Dr. Blair on Dyno Hill- Place your bets!

I still wonder about how good torque-based dynos are at measuring torque. When I see the graphs, I wonder about the scaling of the vertical axis. You might see a large smooth torque curve which might have a peak with different scaling. What I have got with my bike seems to work. I presume there is a torque peak at about 6000 RPM, but I might be wrong. When I race, my revs always stay between 5,500 RPM and 7,500 RPM ( if I over-rev).

The last thing I want to do is muddy things up by adding a gearing change!

This is just a midrange power testof a " Performance" exhaust, using the same gear ratio every time.
I would love to do a top speed test of the various exhaust combos, but there is no safe/legal way to do that here.
The midrange pull on the hill is useful for roadriders as we often need all available grunt to surge past a line of slower traffic on those uphill passing lanes. You want to get out there and go.
Once you get by them all its back to the fun of going around corners too fast!
Midrange is good to have there too.
Top speed doesn't come into play. That's for a track.



Glen

worntorn said:

Hi Bruce
Yes, I'm not getting anything out of this setup so far. In fact the very best result on that hill has been with the standard balance pipes plugged to simulate separate pipes. That showed 113kmh at the top, every time.
The silencers were open peashooters for that combo.

So it seems most Commando owners are doing alright with their early type standard twin exhausts, non balanced, with free flowing peashooters, provided it's a stock bike.
It would be interesting to test some more exhaust types. If I get the opportunity, I will.

Glen

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What is the difference with the balance pipe connected back up on the stock system?
I only ask as I recently fitted downpipes with the balance pipe on my 750

I get 110kmh with balance pipes working.
I suspect that the balance pipes would help if the more restrictive silencers such as the stock MK2/mk3 black caps or the small hole peashooters were used. I would guess that the speed would be somewhat less than 113 or 110 but better than separate pipes with restrictive silencers.

Glen

worntorn said:

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chaztuna said:

Which manufacturer supplied that chart?

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Looks like a Mikuni chart.

Yes, that chart is in my Mikuni Tuning Guide.
I guess Jim borrowed it from there.

Here's the Amal version. Looks different but the info is much the same.

Bruce MacGregor said:

What made the bike get into the 11 second 1/4 miles was the addition of the Dunstall Big valve head.

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Thanks for that tid bit.
I wonder what cam , displacement, and CR for the 11 sec 1/4 bike?
I am going to use all my commando dunstall equipment on a 71. The origin, all of which was from a different 71. The big valve head origin year is not certain. My dunstall cam, PD04ZC, was pitifully mild and not the 75hp R2 B540 WFC which is very much like the 2S combat.

Tried the 250s and that brought the speed back + 1 kmh. The speed at crest was 110 kmh, identical to the balanced pipes/open peashooters.
240s went in next but that slowed things down a bit to 108. Just for fun, I did a plug chop with the 240s and it was tending lean, whitish rather than the grey we look for with modern fuel.
So that's it, the best run with the Dunstall 2-1-2 was equal to the balanced pipes but not equal to separate pipes, or simulated separate pipes.
A friend noted that my temporary plugs are not the same as two smooth separate pipes. This is true, but the smooth separate pipes could only be better. Or are they?
Maybe someone in our club has an old set I could try.
I would also love to try an original set of black caps. A power drop would be expected, but it would be nice to see how much.
The interesting thing is that I can't feel any difference between the combo that runs at 106 and the one that runs at 113, but the GPS gets the same result with the same combo nearly every time.
In about 20 recent runs I only had on outlier and that was only a 1kmhr variation.

Glen

dynodave said:

Thanks for that tid bit.
I wonder what cam , displacement, and CR for the 11 sec 1/4 bike?
I am going to use all my commando dunstall equipment on a 71. The origin, all of which was from a different 71. The big valve head origin year is not certain. My dunstall cam, PD04ZC, was pitifully mild and not the 75hp R2 B540 WFC which is very much like the 2S combat.

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Hi Dave,
Missed seeing you at the local brit meets lately! Probably canceled due to Cobid.
I think I remember from the Dunstall catalog it was 10:1 in the 810 kit,
I bought all my parts from Boston Cycle, Right after the 750 Hondas were the rage.
I kept adding Dunstall parts on my 71 ,750 Commando to keep up with 1000 / 1100 Kawasaki's.
1 had a relative who toured Europe on a later Dunstall bike direct from Paul.
He hated it, brought it home, and bought a 850 Duck.
His Norton had the big valve head, I swapped it out for my head because his exh. threads were bad . So he could sell it and buy the duc.
I really noticed a big difference with that head after fixing it. also I got the Dunstall twin front brake with it!
Great bike ,had to sell it to make a mortgage payment in the late 80's. ( may be still in southern Mass. )
So, I've done the build a fast Norton thing, But this time around I decided to see what the Drouin superchager could be made to.
Like 15 psi boost on a semi stock 850! Yea!
It very quick and still a work in progress.
Bruce

dynodave said:

Thanks for that tid bit.
I wonder what cam , displacement, and CR for the 11 sec 1/4 bike?
I am going to use all my commando dunstall equipment on a 71. The origin, all of which was from a different 71. The big valve head origin year is not certain. My dunstall cam, PD04ZC, was pitifully mild and not the 75hp R2 B540 WFC which is very much like the 2S combat.

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Re Dunstall cam. I strongly suggest when you build the motor to put a timing disc on a check the cam timings- Dunstall's have a habit of being out on this.

And here’s yet another tuning guide for a Mikuni VM that many of us have likely seen before.



One of the features of all these tuning guides that I find somewhat perplexing is the fact that they all show the needle circuit affecting mixture at throttle openings greater than 75%. Without exception any calculations I’ve done comparing needle annular area to main jet area, all show that the annular needle area exceeds main jet area before 75% throttle opening. Beyond 75% throttle opening the needle system is no longer the metering limit since its annular area exceeds the area of the main jet, thus the main jet is the sole metering device in the system at that stage. Perhaps there are exceptions to this (really large main and really small lean needle system), but I’ve not encountered any in the systems I’ve examined to date.

The 2nd image below shows the annular needle area relative to the main jet area for an Amal MK1 Concentric 932 carb and as noted above, by 75% throttle opening the main jet is in sole control of fuel metering. If the needle were raised to provide an even richer condition the main jet would be the sole fuel control at even smaller throttle openings. The foregoing comments support Glen’s earlier contention that when he wacks the throttle wide open on Dyno Hill, the fuel supply is solely metered by the main jet.

worntorn said:

Tried the 250s and that brought the speed back + 1 kmh. The speed at crest was 110 kmh, identical to the balanced pipes/open peashooters.
240s went in next but that slowed things down a bit to 108. Just for fun, I did a plug chop with the 240s and it was tending lean, whitish rather than the grey we look for with modern fuel.
So that's it, the best run with the Dunstall 2-1-2 was equal to the balanced pipes but not equal to separate pipes, or simulated separate pipes.
A friend noted that my temporary plugs are not the same as two smooth separate pipes. This is true, but the smooth separate pipes could only be better. Or are they?
Maybe someone in our club has an old set I could try.
I would also love to try an original set of black caps. A power drop would be expected, but it would be nice to see how much.
The interesting thing is that I can't feel any difference between the combo that runs at 106 and the one that runs at 113, but the GPS gets the same result with the same combo nearly every time.
In about 20 recent runs I only had on outlier and that was only a 1kmhr variation.

Glen

Click to expand...

Nice report Glen. seems fairly conclusive.

Of course, like most scientific tests, it creates more questions than answers, what about ‘smooth’ unbalanced pipes? Or 1.5” down pipes? Or a Maney style 2:1? Or different silencers? Or different engine tune combinations?

I wonder what Blair’s original scope was? I wonder what cam, valve size, CR, the pipe was designed to compliment the best?

Glen - nice test! I'm surprised that the balanced pipes didn't perform a bit better than the single pipes. I might have to remove my balanced pipes and go back to the singles I used to use!

"go back to the singles I used to use!"

...and a LOT easier to deal with!

At what revs is the run started at? I would guess around 4000?

You might find a bigger difference in the max speed between pipes if you could start the run at lower revs, would it pull from 2000? I think the effect of the balance pipe for instance, will be over by 4000 rpm and therefor have little effect on your result.

worntorn, thanks for sharing the tests.

wz507, about the main jet being the sole fuel control at more than 75% throttle:
My guess: the Mikuni graph shows the influence of the taper continues at wider throttle openings as main jet area and annular area around needle act as say 2 resistances in series that the fuel has to overcome?
So the main jet not being the only control there?

"
"My guess: the Mikuni graph shows the influence of the taper continues at wider throttle openings as main jet area and annular area around needle act as say 2 resistances in series that the fuel has to overcome?
So the main jet not being the only control there?"

I'm confused. I thought the carbs on the "test bike" were Amal Concentrics. Are they Mikuni's? On Amals at WOT the needle has no influence.

I agree that a roll-on from 2000 RPM up to whatever RPM it gets to at the top of the hill would be very interesting! See what happens when you start testing, Glen?

Yes the engine of the bike is stock other than the silencers which are open peashooters. Carbs are twin Amals.

I could do 2000 up but the 2-3000 portion isn't something I use on the road, other than for a half second or so at partial throttle when starting off in 1st gear.
3 to 4000 does get lots of use for slower roads. It's not really an rpm range where great power is called for as I would generally shift down if running at that speed and suddenly needing more power.
It would be easy to test it though.
A third gear pull from about 4500-? would also be interesting. Thats a common occurrence, needing to pass quickly on a hill and shifting down to third for more acceleration.

Glen

Glen apart from your dynohill test what do you think of the dunstall system?
It's 40 years since I had one on my mk2a
And the only thing different was I had bell mouths fitted to mine
From what I remember the system made no difference to my bikes performance
I've always wondered about the dunstall claims
When my system fell apart at the back I fitted a single megaton reverse cone mega to what was left of the mid pipe
Again this made no difference to the performance
It made a nice noise on the overun but that was about all
More recently on my 750 I had a large bore 2 into 1 open mega
This felt like it lost power everywhere despite many jet changes etc

At the moment the Dunstall system is securely on there and sounds great, right from idle up.
That part I like.
There is a significant weight reduction over stock as the tubing & silencers are thin wall. This is also a plus.
If a weld breaks or the thinwall pipe cracks then I'll be putting it under a bench.
I wish it gave equal performance to the standard Commando non balance exhaust.
I did not buy into the 1.14 second reduction in 1/4 mile et as this would put a Commando in the same league as a modern 100 HP bike such as the 1200cc Thruxton R.
An exhaust pipe ain't gonna do that!
It will be interesting to see how it performs on the 920.

Glen