did all of you who ran these systems have stock or Dunstalls on?

I always ran mine with matching dunstall silencers.

Good observation about the tuning manual, it's the 1st place I went to before replying to this thread. I'll try to find some original literature from the day because I'm sure it will explain the virtues better.

I have a long way to go before I fire up my bike or I'd try it with each exhaust system for a fresh comparison.

I only ran it with the Dunstall silencers.
I little write up about the development of the system came with the original set.

From my poor excuse of a memory it talked about the whole package being engineered together. There's a lot more to those silencers than one would imagine.
IIRC there is no packing in them. They silence and smooth out the sonic waves with the steel baffle.
Dr. Blair put a lot of effort into the design. It just wasn't hacked together.
That's how I remember it anyway.

As far as length, the whole system extends past the rear tire.
http://woodgate.org/dunstall/index.html

I like the system and would absolutely put it on the 'correct' bike again.

I have mentioned it before to others that was talking about reproducing the 2-1-2 system. IMHO one correction I would make is to the 3 piece front part. I would change to 2 parts with one head pipe and the center section as one than the other head pipe the second part. my past experience had mine shaking apart at the head pipes with the center section hitting the tarmac. I put a wire hanger from the front frame cross member under the pipe as a catch.

That's funny Bill, I never had probs. with it coming loose. I do remember having to keep the cylinder head rings loose and kind of shaking the whole thing into place as I slowly tightened the whole thing up, eventually getting everything including the head rings nice and tight.

Rennie, did you run a 750 or an 850.
I ran an 850 and had trouble keeping the head pipes tight.
I think I was doubling up on the gasket / washer.
I don't recall if I was using collets or not.
Thinking about it now I would have been better off with the longer 750 exhaust nuts. The shorter nuts may have contributed to the whole thing shaking loose.

Mine was/is a 750.

I have no experience with Dunstalls or any other 2 into 1 into 2 system.

However, I don't think the stock 850 system, with a balance pipe, is a true 2 into 1 into 2 system. The scavenging effect (if I can even call it that) of a balance pipe is going to be completely different and much less significant than that of a true 2 into 1 into 2 system.

I did have a stock 850 system on my 73 850, which I replaced with a 750 style separate pipe system - two "1 into 1"s if you will. Didn't notice a whit of difference in power anywhere. Might have been a subtle difference, but if so it escaped me.

The advantage of the stock 750 set-up is the deeper thread penetration for the lock ring, and the lack of two welded joints that can crack. As I stated above, I cracked two pipes before switching to the straight pipes.

I've got two, practically new, 850 lockrings (63998) and collets (63990) for sale if anybody's interested