It must surely be the ambition of every Bantam engine tuner, taking on a new engine project, to design in as high a power output over as wide a power range as can be accommodated within the emerging concept. That unique challenge of taking the most unlikely of ingredients and metamorphose them into something hitherto only imagined. This challenge has always appealed to those who enjoy the somewhat perverse task that converting a humble 4hp run-about into a credible racer presents.
The overarching and limiting influence will always be that of the meagre 3 gear ratios available. Whatever tuning scheme maybe conceived, track performance will always be compromised by that one single factor. Let the revs fall too low towards the base of the torque curve, progress then decays and acceleration falters, we`ve all been there and suffered from those negative effects.
Always the fundamental question is, how can I move increasing quantities of fresh mixture through the engine at low rpm and up to its maximum and retain as much of that mixture as possible for combustion and not have it disappear out of both the inlet and to a far, far greater extent, the exhaust port. The inclusion of a reed valve took care of the inlet side, but could an exhaust valve help as well?
At the time (the mid-90s) I got hold of a second hand spool valve from a Yamaha mx motor and started exploring the feasibility of including a rotary power valve to the new barrel build. However, after much head scratching and cyanoing together pieces of 3mm balsa sheet as the mock-up housing, the entire project was getting so complicated and drawn out that it was, reluctantly, shelved. Even so, the whole engine build still took three years to get from drawing board to track, in part I guess due to being constantly side tracked by yet another “idea”!
A delivery ratio of 1.2 is quite common in a “good“40+hp 125 race engine and perhaps up to 1.3 in a GP winner like the 53+hp Aprilia engine. Those figures imply that some 150-160 cc of mixture is flowed through the transfer ducts in each engine cycle at peak torque rpm. I can`t think for one moment that a “good” Bantam will even flow its own cylinder capacity, having only half the power of the other examples. Not all of this mixture can be retained in the cylinder as scavenge and trapping ratios are invariably less than unity. That is unfortunate but is one consequence of having a big, gaping exhaust port and a highly extractive diffuser on the exhaust pipe, both conspiring to encourage charge loss.
And so, rather conveniently, that creates a neat segue into the limits of a conventional exhaust pipe, and what can be done to off-set some of the specific downsides that so bedevil the Bantam race engine.
By the very nature of its construction the exhaust pipe has fixed values for diameters and component lengths, but engine rpm vary enormously up and down so a pipe optimised for peak torque will be hopelessly out of sync 2,500 or more rpm lower. At peak torque rpm the strongest pulse entering the exhaust will, by definition, create strong reaction from all the other pulses in the pipe. Pipe-dependent two-stroke engines therefor will always exhibit a significant “dip”, at around 2/3 of maximum torque rpm, caused by the returning stuffing pulse arriving back at the cylinder at the wrong time. The stronger the initial pulse the stronger the ensuing disruption.
Honda, along with many other manufactures each with their own systems, added an additional, large volume side branch chamber, the ATAC, to their exhaust pipes, near to the cylinder exit. The stuffing wave pressure pulse returning to the header at the wrong time, caused by the pipe length to rpm mismatch, is reduced in amplitude by the volume of the open side chamber. This effective killing of the of the adverse return wave amplitude pumps up the lower end power by, somewhat counterintuitively, reducing the pipe`s efficiency, a simple device that works very well, particularly on cylinders where no power valve option is available.
Freddie Spencer`s 500cc Honda triple incorporated ATACs, initially on the two outer cylinders and subsequently on all three to great effect, without the taming influence of the ATAC system those machines were reported to be extremely tricky to ride. Clearly Honda found out how to makes these devices work efficiently and to their competitive advantage.
The ATAC is ideal for Bantam style applications where extra power is needed well under the natural power band the pipe and engine spec are capable of. I have seen claims of 25% improvement under the pipe, but as I always caution where such claims are made, 100% of nothing is still zero, but what can`t be argued against is that these devices do work well and are simple in construction, and seems non critical!
The volume of the resonant chamber needs to be around the volume of the cylinder displacement and this seems to“work” every time, going bigger does nothing to help. A slightly smaller volume, say 100cc, may suit a typical race Bantam power output better, so concocting an adjustable volume chamber could expand the range of influence.
The connecting tube from pipe to chamber needs to be as short as practicable and a diameter of about half that of the header pipe dia, going smaller limits effectiveness and larger impairs top end power. Getting the whole thing as close to the cylinder is important but installation restrictions may obstruct this so as always do the best you can. The butterfly valve needs to pivot into the header by about 1/3 when open and in the direction of flow.
The following details were provided by a very credible source and dyno testing confirmed that 28% more power was shown just before the valve was snapped shut. A 20mm tube on a 43mm header for a 166cc (500 triple?) cylinder capacity with a 150cc resonant chamber. A later development on this original concept was the addition of a second, smaller chamber that gave even better band width where a second valve gave access. The two are initially open simultaneously and then second chamber is closed, the resulting smaller resonant chamber closed at higher revs. This double volume system though more tricky to construct and adjust might just be attractive enough to investigate on the dyno.
It is important to snap the Atac valve shut with a solenoid just before its influence on the pipe starts to kill power. Locating the switching point is straight forward on a dyno, test fully open then fully closed where the two curves intersect is the open/close point.
Ideally an rpm driven solenoid switch from an Ignitech or similar programmable ignition system is preferred but a handle bar switch to a solenoid linkage can work, track testing with the rev counter and getting a feel for what is happening is equally effective. What is critical is that the butterfly valve switch operates instantly, if not the resonating volume starts to kill power. Unlike an exhaust port power valve the ATAC opening /closing can`t be ramped up and down.
The great appeal of this exercise is that it works a treat when set up is non-critical in construction can be made with hand tools and is dirt cheap to make, a bean can and a bit of sheet steel, a few bits of electricery , a word with a friendly welder and your there.
Not quite something for nothing but not much money either.
Stay safe, Trevor
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Sun Mar 14, 2021 7:23 am