And quiet flows the duct !

Just found this sentiment ,and did a spot of paraphrasing .

" We are all just a bunch of censored floundering around in the mire of a mediocratic two stroke wasteland------- Bantams ! "


Made me chuckle , Trevor

ps , the new topic title will become apparent in due course , just establishing a " home " ?

Yeah!

It´s what Nick said of something I said and apologised for it at Sept ... Cadwell 2011.

Can´t remember what it was but it is still flowing...

When a Bantam looks at another Bantam it´s like a frog, when it looks at another frog
says, you are beautiful.

A lunatic thinks another lunatic is normal.

Jock says to the girl who asked -- ............

No! I ´d better stop -- this is a repectable site.

Cheers!

JayBee -- John-Boy trying to behave,..!

Courtesy & Property Of Trevor Amos

Hi All ,
Once again thanks are due to the ever dependable Ed for posting up the picture for me !

The pic reveals the true intent of the topic heading , the two profiles are sitting in place on a CAD drawing of the cylinder base/ crankcase mouth
boundry. Two further components are of course required to complete the compound curves and radii , take a look , and if you care to , comment !

Catch you later , Trevor

So is this your air-cooled motor ? or just section´s of the transfer ports ? Are you going transfer "under exhaust" ?
Very interesrting however.. think its the bantam folk that have to push the boundaries now and again.... Regards Mike

well it looks to me as though you are forming a new transfer port area above the exsisting engine case to bottom of barrel area, maybe for multiple transfer ports into a new cylinder sleeve... six transfers maybe, with multiple exhaust ports above... f o s ..??

Mike , Nobby ,
The sections missing from the two rings were installed in the w/c engine in 1994 . The larger forms the transfer duct outer wall and is
bronze welded to the finless barrel , after having end plates welded to each end . The barrel wall was then milled away to blend with
these ducts, bull nosed cutters providing the correct blending corner radii .
The smaller , aluminium ring , fixes to the o/d of the liner after installation ,the radii of the converging passages terminate at the transfer
ports in the bore and the smallest cross section is the port its self . A trickle of Devcon onto the warm barrel then forms a neat radiused
corner blend to finish the streamlining .
The design objective was to enable high speed gas to progress from the crankcase to the combustion chamber in the most turbulent free
way possible and maximise cylinder filling .
It could be that this is the precursor to the ally job . So you see nothing exotic or trick , just doing the fundamentals as efficiently as possible
and that allways makes the power !

More if you would like it anyone , regards Trevor

All looks very good Trevor... So pic outside ring made of steel inner ring alloy - have i got that right?
Also have you high-lighted left side with tippex to show shape better. Does the inner ring come down to
meet piston transfer cutaways at BDC? All my cylinder transfers are sheet steel bent-cut-etc

Cheers Mike

Miike,
All of your assumptions are correct .
The inner ring top curve blends to the liner o/d to make the largest radius flare to an inclined , tangential entry to the bore . This will reduce the tendency for lateral acceleration along the inner wall to disrupt the kinetic flow velocity of the charge sweeping around the outer . It`s a bit like trying to get your car around a 20mph right angle bend at 85mph , it just won`t happen . A straight inner duct wall with a right angle turn to the bore will always suffer flow detachment and create tubulence with internal energies , to the expence of kinetic flow , and the higher the revs climb the greater become the losses . Even at lower rpm the effect can be felt, so a large rad here helps to maintain a wider power band .
The lower edge of the ring is around 4mm from the transfer cutaways with an anlge blending to a radius at the lowest edge where gas swings around from the under piston area .
In a similar fashion , the outer ring curve is followed through both the cylinder wall and liner , the 3mm wall thickness ensures strength and rigidity after the inner cylinder wall is milled away . The big advantage to profiling the ducts in this way is in ensuring that the double curvature , rate of taper and volume are identical on both sides of the barrel , and that can only help in producing the power by eliminating losses and promoting crisp running .

I tried the cut and shut method with sheet steel but the end result was awfull , so i had to think of this better way .

Fantastic! I really missed something in those old days of frigging about with a dentist´s
drill and bits that would snap off in blind holes...
... but I seem to remember not doing very much of it -- I passed that onto some other poor soul.

Well done Trevor -- 10/10...

Cheers!

John,
Thankyou for the accolade , it`s always very gratifying to receive the approval of one`s peers , never had 10/10 before !
I just hope the remainder of the Bantam world , and in particular the" New Build " brigade , derive some benefit from my offerings .

Stay young , Trevor

Excellent! we are all leaning a bit here... This looks like a good way of doing transfers..
I've a dilema now do i cut my (devcon part) transfer walls back to 4mm above piston cutaway
at BDC to improve crankcase port entry? but which will in turn lower primary compression ratio?
Think i'll toss a coin??

Mike

Mike,
Thats not an issue to gamble on , never trade good gas flow potential for the odd cc of volume , you`ll always lose !

Trevor

Good Morning Trevor,

Can any of this process be passed onto a 175 or is this strictly for 125's?


Kind Regards


Eddie

Ed,
All of this topic is applicable to any capacity of engine , after all the gas has no idea it`s flowing in a 50 or a 500 cylinder ! As far as Bantams are concerned the problem is one of installation , particularly in a cast iron barrel on a 175 which is constrained by the limits of the original design . Derek proved that with carefull work with filler , gains can be made but go too far and choking can occur for the higher rev range .
The reason i went to water cooling was not for temperature control exclusivly , but that i couldn`t install the transfer regime i felt was needed , in an air cooled alternative and i would not compromise on that .
Ignore the fact that you are dealing with a Bantam , that can be distracting and " Bantam lore " influenced , it is a high speed , racing two stroke engine , tune it as such , within the constraints of the peculiarities of its heritage .
That is my approach and it seems to work ok for us .

Trevor

Good Evening Trevor,

Thank you for the information, now to take it in and interpret!

Kind Regards


Eddie

Hey Dan , and all,
It might prove useful to compare the transfer duct and transfer port entry on the Dan topic with the Quiet duct topic , judicious use of a spot of Devcon and perhaps removal of the obstructive bridge will , measurably , improve flow and that could mean free horses , no bad thing then , and , at no cost , win win is always good !
There are only two transfer ports to do the job for you so they need to have as smooth and turbulent free path as can be made possible , subsequent scavenging and combustion will improve as a natural consequence . You have some good things going on , congratulations !

Hope that can help a little , Regards , Trevor

Just a small point , and one which dovetails into the ccr debate , flow velocity is largely dependent on the pressure differentual between the duct entry , ie the crankcase and the exit in the cylinder , the port !
I should have mentioned this before , but hey , no ones perfect , so must do better next time .

Trevor

I'll be cutting back my transfer walls then... for Cadwell and see how it goes.
Trevor what is the shape of the transfer ducts in the cases? a nice rounded shape
and keeping the same cross sectional area? Regards Mike

Mike, and all ,
As is customary in my engines every internal step change is rounded and smooth and all radii are made as large as is practical . High gas speeds are always easier to maintain with flowing curves offering minimal resitance with optimum guidance .

The crankcase to transfer duct transition : From crank disc to case / barrel joint face is approx 30mm in height , likewise the crankcase wall to transfer duct outer wall is 30mm - ish , the two positions are connected by a 47mm radius which blends to the duct wall , which its self angles from the vertical by 6* . As i`m sure you know ,
from crankcase and under piston area then to cylinder , gas has to flow verically and pass through at least 180* to complete it`s journey and do it in an infinitesimally short time scale , any interruption to this reduces efficiency . Correct coreography of the whole event offers maximum top end , power band width and general sweet running , and we all want that ! What happens in the cylinder , and how much of this hard won gas remains to be combusted at is another matter altogether altogether !
Simple things Bantams are , so why is it so damn difficult to make them go as they should ?!!

Regards , Trevor

Sure! -- it is difficult to make Bantams go as good as they should, Trevor...
That is the whole point - is it not?
Anyone with a lick of sense can see how the whole package as a racing motorcycle
has been advanced*** -- within a tight formula -- by part-timers... not 'Works´ backed -- not factory sponsored -- hardly even ever dealer supported, has to admire what has been achieved.
I think you lot are bloody marvellous -- and with what I have read on here I wish I
could start all over again. So wonderful to see those who´d retired coming back and
doing well. I´d like to do the same but to do so, I´d have to forge my birth-certificate --


--- AGAIN!

Cheers!

*** like Ned´s fairing work -- bluddy marvellous! Mick Potter´s advice on suspension
and handling and a host more Tom and Brian´s contributions etc...etc ....etc .......

PS -- do not forget that the heavier you are the harder you hit the ground -- it is something to do with the force of gravity (g) -- I think! Actually the force equals
bodyweight when 'g' = 'a' which became `g-force´ ....

Hi Trevor.

I have watched this post from the beginning, waiting to see if anyone else will give any technical input. No one has so it’s time I did.

I have been looking at the insides of engines (4 stroke & 2 stroke) for over 35 years. From the time I first saw a transfer duct to the present day, they have evolved.

Firstly they hugged the cylinder liner & turned sharply in to the transfer port

Secondly they angled away from the crankcases and then turned more gradually into the transfer port.

Thirdly they became tapered (linearly) along the length of the duct.

Fourthly they became fully semi circular.

Fifthly they became fully semi circular with a twist that turns the duct from 90 degrees to the crankshaft into the direction the transfer charge needs to be going into the cylinder.

I have had no input into the design if modern transfer ducts but I believe I understand the dynamics.

The duct that you have posted with the help of Edward doesn’t represent the latest in transfer duct thinking.

There is no doubt in my mind that you have produced the most powerful Bantam to ever grace a race track, but more is possible.

The transfer duct leaves the crankcases at 90 degrees to the crank because that is its easiest exit. The transfer duct twists in order to deliver the incoming gas in the correct direction. The transfer duct reduces in cross section along its length in order to increase the gas speed.

In nature a river will in its meanderings produce a gentle curve. The water on the outside edge moves faster than the inside edge, but all the water arrives at the same place. All the water arrives going in the same direction.

As you say quiet flows the duct, everything that is done in any duct must not cause turbulence in the gas flow.

Be it water or gas the dynamics are the same.

I look forward to yours & others comments.

Keep it rubber side down.

Mick.

My bit of 4 stroke diesel tuning of inlet ports was such to indicate that the turbulence created for use with one shape of combustion chamber -- and its compression ratio -- did not suit another chamber shape at all (with the same compression ratio) -- and, of course, the mode of injection rate of the fuel was a big, important factor in that tuning.

Not miuch use to anyone on here Mick -- whilst the Bantam formula is carburettor and not direct-fuel-injection, I do think it is the way to go -- at some time .. Of course there would then have to be a forced lubrication system which would make things a bit more tricky but I would like to see it done.

Cheers!

Hello Mick,
Thankyou for the nice compliment , and , coming from someone whoose opinions command respect and are based on knowledge and experience i don`t
feel i could argue with you !
Just prior to this post i checked the date on my original drawing of the tranfer duct profiling , and the annotation was for 1993 , 19 years ago , so
considdering that time scale i reccon i did a pretty good job of analysing what would work well . After saying that , i whole heartedly agree that things
have moved on , and i recognise several improvements that can be incorporated in my next effort . The current barrel update was conducted in 2007 , the
power and rpm level improved a little with no noticable reduction in the power band , no mods since then however .
I hope to get the engine back for the coming winter and to incorporate some improvements , but have to be carefull not to sacrifice torque for
meaningless rpm . To this end i always tune for a torquey power delivery and top end will come what it may , Mark always seems to run higher gear
ratios than most others , so i guess it works , and he reports " power all the way " !

Stay loose , regards Trevor

All,
It just goes to show that bigger , is frequently , not always better !

On another two stroke site i found the following , and it has echos for this topic , our guy ups his exhaust timing and then carves a bigger entry to his transfers
and reduces the inner radius of the duct . Instead of improving on his 30.4 bhp at 12000rpm , he gets only 27.5 bhp at 11750rpm !
So be careful with grinding out those holes in that super barrel !

Stay easy and loose , Trevor