Commando exhaust design ?

Re the claimed 1.15 second improvement with Dunstall 2/1/ 2 exhaust. I tried one of these on Dyno Hill. It slowed the bike down a bit, not too much, about 3kmh slower than standard non balanced exhausts with open (fluted) peashooters.
With any reasonably free flowing exhaust you are playing with a couple of horsepower, a few at most.
The 1.15 second reduction would require and additional 20-25 bhp output, not going to happen on a 50 horse motor.
In fact it lost 2 HP.

Glen

worntorn said:

Re the claimed 1.15 second improvement with Dunstall 2/1/ 2 exhaust. I tried one of these on Dyno Hill. It slowed the bike down a bit, not too much, about 3kmh slower than standard non balanced exhausts with open (fluted) peashooters.
With any reasonably free flowing exhaust you are playing with a couple of horsepower, a few at most.
The 1.15 second reduction would require and additional 20-25 bhp output, not going to happen on a 50 horse motor.
In fact it lost 2 HP.

Glen

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I think of exhaust systems more in terms of resonance than gas flow. If an exhaust system is restrictive, you usually lose a bit off the top of the rev range,. Advancing the exhaust valve opening point makes the exhaust system work harder. On every second stroke, there is a bit of mixture which flows into the top of the exhaust pipe, then gets stuffed back into the combustion chamber. It is the Kadency effect. With a two-stroke, that effect is greatly amplified by the expansion chamber. But it still happens with a four-stroke.
With my bike, my cam is almost standard 850, but it is advanced 12 degrees. It works very well, but the exhaust noise is too loud. Instead of my inlet opening at 53 BTDC, it opens at 65 before TDC. Instead of my exhaust opening just after 90 before BDC, it open at about 92 before BDC.
I only ever worry about the valve opening points. They have the greatest effect on performance.
With any exhaust system, the cam timing and jetting must be optimised to suit it.
Also for me, because I use methanol fuel, engine heat is not such a big problem

acadian said:

I'm more curious about why they kept the damn mainstand attached? Look how close that thing is to the pavement, could be a fine way to get chucked if it catches

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Production Racing, exactly as supplied.
Laverda were not fools, that SFC was built for Endurance Racing and would have been testing the tyres of the day.

Glen

The post by Ken in which he posted a copy if the original Dunstall brochure states “This system only works with genuine Dunstall Silencers and not at all without Silencers as it is computed utilising pressure waves reflected back from decibal silencers.” Genuine decibal silencers have a perforated cylinder and a blank end as well as the final baffle. Pea shooters are more open. So in effect your dyno hill test only confirms what Dunstall states. You would need to do a run with decibal silencers fitted to verify one way or the other.

David.

I didn't use peashooters on the Dunstall setup. It came with long megaphones that we later learned were the correct Dunstall items. A bit of polishing to remove old burned on oil brought up the Dunstall Decibel stamp.
The Dunstall exhaust , while not as effective as stock open peashooters on separate headers, would easily knock 1.15 seconds off if it was replacing an exhaust using stock Black Caps. Maybe that is where the impressive number comes from.


Glen

Loved seeing this string--I remember seeing that 2 into 1 into 2 system back in "the day" and doubting the performance claim, but mostly just thinking "too ugly!"