Dr. Blair on Dyno Hill- Place your bets!

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I suspect that the HP increase with the 2-1-2 exhaust as detailed back in the day was accompanied by other changes - main jets, adding velocity stacks, change of muffler, etc - stuff like that. I don't believe a HP increase occurred simply by bolting on the 2-1-2 exhaust with everything else left as OEM.
The Dunstall ad claimed " Testing resulted in a 1.14 second reduction in 1/4 mile time, just by installing this exhaust on a stock Commando!"
The ad was from 1970 so it would be referring to the 750, however that claim seems pretty wild given my 850 results.
Really looking forward to the SS pipe test. I have read that these pipes give a bit of a midrange boost. Will soon find out!
Hot weather here for a few days, will wait for cooler conditions. I try to do the runs in mid 60s f temps.

Glen
 
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I'm all in favour of your effort, even though it kind deflates my perception of what i accepted as the facts. right now my facts say i got a busted norton head and have to ride one of the other bikes...:oops:

how about teasing us with a video run?
 
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You have the exhaust system that Dunstall was selling.

Is it really to the spec Dr Blair designed?

Did Dr Blair claim these increases in performance with no other mods, or was it Dunstall’s sales talk (it was).
 

cliffa

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Depends on the type of road and rider.
On really twisty roads ( not twisty by US standards..) it is 10 % bike, 90 % rider.
In the Alps, no matter how much HP you have, or how fast you THINK you are,
the locals will piss on you ..
Yes, I can vouch for that having been pissed on several times by riders on SuperMoto type machines ( KTM, Husqvarna, Husaberg etc.) Now I just try to enjoy the scenery. Unfortunately the scenery often includes bunches of flower at the edge of the road...
 
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"Is it really to the spec that Dr. Blair designed?"

I don't know. If someone has the dimensions from Dr. Blair's writings, I'll check and compare.

Glen
 
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I'm all in favour of your effort, even though it kind deflates my perception of what i accepted as the facts. right now my facts say i got a busted norton head and have to ride one of the other bikes...:oops:

how about teasing us with a video run?
I don't have any proper video equipment. The smartphone works (sort of) but makes a blurry picture along with a lot of wind noise.
There have been complaints.
One YouTuber asked-
" Was this filmed with a potato?"

Glen
 
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Maybe, if the wildfires are out by then. Very smokey up there right now. Some days it registers 20x safe pollution levels !

Glen
 
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Today I fitted the complete original exhaust from an 1973 850 Interstate. My friend says the silencer is of the straight thru type, however the bike runs much quieter than with my open peashooters.
I left the 260 jets in as that is what he used also.
People might recall that the bike , with balanced pipes and open peashooters, pulled from 100 kmh to 110 in top gear on this 18 percent grade. With the balance pipes blocked , it did even better, 113 at top.
The Dunstall 2-12 setup managed 110 kmh best after getting the mainjets right.
This stock Interstate exhaust made the bike feel sluggish. I could feel that immediately, but we know how inaccurate the arse dyno is.
This time arse and hill dynos agreed.
100 kmh at bottom netted 96 kmh at top!
I tried smaller mainjets but it made no difference.
No wonder Commandos lost their performance as the years went on!

Will try the open peashooter silencers on these Insterstate headers next. That should bring the power back.

Glen
 
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I managed to separate the silencers from the headers today.
The silencer is not straight thru as my friend thought.
It has a very restrictive baffle. That explains the very quiet running and power loss.
I've now fitted my open peashooters to his separate Interestate pipes, will see how that compares to my crudely blocked balance pipes, which gave the strongest hill climb to date.

Glen
 
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The pipes and silencers for testing.
I'm getting good at exhaust changes as well as jetting changes. Could be useful in the next life.

 
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It went slower than with the balance pipes. Four runs were made, all netted 108 kmh at top. I then tried jetting up and down, both slowed the bike even more, so the 260s were correct.

The balance pipes with the same silencers netted 110kmh. The balance pipe with cross pipe plugged netted 113 kmh, the very best result found.
The separate Interstate pipes with these open silencers fitted also netted 113.
It seems most folks already have the best combination for midrange power, separate 1&3/8" pipes with open peashooters.
I tried nine other combinations and they all came up short.
I did not have a Maney type exhaust to try, that would be interesting.
It appears there are lots of ways to slow things down with the exhaust configuration but getting more out of a stock bike with a stock pre black cap exhaust is tough to do.
The quiet silencers killed performance the most.

Glen
 
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I don't think there are any of the Maney type close by.
Ken and JSeng are a whole country with locked border away.

The next step is to calculate bhp differences in the various setups.
I think I can do that by adding weight to the bike and using the known figures for crankshaft HP and Torque and weight.
I'll dig out the torque curve for the 850 engine to get the HP at the hill rpm.
Next is to divide the weight of bike and rider by the hp number.
This gives a lb per HP ratio, probably about 15lbs per HP at that rpm .
So adding 15 lb or subtracting 1 bhp should give the same speed reduction at top of hill.



Glen
 
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