Time,timing,time area ?

Once the ex. pulse is underway, and it takes a while to be fully formed from the initial port opening, the hot pressure wave propagates down the header pipe, initially at sonic speed , with heat loss and expansion the pressure in the cylinder soon deteriorates to almost atmospheric as the wave reaches the first part of the diffuser. The wave continues its journey, reducing pressure in the header as the trans. ports open. This negative wave needs continuing gradient increases to maintain energy but duration diminishes as energy is used up. Hopefully this negative wave can extend all the way back through the cylinder ports and to the carburettor to maintain useful air flow. The wave then reaches the end of the diffuser and propagates towards the rear cone without further wave reflections. All gas contained within the header now remains after the trans. ports have closed, the rear cone now sends a large reflection back toward the cylinder. This reflection is of reduced duration after having dissipated so much in getting this far, the wave passes the parallel section of the pipe and hits the diffuser in the reverse direction. The diff. then acts as a baffle and reflects a small secondary wave, but the main wave makes its way onward to the ex. port and just before the port slams shut, the waiting header gas is forced back into the cylinder to augment combustion . The remaining wave energy continues to reflect back and forth and eventually disappears out of the tail pipe and the whole sequence is repeated for each rev of the crank, so don`t shut the throttle or this will all go to hell in a hand cart !
The pressure wave picture clearly shows the trans. port reverse flow with a spike at the A port, the first to open, and the higher the port goes the more the cylinder pressure impacts upon the flow. If the port is open when residual cylinder pressure exceeds scavenging pressure, flow cannot take place and will not do so until the ratio of trans. to cylinder pressure reduces to at least ONE. The real downside to this is that the trans ports offer another potential escape route, in addition to the ex. port, and spent gas enters the transfer ducts, this will reduce transfer time/area before fresh, meaningful, gas flow can take place . Subsequent combustion will have to commence with a contaminated charge and power will be compromised, and will do so at higher initial temperature, extreme cases could promote deto which will then be combatted with a rich mixture and/or lower comp. ratio, in both cases reducing power.
As far as gas flow is concerned, the number of degrees the port is open is secondary to the actual, real time, the port is effectively open, reverse flow from excessive trans. timing reduces this time. As engine revs reach a peak above the blowdown available, the transfer time/area suffers further and power drops steeply . This may seems counter intuitive but it happens in reality and the sim image reveals the phenomenon quite clearly .
There are a lot of other additional events going on whilst all of this is happening, some of which have been touched on in earlier posts, but it all goes to illustrate that 2t gas dynamics are not simple, it only seems that way, and as Jan Thiel once remarked, get the Aprilia outside the rev band, both below and above, and the engine just stops working , not a lot of good for a Bantam engine !

Trevor

Thanks Trevor!
I regret having never got into 2 stroke tuning and as I have said many times I left it to Derek Neil who was provided with info from his brother Colin (who raced a Bantam)  who worked for Dr Joe Erhlic on the famous EMC ...

Like I said...  I could only afford time to race and sometimes not then by reason of Simms wanting me over the weekend.

I often wondered about extra cooling of the transfers ... With some of our turbocharged  diesel engines we had an  intercooler cooling the extra-pressured air charge and wondered if something similar could be done on a Bantam. I guess the W/C Bantams have this cooling  anyway -- adequate for requirements like?...  

Yes, a thank-you to Edward for providing a good service. I hope that one of these days I´ll get the hang of sending pics but at the moment I am in trouble with `lodgers´ on my e-Mail software and my PC has to go the dealer to get sorted. It was my own fault, of course, but I had what appeared to be genuine e-mails from known friends which when opened turned out to be cyber-business offers. Of course, I should have deleted but like a dumb-dumb I pressed the wrong key.

If you don´t know this already there´s `Fanbox´who does this -- apparently they are not breaking the law but it has given me a lot of aggro and already cost me in having it partially sorted so I could actually send an e-mail. ***Fanbox keep sending me advice messages saying I am in credit by about $650 and want me to co-operate but my computer dealer/expert warns against it....

Until next time, Cheers!

PS -- Who did all the winning at cadwell?
***I wonder if any others posting on here have been hit with this problem.

Life continually presents us with tricky concepts, those niggley things we feel we should know but can`t quite get a handle on. Angle/area , specific time/area are just such cases in point , a bit of geometry a bit of maths and at the end what does it really mean ?
Time, astronomers use a base value of light years, with Bantams, milliseconds for internal events, it`s all a question of scale.

When assessing barrel porting and their suitable dimensions , the first requirement is to calculate angle/area, and this has no time element, it is simply the integral of the area of the port, or ports, multiplied by the number of degrees of crank rotation from port opening to closing.

Time/area involves rpm, the time element and the real reason you have a rev counter fitted, this is calculated by taking your angle/area and dividing it by rpm. Next, by using the time/area number and dividing that by the cubic capacity of the cylinder in question ( don`t forget to include the combustion chamber volume ! ) we arrive at STA, or specific time area ! Because of the relationship between port area and time it`s easy to see why gas flow is not critically dependant upon how many degrees that the port is open for , but on the real time that it has been open for, double the rpm and you halve the time the port is open, another reason to avoid meaningless revs !

Exhaust blowdown measurement is vitally important and is the portion of the port that is open at the instant that the first, or all , of the transfers open, and is arrived at in a similar way, and it is worth repeating that too much or too little blowdown seriously alters power production and delivery.

The synergistic balancing of all of these calculations is at the heart of producing a powerful and well mannered Bantam engine, the quest for a big dyno number is not where its at, power range is far more important for quick lap times , and notice I said range and not band, a different concept ?

More suitable power can always be found with a calculator, when all else fails !

Catch you all later , Trevor

Time was ...

... oh heck! Soppy nostalgia again...

I used to go to Brands and watch the revs climb to 8000, change up and repeat the procedure -- or not -- depending on what the `new´pipe was doing. I remember one pipe being hopelessly wrong with me making several changes of sprockets until realisation hit that Icarus-1 was always going to `burble-burble-brrp! -- burble-. burble´ with the wrong pipe....

If things looked better than last time I´d be slipping the clutch like mad and getting it to max revs, another 400 of them, and mentally noting which part of the circuit it was that max revs came with the clutch fully engaged. Then keep on doing it lap after lap whilst memorising where it was happening and how long it was taking to happen for my notes later....

All very subjective -- no maths,no dyno testing -- just the straight gut feel each time with me making notes of gear changes and max revs (fully clutched) at which part of the circuit. This gave a good comparison of whatever was new with what had gone before.

Then by the time Andy lent me his 250 ABS I was quite good at this test-recording and we managed in a relatively short time to get the ABS as quick as a Greeves Silverstone without any dyno testing.

Really crude but we did´t have the facilities and I did know I was racing when --for instance -- I was alongside John Senior on his Greeves Silverstone ....

I should delete this -- sickly nostalgia again,

Cheers!

Message for Mike Powell only.

2 issues
1) reference our discussion on your current bikes and reed induction you never responded to.
ln the very well presented Video you have posted on tube and face book, I have to say is excellent, If I were to criticise all I would say is it was a shame, it did not show more or other Bantams racing that day, in the paddock, it seems to show just two. ! it was also good to see both bikes back in their original fairings. I have to say they look so much better than those beautifully made abortions you had changed to, my compliments to you for junking them. Anyway to my second point, both bikes In the video I assume are your current ones, but both are piston ported, your sketches and comments are about reed induction barrels !.

2) The deformation post "alter ego", remove it please.

mjpowell wrote:

Derek things I definitely know about my 'current' championship winning bike
Is it uses the barrel I took to the first midland meeting and you had a good
Look at it and posted some pic's on line and on facebook under your female
alter-ego Bsa Bantam Racer. This bike 001 is a little faster than my reed valve
barrel 0.3 sec at Cadwell and my spare bike 003 is a second slower round Mallory.
Hope that sorts your memory out.
Mike

Derek

Trevor, im sure ive read somewhere ( probably jennings 2 stroke book )that the profile/shape of the transfer port tunnel is only important for the  low to mid range power characteristics of an engine and at higher revs it becomes less important, is this still the  current thinking? if you could redesign  the shape/ profile of a cast barrels transfer tunnels what kind of shape would you consider to be a better?( only allowed two transfers) You have already mentioned that a good bantam engine would be one that has a broader power spread rather than high hp so this transformation of the fuel from the crankcase/ up through the transfer tunnels to the port window, to effect this, must be vitally important.Also would the volume of the transfer tunnel have any influence in trying to achieve a better spread of power. thanks, nigel

Hi all
I too would really like to see what Trevor advises on this, I spent hours on the Dyno doing experiments with the inner profile of the transfers, with advice from Trevor, and certainly got some big jumps in lower end power -increasing the spread of power but more importantly the torque curve.

reducing the volume of the duct always seemed to be at the expense of max power, !! not sure this is an accurate real world stance and it may be to do with the confines of my D1 cylinder, that is a lot smaller than the capabilities using a D10/D14 cylinder and liner it, or even the alloy BTW barrels (if you can afford them!) that have huge possibilities, I think I achieved a compromise with my new Pipe, that was so much better overall the last meeting I raced it, we eventually removed all bar tiny pieces of Plastic padding over painted with araldite, from the ducts inner faces ending up with an almost straight wall, after a largish radii at the entry to the duct inner side just like a dolphin nose ! putting it back in all the way up will give a huge jump in torque, but kill top end (this is only on a cast iron cylinder.

looking at the cast iron barrels by far the best transfers I'm sure! are the D14 in std trim, but Im sure a lot of the barrel vary considerably, getting hold of a with careful work they can indeed be improved. finishing of what I found with more of a results orientated stance was that transfer volume affected power more than the time they opened, but with the biggest volume it was difficult with the constraints of the cast cylinder to get any king of reasonable profile.

regards Derek

Nigel, Derek,
Over the past 30yrs or so 2t power from race engines has increased by around 50% but rpm levels by only 15%, I have an original Yam TD2B barrel and the inlet and exhaust port would not look too out of place in a modern engine . However the transfers are radically inferior compared to 2013`s best offerings . Jennings analysis is as outdated as the Yam. barrel, to suggest that a sweeping turbulent free duct can be equalled by a parallel, flat sided alternative is not realistic , and represents outmoded thinking. After saying that we have the advantage of knowing what we do now, he did not, so the best I can say , and this applies to a lot of what Bell has written as well, is that these books are ok for back ground reading, but go else where for the real deal ! I recon there is more forward thinking stuff here on the Bantam forum than in almost all " tuning books" on 2t engines from the recent past !
The outer duct wall radius needs to sweep in a continuing curve, but the inner wall in very important, and the radius should be as large as the physical restraints of the original design will allow. A large rad will discourage flow detachment thus greatly reducing pressure absorbing turbulence and the Coanda effect encourages flow to cling to the duct inner wall , thereby meeting less resistance and being guided to where it is required to go, and transport a greater mass of mixture into the bargain , a real win win. It is the inner wall that also determines the outer curve as there should be a constant reduction in the duct cross section from the crankcase right the way to the port opening. This is all well and good but we have to deal with a Bantam barrel , and Derek is correct to say that the D14 is the best cast iron option, with the BTW, top of the heap, and you pays your money and takes your pick here ! The Aprilia , as a 125, produces more power than the TD2B did as a 250 twin, and the 50cc Kreidler, produced more power than almost all race Bantams , so we still have a long way to go yet and it is the transfer/ scavenging phase that should receive the most development. I would suggest finding out just how far it is possible to re-profile the outer wall in a cast iron barrel leaving a minimum wall thickness and then , as Derek has successfully done, build up the inner wall to promote a more flowing form . Prior to attempting this, determine the area of the port you intend to use and try to ensure that both are the same and that the duct entries have the same areas. Always leave a radius at the four corners of the duct, to aid flow, tight corners create turbulence and kill flow at this local level, and never leave sharp inner edges to trip up flow, eliminating them always pays off .
Duct volumes have been reducing as a result of recent test results, and it stands to reason that if a large volume of fresh charge is sitting in a duct it will take a lot of energy and time to get it moving into the cylinder. Bantams with their modest power and rpm levels are going to struggle to achieve a meaningful flow rate if the duct volume is excessive , optimising the mass to be moved with the pressure difference available, at any particular rev point is tricky to arrive at when having only 3 gears to play with . And it is very well worth remembering that whilst the port is fully open for a few degrees of crank rotation, the duct is open all of the time. The Honda RS producing 40hp has quite modest duct volumes , so a Bantam at just a half of the figure , won`t have need of excessive ones to be able to perform well within it`s own class .

Although a different topic, it`s also of benefit to point out the woolly thinking on exhaust systems from that era. Whilst the tuned length is based on temperature and port timing, and arrived at within a mathematical formula with a one size fits all constant of 1700ft/sec , the header length is an arbitrary function of ex.port diameter , with no insight as to what that diameter should be ? Examining bell we find almost identical theory with his solution for ex. pipe dimensioning ! Their stuff, by and large, makes for interesting reading but care must be exercised to avoid costly dead ends .

I`ll take some more pics of the Honda barrel to show the trans. ducts and ask Ed if he wouldn`t mind posting them on here for us .

Trevor

Thanks Trevor, good reading.I have been trying to find my silicon castings i made of both my bantam barrel and rotax barrel transfer ports, but they have disappeared. i also deleted my aprilia files which showed the various cylinder head measurements, although i thought maybe those sort of radi ect would not be relevent in terms of the cast iron barrel anyway. looking forward to seeing the honda barrel pics.
I would like to take this opportunity in thanking you greatly for all the time and effort in these posting as i know we all have other important things that need servicing first. The thing i find strange again is the amount of viewings of these posts with only a few people contibuting there ideas or experiances. Still, thanks again.

Nigel,
Back in the 60s George Todd was a one of a number of personalities answering questions on a forum in the " Motor Cycle " weekly paper.
Lots of us showered questions on poor George and he was by far the most popular forum member , enthusiasm for all matters Bantam was enormous and we all gained top class information not available anywhere else . The contrast between then and now couldn`t be more starkly reflected than in the absence of contributions, questions and experiences here on this forum, it certainly puzzles me ?
Shame about the mouldings you made of those trans. ducts, they were quite revealing and put the poor Bantam firmly in the category of commuter bike, have another search I`d love to see them again .
Thanks for the nice words, I have to say that it is something I enjoy doing and if just one reader benefits it becomes worthwhile

Cheers, Trevor

make that one more Trevor, I have certainly gained a great deal from your contributions, both on and off this Forum, and at the midlands meetings, I certainly know My bike is more powerful, produces more torque, and I better understand, what was always missing, anyway I have some really nice photo's for you to look at and enjoy, hopefully Ed will be along and post them soon, or please look on my face book page, as they are already loaded on there.

see if you can spot/name the Bantam racers from years gone by, could you believe there were more than Five former Bantam racers in the paddock at Silverstone BSB.

best regards Derek

Courtesy & Property Of Trevor Amos.

Trevor Amos wrote:

, power range  and notice I said range and not band, a different concept ?

More suitable power can always be found with a calculator, when all else fails !

Catch you all later ,     Trevor
 

hi Trevor

Could you explain more about this point range and Band !  please, as always believed they were one or the same thing !.


also how about a timing and area calculation based on a current engine, so we can actually understand/see what you are explaining.


rgds Derek

Nigel,
The pics are , quite obviously, various trans. duct mouldings taken using Vinamold, the small one is the RS125 cylinder , the others are various TZ125 arrangements . This is the sort of thing that high performance passages should look like , the triple Yam setup is perhaps a complication too far however , but make interesting viewing . The photo`s are still in the camera, but will get them up when I can, same with you Derek, been one of those days but I will get there !

Cheers Trevor

Courtesy & Property Of Trevor Amos.

Thank you Ed,

The first picture is of a barrel from the Honda 500cc GP engine, with the head still in place, and the power valve mechanism can just be made out, what is surprising is how similar the standard, over the counter porting, is to the works one. For anyone interested , the diameter across the cylinder base/ transfer duct interface on the std. barrel is 103mm, so using the bore dia. of 54mm a reasonable job of scaling the picture could be achieved to determine the other pertinent dimensions . Viewing these images, together with the Vinamold shapes will provide for an overall impression of good practice . Making a reasonable job of replicating one of more of these ports/ducts in the Bantam barrel can`t be too bad an idea , for you know they will be efficient !

Cheers, Trevor

Evening Trevor, look what i found under the staircase....

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sorry the bantam cast is a bit dirty.
so Trevor,
it looks like the outer wall and roof angle of the bantam transfer port could possibly be adjusted to follow that of the rotax barrel,the inner profile with the addition of may be some kind of epoxy resin could again be adjusted to follow that of the rotax barrel. The thing is that the inner profile would may be make the transfer port area smaller and that seems difficult to accept that this would be an improvement as you would think that the bigger the better.But obviously this is not the case as the new barrel profiles show. I can see from your barrel pictures that the large radius on the bottom of the barrels on the entry into the transfer tunnels of the modern barrels would probably take up alot of the bantam transfer tunnel entry if you tried to adjust to suit.... So im assuming that in standard form the bantam transfer port flow must be pretty inefficient in comparison to the point that by even making them smaller but flowing better is whats required.

Looks like I wasn't too far off the mark

Glad you persevered Nigel ,
Crikey that Bantam impression reveals just how poor the standard duct really is, when compared to the Rotax , ok for trundling to work on a commuter D7 and 6500rpm but not a lot of good when asking for decent power at 10,000rpm . Interestingly, the B port in the Aprilia barrel has a inlet/outlet ration of less than 1:1, which suggests that unhindered flow is far more important than just area alone, and the correct profile promotes and maximises the best flow . It is pretty easy to keep a race engine with six speeds constantly in the best power range but doing the same with only three needs a different approach and good power at lower rpm becomes crucial . Big ducts with slow flow rates at low down revs, that contain a large mass of inert mixture prior to opening, become a handicap so perhaps you won`t be too disadvantaged by being restricted to the old iron barrel set up that will peak at around the 9000rpm rate . The Honda ducts are helping to produce over 40hp at 12500rpm, so the modest 20+ for a Bantam will not need that geometry to get the best from it .

As you say Dan, not too far out, but looking at you inserts it might appear that the outlet c.s.a is greater than that of the inlet, which is not so good , but perhaps this is just an impression created by camera perspective at obscure angles ?

Catch you all later, Trevor

your right trevor , i should have added that the photos are of a scrap one that i made to see if i could make it! then i changed the sizes etc on the real inserts, i dont think i took a picture of the finished job.

Evening one and all, are the Jennings port time area requirements /calculating formulas still valid? Or is there more modern formulas to be used?

Evening one and all ,
                             Nigel , Jennings took his ideas on STAs, and a lot of other stuff, from the 1971 SAE paper published by Naito and Nomura of Yamaha, he distilled what they offered and came up with, in 1973, what has remained with us ever since . The trouble is he examined the available race engines and retro fitted stats. to suit his ideas , that was fine for then . Strategically , he ignored bowdown  t/a and concentrated on timings , to be fair trans. STA was pretty reasonable but has extended for todays analysis . A 2t race engine of today would not compete with one from then, if all of Jennings statements were followed .
Formulas for determining port areas and timings are simply maths and geometry so remain absolute and timeless . I`m not denigrating Jennings but it`s like comparing Concord with a Tiger Moth, both fly, but the comparable technology stops there, leave each to it`s own era .

The unique and specific needs of a Bantam , both 125 and 175, require very careful application and blending of all engine parameters to develop good performance . No point in taking the pipe from a RS125 and expecting 40hp, it won`t happen, but a formula could suggest other wise !

Cheers Trevor

As usual Trevor, so profound! That last sentence, I mean -- especially when reading some of the Elitist´s Papers..

Go well and keep well, with more power to your elbow....

Cheers!

JayBee

Courtesy & Property Of Trevor Amos.

Interesting graphs Trevor! 10% improvement on the 12 to 14 shape --´at 5500 rpm though... ?? and I make that a difference of 82.7lbs/in²(14) to 75.4lbs/in² bmep ...
 The difference in port shape definitely explains why we only had about 12horsepower at 8,400rpm  -- wind behind,  of course, and I wonder what we had in the way of torque because Icarus-1 was at times seeming competitive against W/C engines reputedly getting around 18horsepower at 10 - 11000rpm..  


  Was that QUB a flat-top piston?  or was it slightly domed as the Bantam?  

The racing QUB was a 250 wasn´t it?   And had the reed-valve which was Blair´s big innovation -- if I remembrer correctly.

Cheers!