Dunstall-type 2 into 1 into 2 exhaust

The Dunstall exhaust on my Norton is 120mm off the road, less with a rider. My exhaust does appear to be tucked up a bit higher than Reggies and the Dunstall advertising photos.
I have just found an old Dunstall catalogue, copyright 1974. A 2 – 1 - 2 exhaust system similar to the Norton system was also marketed for the XS1 and XS2 Yamaha and the 1972 T120 Triumph. The photos of these bikes show the exhaust much tighter under the engine and much neater than with the Norton.
Regarding the Norton system the catalogue states “NORTON POWER STREET EXHAUST SYSTEM part number 1175.
This unique and unusual exhaust system designed in conjunction with Dr Gordon Blair at Belfast University, offers greatly improved performance due entirely to the efficiency achieved by the design. It consists basically of two small diameter pipes coming from the exhaust ports and joining into one larger pipe which eventually splits back into two to join the Dunstall decibel patent silencers.
The first road tests carried out by an independent journalist, proved its incredible efficiency by knocking 1.14 seconds off the standing quarter mile of a Norton Commando just by changing from the stock system to this new Dunstall Power set up. “
Then highlighted
“Blairs exhaust system for the 810 is a marvel. Two short header pipes converge just in front of the forward engine mount, a single larger carries the exhaust below the engine, and splits back into two mufflers right under the transmission. According to Dunstall, the system is worth five mph in the quarter and 5mph on top. It may well Be.But performance improvements aside, the pipes are worth theor weight in gold in terms of exhaust note and solid noise reduction. At last! A high performance exhaust system that honest-to God-muffles”
"Extract from “Cycle” Road Test April 72".
ando

A while back there was a very competitive 900cc 750SF Laverda running in our historic races. It also had those very short header pipes. I have never worried much about tuned lengths, only ever about what would fit well on the bike, mainly because the exhaust operates over a wide range of frequencies. Mine has been all 'suck it and see'. With the two-strokes, expansion chamber design is handled by computer programmes. There is probably some way the 2 into 1 exhaust systems can be improved with a different approach..

Re; “The first road tests carried out by an independent journalist, proved its incredible efficiency by knocking 1.14 seconds off the standing quarter mile of a Norton Commando just by changing from the stock system to this new Dunstall Power set up."

I do remember reading that road test; I also sold a 2-1-2 a long time ago to someone who fitted it to a Norton F/Bed by carrying out I think judicious pipe re-bending , as the pipes were a bit blue!

I had a Dunstall 2 into 2 bought from the man himself when he was in Well Hall road... Did see dyno figures which claimed 5bhp increase mid range and that was probably right. Little on top end but the whole system was badly made . Could never get it seal the method of mounting the silencers was an abomination... threw it in the dustbin .

Fitting a 2 into 1 into 2 system is more likely to reduce top-end and fatten mid-range. At what revs did they claim the 5 BHP increase ?

That is exactly what it did ... i cannot remember the source for the dyno figures , i think it was claimed at around 4,500 but its a long time ago.

pictures-dunstall-exhaust-t8494-15.html



:lol: :lol: :cry:

The Bloke who won the wet ( 2002 ? ) C M R R Pukekohe thing on his ride to work Copmmando , had Dunstall Pipes , with the ' then new ' F1 ' cans ' upswept , like the latest dewcati .
It also had NS ?? 250 homda suspension & a Alloy Cradle , discernable by its steady & constant note & pace , while everyone elses balls had shivled up in the cold & wet . Metaphorically
also .

I bought my commando from the original owner who bought the bike from a Dunstall dealer in London while in the service. The title days the seller was "Paul Dunstall London" and it had the 2-1-2's with the Dunstall center stand and I promptly pulled both off.

The center pipe was bashed and scratched on the bottom and the original Dunstall mufflers were rusted out and the center stand looked like it was made by 9TH grader in shop class. I kept the pipes and center stand in hopes of reworking the pipes (I like the looks) and fabricating a better center stand.

Unless the pipes provide more ground clearance I would not waste the time and money. Can't say much for "Dunstall quality".

oldmikew said:

That is exactly what it did ... i cannot remember the source for the dyno figures , i think it was claimed at around 4,500 but its a long time ago.

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A common mistake that many people make is when they try to go extreme top end with their motor. My short stroke 500cc Triton was originally built by a friend of mine. He had all the theories. The stroke was 63mm, the cams were very long duration with not so much lift. The gearbox was 4 speed close ratio. And the pipes were fitted with 4 inch megaphones. And it was on methanol. When he owned the bike, it almost killed him at Bathurst in the 50s, so he did not race for about 20 years even though he was an excellent rider. When I got the bike, it turned me into an instant dud and I crashed all over the landscape. It was exciting to ride, but who needs that much adrenalin and anxiety ? In later years, I rode it when it had been fitted with a 5 speed box, straight pipes and using petrol as fuel - it was not so bad.
' Torque wins races'.

I cannot remember why, but in the old days I did not much believe in Paul Dunstall. I think it was because his hot-up bits were so tame. But in those days, I was not really aware of how good Norton twins are. They seem to be much better than the old Triumph 650s. I am amazed at how responsive the Commando 850 is to mild tuning and I had a lot of very good Triumphs.

I think the secret is in the head... tricky subject exhaust design.. somewhere on the shelves i have a copy of a book called the scientific design of exhaust
systems... written before the age of computer modelling its basically the log of a professional engineer 's experiments.. there seemed to be no overall magic
formula for calculating pipe length, id or what not.. My ES2 absolutely flies on a model 50 exhaust pipe which is of much narrower bore..

A friend of mine was the top Victorian A-grader on Manxes in the 60s. I was in his workshop one day and noticed the exhaust off the 500 Manx hanging from a rafter. I said 'Gee, that's a skinny exhaust' and he said 'you notice too much'. However looking at that Dunstall exhaust in the photos, I think the skinny centre section would stop it from working at revs over 5000 RPM.
A few years ago I was involved in working on a Manx which Mike Hailwood rode at Winton. I read an article about it recently, and it was claimed that it went faster with a fat exhaust. So a lot might depend on who is telling the story or who is on the bike ?

acotrel said:

So a lot might depend on who is telling the story or who is on the bike ?

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Well yes , smiles-but not entirely. The Dunstall system was designed by Gordon Blair, but I think the silencers were Dunstall stock items that predated it ,so he may not have had a complete free hand. It is significantly longer than the standard exhaust which would affect the revs at which the negative shock wave gets back to the exhaust port.. maybe that is why it did little on top end.

Wasn't Gordon Blair the guy who was at Queen's University Belfast ? I believe in academics up to a point, however many of them only know what they have been told or have read. I think with exhaust systems, use of theory is possible, but I've never seen it expounded to the point that would help development of a 2 into 1 exhaust for a four-stroke. With my own system, I only know from experience what DOES NOT work.
I once bought a book on motorcycling handling from the Society of Automotive Engineers - the upshot of it was 'we don't know what we are talking about'.

Acotrel
it would be to your everlasting benefit to read up all you can find on Prof. Gordon Blair, and then perhaps give us your assesment of where he stands as a person who had a considerable understanding of gas dynamics as applied to the internal combustion engine.
If your numerous posts on a variety of topics are anything to go by, I would without hesitation say that he had more in his little finger than you have in the whole of your body!
As it was originally designed, the Blair 2 x 1 x 2 system for the Commando did all that was claimed for it. Whether it was made correct to design is another matter, likewise the finish of the final product may have dissapointed many purchasers, but in both cases these were down to Dunstall, and not Prof. Blair.
The installation of the exhaust was another bone of contention, and may well have been the deciding factor for many who were initially attracted to the design.

acotrel said:

A friend of mine was the top Victorian A-grader on Manxes in the 60s. I was in his workshop one day and noticed the exhaust off the 500 Manx hanging from a rafter. I said 'Gee, that's a skinny exhaust' and he said 'you notice too much'. However looking at that Dunstall exhaust in the photos, I think the skinny centre section would stop it from working at revs over 5000 RPM.
A few years ago I was involved in working on a Manx which Mike Hailwood rode at Winton. I read an article about it recently, and it was claimed that it went faster with a fat exhaust. So a lot might depend on who is telling the story or who is on the bike ?

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As the proof is in the pudding, or, in this case the speed traps at say, the IOM, Hailwood's single 5600 Manx always seem to go faster than anybody else's through the speed trap, whether that was he came out of the bend faster and was able to get a higher top speed - well, we will never know.