Dr. Blair on Dyno Hill- Place your bets!

I will try this system on the 920 when it is running. Who knows, it might work better on that engine?
The interesting thing is the reality vs advertising.
The Dunstall catalogue claim of a 1.14 seconds et reduction , or an 11.5 second et by adding this exhaust to a stock Commando, did not materialize for me.
That number is pretty wild as it equates to roughly doubling the horsepower of a standard Commando.
The other claim that was sometimes made is " 5 HP added to the midrange "
That seemed quite possible.
In reality the system made a little less midrange power than a standard non balanced Commando exhaust.
At least it is light in weight!
Because it is thin wall, it's bound to be light in weight, but the thin wall does mean that durability is much reduced.
Oh well, gotta put up with that to trim the weight.
Checked out just how much weight is saved last night.
The stock system is .080" wall whereas the Dunstalls is .060" wall. However, the Dunstall uses more pipe and larger pipe.
In fact, the Dunstall ( pipes only) is 5 oz heavier than stock Commando.

So it's a performance mod that is a bit heavier and makes a bit less power but is far less durable plus requires replacing the excellent Commando centre stand with a crap stand.
In fairness, the Dunstall silencers are probably lighter than stock Commando. I have pattern Dunstall type here and they are lighter than the open pea shooters.
I won't use them though as I have some Emgo 16" reverse megaphones that are about half the weight of the Dunstall pattern silencers.

Glen

"When I was young and inexperienced, I would have raced on the IOM."

Yep, I would have done it in an instant had I been able to finance it in my teens/early 20's...and probably wouldn't have survived!

Like many things most of us did at that age, I look back and wonder, "Who WAS that guy???"

Sadly, I have a similar relationship with mirrors - "Who IS that guy??"

worntorn said:

I will try this system on the 920 when it is running. Who knows, it might work better on that engine?
The interesting thing is the reality vs advertising.
The Dunstall catalogue claim of 1.14 seconds, or an 11.5 second et by adding this exhaust to a stock Commando, did not materialize for me.
That number is pretty wild as it equates to roughly doubling the horsepower of a standard Commando.
The other claim that was sometimes made is " 5 HP added to the midrange "
That seemed quite possible.
In reality the system made a little less midrange power than a standard non balanced Commando exhaust.
At least it is light in weight!
Because it is thin wall, it's bound to be light in weight, but the thin wall does mean that durability is much reduced.
Oh well, gotta put up with that to trim the weight.
Checked out just how much weight is saved last night.
The stock system is .080" wall whereas the Dunstalls is .060" wall. However, the Dunstall uses more pipe and larger pipe.
In fact, the Dunstall ( pipes only) is 5 oz heavier than stock Commando.

So it's a performance mod that is a bit heavier and makes a bit less power but is far less durable plus requires replacing the excellent Commando centre stand with a crap stand.
In fairness, the Dunstall silencers are probably lighter than stock Commando. I have pattern Dunstall type here and they are lighter than the open pea shooters.
I won't use them though as I have some Emgo 16" reverse megaphones that are about half the weight of the Dunstall pattern silencers.

Glen

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As a side note the down pipes on my dunstall system rotted out very quickly
But the silencers held together for many years
I had them on my 860 Ducati and later they went on to my mates BMW r100 up until about ten years ago when the baffle inside one of the silencers broke loose, they were still relatively rust free considering their age

When I first started racing my Triton 500, I tried using the exhausts it came with. They were Triumph racing kit pipes from 1953 and had 4 inch megaphones. It was impossible for me to get anything like a decent lap time on Calder Raceway - the bike was unrideable. I fitted a 2 into 1 pipe and lost a bit off the top of the rev range, then went to work on getting it back by modifying the pipe and changing the cam timing. I got quite a lot more midrange, but the bike was always noisy. I eventually became competitive, but it was not easy.
You can consider developing a bike to be an exercise in pattern recognition. There are about 6 variables which interact. If you change one, you change the balance between those variables. If you expect a performance increase just by bolting a 2 into 1 exhaust onto your bike, you probably expect too much. The bike is more likely to go backwards before it goes forwards.
What you pick up on the hurdy-gurdy, you often lose on the merry-go-round.

Al
Just for your information.
ie Bob Mac specializing on one type of bike to be fast.

First 100mph lap of the IOM
Gilera 500 four cylinder dustbin fairing

Started racing on a BSA Goldie
AJS 7r
250 350 500 Manx Norton
350 500 Gilera Fours
Bianchi twins
Honda 250 350 (285) fours
Unfaired
full dustbin fairing
dolphin fairing
Fast fast rider on any bike

acotrel said:

I was reading about Dave Croxford. He was used to crashing, but when he went to the IOM he was pertified. And I don't blame him. You would need to be prepared to die.

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Not too ‘pertified’ to get a joint first place, nor to start in ten TT races over eleven years.

It was a 500cc though not a 350 - amazing with the road and tyre technology

Triton Thrasher said:

Not too ‘pertified’ to get a joint first place, nor to start in ten TT races over eleven years.

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But coming from someone who admits to being petrified watching a video!

Dave laughed when told about that comment

Thanks Fred
Amended to 500. His first TT win was on the 350.

Triton Thrasher said:

Not too ‘pertified’ to get a joint first place, nor to start in ten TT races over eleven years.

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Apparently the IOM still worried him. And I don't think too much else did. When our guys sailed to the IOM from Australia in the 50s they used to spend their time on the ship, studying maps of the IOM race circuit. One of my friends was quite quick - the first time he went there he did an 84 MPH lap. That was his fastest that year. I think a lot of people who have never raced, don't see anything wrong with the IOM.

FastFred said:

It was a 500cc though not a 350 - amazing with the road and tyre technology

Click to expand...

I was reading yesterday about Croxford riding against Kenny Roberts when Roberts had one of the first Goodyear wets on his bike. Ron Tommbbs had them on the Hendesron Matchless in the sixties. They were the first gum-ball tires we saw. What changed almost immediately was the angle of lean on the bikes - increased considerably. In those days it was almost impossible to lean a Manx. When the guys fitted 18 inch rime to use the better rubber, the footrests came closer to the road.

Seeley920 said:

But coming from someone who admits to being petrified watching a video!

Dave laughed when told about that comment

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Last month's Classic Bike magazine, with the Fastback Commando on the cover.' Dave Corxford at 80.'
If he hadn't been worried, he would have been stupid.
'Safe' is a situation or condition where the risks are minimised to a tolerable level.

Ref Isle of Man,to clear a few misconceptions. I raced in the Manx Grand Prix for 10 years, late 60s to late 70s, first 5 years on a Norton 500 Domi twin. In those days there was early morning practice often with fog on the mountain, ( no helicopter then ),rain or the sun flag sometimes all in the same session! Race starts were in pairs at ten second intervals and it was common to race for miles without seeing another rider, latching on to someone who was a little quicker was good fun and instructive. When I retired from work I went marshalling to put something back into it, the first year you are always placed with experienced marshals. I then did the Incident Mananagement Course, a full days training, includes helmet removal, CPR, scoop stretcher use, helicopter loading, radio use and so on. Once IMC rated it has to be renewed with a refresher course on a regular basis to keep up to date with any changes so don't think you can just put on the orange jacket and stand there looking good.

acotrel said:

Apparently the IOM still worried him. And I don't think too much else did. When our guys sailed to the IOM from Australia in the 50s they used to spend their time on the ship, studying maps of the IOM race circuit. One of my friends was quite quick - the first time he went there he did an 84 MPH lap. That was his fastest that year. I think a lot of people who have never raced, don't see anything wrong with the IOM.

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Al I'd be very surprised if those blokes weren't studying maps of the IOM !!!

Turn right at Ballacraine and keep the sea on your left.

Dogsbody said:

Ref Isle of Man,to clear a few misconceptions. I raced in the Manx Grand Prix for 10 years, late 60s to late 70s, first 5 years on a Norton 500 Domi twin. In those days there was early morning practice often with fog on the mountain, ( no helicopter then ),rain or the sun flag sometimes all in the same session! Race starts were in pairs at ten second intervals and it was common to race for miles without seeing another rider, latching on to someone who was a little quicker was good fun and instructive. When I retired from work I went marshalling to put something back into it, the first year you are always placed with experienced marshals. I then did the Incident Mananagement Course, a full days training, includes helmet removal, CPR, scoop stretcher use, helicopter loading, radio use and so on. Once IMC rated it has to be renewed with a refresher course on a regular basis to keep up to date with any changes so don't think you can just put on the orange jacket and stand there looking good.

Click to expand...

Last year they were desperate for Marshall's. Mine and my mechanics wives signed on and watch a safety video in a tent in the top paddock/grandstand like many others. They ended up at Hillberry and Cronk ny Mona not quite sure doing what but making the numbers up. The Marshall's do a great job for the love of it and I thought the flags were brilliant throughout practise and races. Safetywise it's a different deal to yesteryear as you will know Dogsbody, the slightest sniff of damp and it's off. It's a big responsibilty as Clerk of Course and Gary Johnson has done a fantastic job in the last few years keeping the racing going. Perhaps we should return to the dyno thread though...Apologies

That's alright, I'm all dyno'd out until some new silencers arrive.

Using a hill as a dyno is a good idea. But how do you calibrate it ? If you are only looking for improvement, you might not need to ? Some of the inertia dynos are calibrated relevent to the specified horsepower of the Yamaha Vmax. They use a fiddle-factor to get the dyno to read right.

No calibration necessary, just record speed results after each change.

Glen

worntorn said:

No calibration necessary, just record speed results after each change.

Glen

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Al is right about the calibration
How do we know that you are not eating a load of pork pies between each run???