Dr. Blair on Dyno Hill- Place your bets!

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worntorn

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A friend gave me a Blair/Dunstall 2 into 1 into 2.
He ran it on a Dunstall 810 in the 1980s, then removed it in favour of reinstalling the stock centre stand. This exhaust has been under his bench for 35 years now. He decided he wasn't going to get around to using it again and thought it might be good for my lightweight project bike.
I'm going to temporarily install it on the MK3 to see how it fares on Dyno Hill.
Dunstall literature claims a 1.14 second reduction in 1/4 mile et just by installing this exhaust on a stock Commando.
That seems very hopeful.
If it gets a couple of extra kms per hour at top of Dyno Hill, I'll be happy. It is 3.5 lbs lighter than the open peashooter balanced exhaust I have for the MK3.
Place your bets!

Glen
 
If the part which is the restriction is not smaller in cross-sectional area of the two header pipes, it should perform very well. I'd be very interested in the torque readings when compared with separate pipes. But I often wonder about the torque readings which come from dynos which are based on torque. I wonder about calibration - whether it is absolute or relative.. I read somewhere that some dynos were made to read values by calibrating them with a bike of specified power, by using a fiddle factor. Most times it does not matter because you are only looking for an improvement over subsequent runs.
 
Funny thing, my older brother did a trade back in '77 and brought home his '70 750 "S" that had a beautiful fireflake blue paint job ( gel coat really ) 7 super nice chrome and a dunstall 2-1-2 ugly black painted exhaust that was missing the silencer on one side. It was light though-(I think that he had a muffler shop hand make a copy and it weighed a ton). It didn't stay on his bike long as he wanted the center stand. I believe he replace it with a 2 into one Hooker header that they used to have back then....didn't really sound right though--that however is neither here nor there with regards to your post......I remember he had to adjust the 2-1-2 a whole bunch to get it to fit and stay...the clamps had been replaced were the wrong type. ( he used to cuss at it a bunch)
 
I assume you have the Dunstall silencers along with the pipes? I've got the original set on my 750.
It's not that hard to set up once you've dialled in the method. Removed and replaced mine many times.
Be interested in your results.
 
I spent the afternoon getting the headers out of the collector pipe. One took maybe 30 minutes of struggling to come free, the other was still welded in with rust after 2 hours.
I finally cut the pipe on the bandsaw, peeled out the rusty inner part from the collector, then tig welded the cut off collector bit back on. The side which came out ok was only in the collector by about 3/4". The side that had to be cut was in a full 2" and rusted right in, so cutting was the only way.

When first applying pressure to the end of the header up at the exhaust port end, when trying to get things apart, the welded straight stub and flange came right off!
This was strange as the welds all look ok.
Turns out the weld was almost entirely situated on the curved header pipe. It was just laying there beside the stub with only a tiny bits of weld here and there connecting to the stub and even those bit were only fused a few thou in.
On looking at the stub after it came off, there was virtually no weld pentration into that piece. If this is way Dunstall welded, no wonder the welds broke.

Does not inspire confidence!

Glen
 
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luckyed said:
I assume you have the Dunstall silencers along with the pipes? I've got the original set on my 750.
It's not that hard to set up once you've dialled in the method. Removed and replaced mine many times.
Be interested in your results.
Click to expand...
My friend said these are not Dunstall silencers. He also said they are more open and louder than Dunstalls. These just have a perforated 15/8 tube running through, no cap at end.
I'm also hoping to try a pair of the little Emgo reverse cones on here. If the performance and sound is OK, the Emgos would give a further 5 lb reduction vs the Dunstall look alike silencers.
The exhaust tubing is all Dunstall.

Glen
 
FWIW, you should be prepared to do some Amal main jet/needle position) adjustments for optimum performance with the 2-1-2 over the standard 750-style pipes.
 
This is an 850 that normally runs balance pipes and open peashooters. I expect the 260 mainjets I have in there now will be close but plan to experiment
Glen
 
My Maney style pipe requires leaner needle settings vs 2 into 2 peashooters (no balance pipe). Main jets stay the same.

Very much looking forward to your findings Glen...
 
Fast Eddie said:
My Maney style pipe requires leaner needle settings vs 2 into 2 peashooters (no balance pipe). Main jets stay the same.

Very much looking forward to your findings Glen...
Click to expand...
And my same pipe with Amals instead of Keihins needed 2x richer mains on both sides and a 107 needle jet on the right.

Different motors and heads...
 
I had one of those systems
I never noticed any difference
So I'm interested to see your findings
 
gortnipper said:
And my same pipe with Amals instead of Keihins needed 2x richer mains on both sides and a 107 needle jet on the right.

Different motors and heads...
Click to expand...
Absolutely, these things are FAR from a one size fits all kinda thing.

When I tried my 2:1 with one engine set up it produced very noticeable gains with the arse dyno and the real dyno. Then when I tried the same pipe on the same motor but with changes to cam duration and head porting it made zero difference vs the peashooters.

On the 920 I can’t tell any difference with the arse dyno between the 2:1 and 2:2, but haven‘t tested back to back on the real dyno yet.

Take nothing for granted !
 
I have a 2-1-2 Dunstall type exhaust system built by Viking Exhausts in New Zealand, with the incremental pipe diameter increases built in but with peashooter silencers which are 1 5/8" bore.
Running this on my 920, my "arse dyno" told me less torque and lower rev power, but once the engine hit 4,500 to 5,000 rpm it pulled really well, much better than using 2-2 exhausts, right up to 7,000rpm where it seemed it would wanted to just keep going. I had to make myself change up a gear at this point to preserve the motor, but was tempting to let it keep revving.

So I would guess that there is likely to be +5 bhp up on the standard exhaust configuration.
 
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Reggie said:
I have a 2-1-2 Dunstall type exhaust system built by Viking Exhausts in New Zealand, with the incremental pipe increases built in but with peashooter silencers which are 1 5/8" bore.
Running this on my 920, my "arse dyno" told me less torque and lower rev power, but once the engine hit 4,500 to 5,000 rpm it pulled really well, much better than using 2-2 exhausts, right up to 7,000rpm where it seemed it would wanted to just keep going. I had to make myself change up a gear at this point to preserve the motor, but was tempting to let it keep revving.

So I would guess that there is likely to be +5 bhp up on the standard exhaust configuration.
Click to expand...

Martin
What carbs are/were fitted with this system ?
 
trident sam said:
Martin
What carbs are/were fitted with this system ?
Click to expand...
It was when I had the Amal Concentric Mk2s fitted (which I never really got on with).

Dr. Blair on Dyno Hill- Place your bets!
 
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