Long Stroke Potential.

Nigel, here is the reply to your last question.

I have long considered the 58mm stroke option for 125 Bantams as offering some tangible advantages over the more traditional 54.5mm, and even more over the old Yamaha option of 50.5mm. The extra torqiness inherent in a longer stroke, for the same capacity, is an attractive bonus when restricted to having only 3 gears to use.. The downside being the higher piston speeds, and therefore, friction losses and heat increases, but if you lose 5% then gain 6% you come out ahead in the power game!
Many years ago ( 1973 in fact ) Steve and I created a short rod, long stroke engine using the Bultaco con-rod that had centres of 116mm. Not the same thing of course but a useful indicator of alternative options whilst retaining the original 58mm stroke. The engine surprised us both in the significant performance increase that we achieved. Steve came within an ace of winning one of the hour long Enduros at Snetterton once , so it had durability as well.
What for me could make this a viable option is the fact that today within the existing rules a 55mm piston can be used. That would bump up the capacity by a useful margin, but more importantly, port areas for similar timings are increased. And, the circumference of the 55mm bore allows for plenty of port width.
with suitable STAs calculated, and the extra flow rates, it could all work very well, at least on paper. So what`s not to like.
Long stroke would not be the way to go in a full house GP style engine, but we are not at that level, just simply looking for more modest improvements. The big worry could be increasing thermal loading and it`s control, particularly with air cooling.
The GT 186 had a longer stroke at 62mm, and coincidentally, also used a 116 con-rod, perhaps a touch too short but needs must back in those days of limited options. There wouldn`t seem to be much future in juggling bore and stroke on a 175 because you come up against the 190cc capacity that most are effectively at already and of the obligatory 58mm stroke. The biggest problem confronting the existing 175 is the 125mm con-rod. Junk that and stick in one from say, an RD400 at 115mm ctrs, and performance will step up! Nothing much to do with case volume or primary compression ratio but using fundamental geometry to facilitate advantages in several other important areas. There is not another comparable race engine that I am aware of that uses such a heavy, long rod. No one would ever scratch design a race engine using such a con-rod. The bantam engine is a flawed design; no sense in helping to keep it that way!

Going back to 125s, I would use the 55x58x115 configuration, full high flow water cooling, reed valve, 5 pre-fabricated, fully profiled transfer ducts and exhaust duct, coordinated transfer stream regime, power valve control by a programmable ignition system, the full works. It would involve a huge amount of design, manufacturing and be expensive, 30hp no sweat BUT would the gearbox take it?
I have serious reservations about that, so I won`t be going beyond the paper stage!

Below are appropriate piston speed numbers for all three strokes in mtrs/sec at 10,000rpm.  

58mm.......19.3
54.5mm.....18      Aprilia engine at 13,250rpm peak........24mtrs/sec
50.5mm.....16.8

Trevor

Transfer Port Timings.

Here are a few numbers to juggle with; using the constant of a 12.5mm high port, and measured from BDC. There is no particular significance to that dimension, purely arbitrary. As you can see I have used some of the more frequently used combinations of stroke and con-rod length. the timings vary by quite a bit making the choice of rod, to a given stroke, quite important, and something to be seriously thought about.
The potential for the 125 long stroke option begins to look quite attractive, see what you think?

Stroke/rod                                    Timing Duration
58/125                                                122.8
58/115                                                124
54/110                                                128.25
54/104                                                129
50.5/110                                             131.8
50.5/105                                             132

Trevor

Back one notch to piston speed, Trevor -- 15m/sec was our limit but of course it was a diesel in production with reliabilty in mind....

Never had a short-stroke Bantam. Both were long strokers. With my second Bantam `Icarus-2´ which was very roughly tuned by me the max revs were over 11,000rpm and the torque better than `Icarus-1´ but the reliablity was almost zero. This was because I was still using standard BSA pistons. The Icarus-2 was short-lived anyway because Andy´s 250 (with Alpha-based motor) took over completely -- but to think of me at that time talking of "squeeze-cast" pistons to otzher diesel makers -- and still using BSA Standard pistons on the Bantams -- has me cringing about my professional past just now.!

It was amazing, how much those pistons would stretch without actually cracking. Rather than complete engine failure (sometimes was with accompanying seizure...) it was the noise that would have me `retire´ to DNF ... Inside the fairing the rattle was awful! The piston would have battered the head but it was the gudgeon-pin holes to provide the awe: they were elongated ínto beautifuly shaped ovals -- a real wonder of science!

Curling up again,

CheerS!

Thanks Trevor, very interesting information. looks like the piston speed  for the 58mm stroke at 10,000 revs is not much higher than that of the 54mm stroke, perhaps with the better made pistons of today, its not too much more of a mechanical strain on the piston after all. I suspect that the different length conrods have varying dwell characteristics that maybe have significant impacts on the engines performance and also calculations for ignition timing/ tuned exhaust lengths.

Exhaust Port Timings.

Here are a few more numbers to juggle with. This time I have taken as the constant, 50% of the stroke from BDC, another arbitrary selection. The mix of options remains the same. It is fascinating to see which of these provide the longer timing, but also how close the respective timings actually are.
Merely numbers on paper of course but the permutations are intriguing.
Just as important as well is the actual cylinder height above the port up to TDC, and its influence on blow-down.
Residual heat remaining in the exhaust gas, after soaking into the cylinder wall, head surfaces, plug, piston crown, need to factored in. As does gas expansion and attendant pressure reduction as the piston descends to exhaust port opening.
Timings and time, more stuff to worry about?

Stroke/Rod/Port Height                                      Timing Duration
58/125/29                                                              193.3
58/115/29                                                              194.5
54.5/110/27.25                                                       194.2
54.5/104/27.25                                                       195.0
50.5/110/25.25                                                       193.2
50.5/105/25.25                                                       193.8

Trevor

Just had a nice email from Derek giving me the heads up on one of the 2015 KTM dirt bikes. Guess what, a new 2T 150cc engine has the bore at 56mm and stroke 58.6 stroke and rod length of 117mm!
KTM engineers know a thing or two about making both 2Tand 4T engines go very well indeed so perhaps there can be some encouragement, even endorsement, to be gained from their example when thinking about a 138cc, 55x58mm-125 Bantam engine? Will someone take up the challenge for 2015?

Surely, it is now time that Derek was accepted back into the Bantam, forum fraternity, his forced absence is just silly now and the forum is far duller and less well informed as a consequence of his being prevented from contributing?
How about it, he`s paid his dues!

Trevor

Hi Trevor,
having used a long stroke 125 for the past 3 years and some time in the past i can confirm modern pistons do take the extra load at over 10,000 rpm mine on one pipe would rev happily to 13,000 with no damage but most of the time i tried to keep it below 11.500. 115 rd400 rod 54mm wiseco piston, crankcase reed. the extra torque helped with someone of my weight and size get out of all the modern slow corners.
Andy

Great minds thinking alike Andy?

Given that there is a time gap of almost 40yrs from my engine to yours, it might appear that things have come full circle? But it is reassuring to have your conceptual endorsement. What I found, back in 72/73, was how easy it was to achieve the step up in power and torque we did from implementing the design geometry changes. One restrictive factor back then, was the original 52mm Bantam piston with its puny gudgeon pin forcing the use of bronze small end bearing in the Bultaco rod.  Today the piston choice is almost limitless.
To be fair though, much later, when I went to the 54 stroke engine with the same 116 rod and Wiseco piston there was a further improvement, but this could have been due to incorporating extra transfer ports and area?

Given my experiences with the current water-cooled 54x54 reed-valve engine, replicating that but with a 138cc-55x58 configuration would provide for exactly the characteristics you describe, and would be my preferred option.

In all of the various engines I have constructed, even the retrograde 50mm stroke jobs, never have I managed to get 12,000rpm never mind the dizzying height of 13,000rpm out of them, so you have that experience which I cannot share!

Again, like you, I use the pipe to give a really broad spread of power but it does cut the peak some what, but then racing is about the lowest lap times and not necessarily the highest peak power. Everything always comes back to the severe curtailments forced by 3 speeds only.

Trevor