Dunstall Ex 2-1-2

Geoff Lather · Dec 1, 2018

Does anyone know if the Dunstall 2-1-2 ex system really did work well or was it all just marketing B.S.?

Thanks Geoff

Bernhard · Dec 1, 2018

Dunstall’s only claim was it produced 4-5 more hp in the mid range.

Also it was lighter than stock.

baz · Dec 1, 2018

I had one fitted to a stock 850 mk2a I can't remember it making any difference to performance
It rusted through very quickly ,first the silencers then the Y join at the front literally disintegrated

Bernhard · Dec 1, 2018

Ha! The rust, I forgot all about the wonderful rust!

Peter R · Dec 1, 2018

If my memory serves me right , this Dunstal system was designed by the team of Dr, Gordon Blair at Queens university Belfast, This guy surely knew what he was doing.

sportsroof · Dec 1, 2018

Had one on my 750 in the late 70's, performance the same as far as I could tell, sounded wrong to me and eventually the centre pipe broke. I found a vacuum cleaner pipe was the same size and had the same bend so I stole that from a vac at work and rode it around till it broke again. Binned it and went back to standard setup. Oh, and you had to lose the centre stand to use that Dunstall system, not good.
Cheers, Martin

baz · Dec 1, 2018

Peter R said:
If my memory serves me right , this Dunstal system was designed by the team of Dr, Gordon Blair at Queens university Belfast, This guy surely knew what he was doing.

Yes it was designed by them, maybe
it needs camming up a bit to work?
My mk2a was completely stock including air filter

ILLF8ED · Dec 1, 2018

I tried the system on a ‘73 750 with hi compression head. It worked pretty much as advertised with maybe a bit more power upper midrange. The build quality is really poor with welds cracking. Went back to stock after a few months and got my center stand back. This was around 1983.

eskasteve · Dec 1, 2018

I put one on my 72 Combat. Once I got the jetting sorted out it worked very well. Before changing the jetting it only performed slightly better than stock. The build quality was very poor with a minimal amount of chrome causing a lot of surface rust within the first few months. It broke twice at the front y pipe then again at the rear y within the first year. Probably about 15,000 miles worth of riding. I missed the center stand also as weighing only about 145 at the time I needed the stand so that I could give that high compression beast all of my leg attention. After the rear Y broke I yarded it off and chucked it into the trash. Good idea with poor execution.

baz · Dec 1, 2018

Bernhard said:
Dunstall’s only claim was it produced 4-5 more hp in the mid range.

Also it was lighter than stock.

Was there ever any actual Dyno tests to back up these claims?
4-5 bhp increase in the mid range is quite a lot

kommando · Dec 1, 2018

If Dr, Gordon Blair at Queens university Belfast says it added 4 to 5 hp that would be good enough for me. He was world famous for his work especially on 2 strokes.

https://daro.qub.ac.uk/pages/2016-rebrand/news/obits---all/obits-professor-gordon-blair

1up3down · Dec 1, 2018

I put one on my 72 Combat with lightened rockers, ported and polished head in an effort to extract even more speeding tickets. It worked! I lost my license three times in two years and spent a lot of time hitchhiking.

eskasteve · Dec 1, 2018

1up3down said:
I put one on my 72 Combat with lightened rockers, ported and polished head in an effort to extract even more speeding tickets. It worked! I lost my license three times in two years and spent a lot of time hitchhiking.

I recently got rid of my BMW S100RR for the same ticket earning issue. Not a good thing to be hitchhiking at 60 years old. None of the "interesting" strangers stop to pick you up or offer you candy. Being older BITES!

Reggie · Dec 1, 2018

In 2011 / 2012, I had a 2-1-2 Dunstall type exhaust system fitted to my Commando. It was made by Viking Exhausts in New Zealand and I had reverse cones fitted instead of Dunstall decibel silencers.

My impression of it was that low to mid-range was down a bit, but when the engine got into the region of 4,500 / 5,000 rpm it pulled really well and felt as if it wanted to continue revving right past 7000. I have no dyno figures to back up or confirm my impressions.

74ok · Dec 1, 2018

from what i read, there can be road clearance issues with the main pipe running under the frame

Reggie · Dec 1, 2018

84ok said:
from what i read, there can be road clearance issues with the main pipe running under the frame

Not with mine. I asked that the down pipes fitted quite tight to the down tubes and that the pipe under the bike sat as close as was sensible. Also when I had a 22t gearbox sprocket or 23t on, the chain would scuff the "Y" pipe a bit, but I realise that the "Y" section on mine sits a bit higher than a standard Dunstall 2-1-2, but that was done at my request. I never had any issues with the pipe hitting anything and have to say that mine never came undone or dropped to bits.

The issues that I had was that the silencers didn't sit at the same height at their ends until I had the rear "Y" section modified and it seemed to me (I may be wrong) that the silencers seemed to be made of reasonably lightweight metal as I managed to put a small ding in one whilst fitting the system when it was new. If I recall correctly, the header pipes are 1 3/8”, and 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 silencers which of course also have 1 5/8" bores.

If you go to page 9 of "Pictures of your Norton Commandos" and scroll down you can see the system I had on my Commando.

74ok · Dec 1, 2018

https://www.accessnorton.com/NortonCommando/pictures-of-your-norton-commandos.5804/page-9#post-64133

Reggie · Dec 1, 2018

Thinking about it, I suppose if there are speed humps aggressive enough, it would be possible to hit the single pipe that runs underneath the bike, but here in the UK, speed humps are usually built at a reasonably sensible height......although what that means is quite subjective I suppose. Also, most of my riding is in the countryside where there are very nearly no speed humps.
Thanks for posting the picture. I do have some other "side on" pictures somewhere that show how high / close the pipes come out below the Z plates.

acotrel · Dec 1, 2018

Peter R said:
If my memory serves me right , this Dunstal system was designed by the team of Dr, Gordon Blair at Queens university Belfast, This guy surely knew what he was doing.

So you believe in experts ? Many university professors are simply lunch-time heroes. If a 2 into 1 exhaust system has any restriction whatsoever past the first collector, you lose top-end. With the Dunstall 2 into 1 into 2 system, it would probably depend in the pipe diameter between the two collectors. I recently saw a similar system on a 900cc Triumph road racer. The owner agreed with what I said about advancing the cam timing to get it to work properly. He had the exhaust valve opening at 94 degrees before BDC.

oldmikew · Dec 1, 2018

Dr Gordon Blair was not an 'expert' but Dr Gordon Blair. He also designed the Q.U.B Seeley engine and was a noted authority on exhaust systems. Had one on mine , yes the design was fine the build quality atrocious .I junked mine . One curio no one has mentioned is that all though it worked well with the 'patent ' decibel silencers , when fitted with the original peashooters there was a noted loss of power...

Dunstall Ex 2-1-2

Similar threads