As you may be aware Nick's new motor is only for Non BHR events!.........
An interest topic to add is this one by Trevor on the latest pipe technology and how it could make worthwhile improvements!.............
Rex has very generously allowed me to use some his 2016 186cc engine data and pipe drawing in the preparation of this article. The dimensions for exhaust duct outlet, centre section diameter, and tuned length remain unaltered.
It is important to realise that you have to stay within certain percentage lengths for all elements of an exhaust system; starting at the cylinder port and on to the rear cone/tailpipe junction. You cannot make one part longer or shorter without losing power somewhere, or worse still, not making it in the first place. For instance, an overlong header delays the beginning of cylinder depression too late in the cycle, and too short a diffuser creates steep angles that have greater amplitude but of such brief duration that serves only to narrow the optimum power range. There is always a conflict between power range and peak power within the constrained scope of three speed gearboxes!
The dilemma we face is that there are a fixed set of lengths for the pipe, so in the case of the tuned length (t/l) the criteria for the calculated dimension are only met at around optimum torque rpm, and then only after a period of constant running that establishes the relevant time and temperature.
It is the depression at the cylinder/exhaust port interface which establishes the pressure differential with the crankcase that moves fresh mixture from the transfer ducts in to the cylinder. However, the point of maximum depression, in relation to BDC, changes with variations in rpm, thus temperature and pressure, as for instance during acceleration and gear changing. Whilst it is the returning pulse from the rear cone that re-cycles washed through mixture back into the cylinder via the exhaust port and effectively `plugs` it just prior to the port closing ; it is diffuser action that draws mixture into the cylinder during the transfer phase. Nor is it just a matter of mixture drawn into the cylinder, but of mixture drawn through the cylinder. Some stays there, some exits and is returned and some is irretrievably lost, particularly at engine revs outside the optimum range! The pipe is an influence one way or another on all of these happenings.
Notwithstanding the conditions in the cylinder at the point of exhaust port opening, it is blowdown time/area that determines the cylinder pressure at transfer opening. Blowdown, critically depends on four principle factors, exhaust timing, transfer timing, effective exhaust port area and exhaust flow co-efficient. Another apparent ambiguity is at play here also. At low rpm around the base of the power range rpm there will be more blowdown time/area than the engine needs, at high rpm there isn`t enough and blowdown may continue after the transfer ports open, they then become convenient, additional exhaust ports. So in one sense it can be argued that the exhaust gas determines how much time/area is needed to evacuate the cylinder effectively?
All of these complex, interrelated flow and pressure fluctuations are taking place inside the engine that is equipped with an exhaust pipe that has pre-determined, rigidly fixed dimensions, and that seems to be a direct conflict of intention.
The last 10 years or so has seen a burgeoning of worldwide understanding of the physics of wave action in exhaust pipes, which has been painstakingly arrived at by both countless hours of computer simulation and subsequent confirmation during dyno sessions and then track testing. If all three correlate pretty well then reliable parameters can be established. Linear dimensioning is now reasonably straight forward, but determining diameters is less clear cut, particularly so with low BMEP engines such as the larger 186cc Bantam. By definition, the amount of energy remaining after blowdown for the pipe to work with is comparatively modest and must be utilised to best effect, energy that can be utilised to work for us is a precious commodity in Bantam engines!
One thing is for certain, the old generic formulas dressed up in 2stroke tuning packages offering the last word in pipe design are, in the overwhelming number of instances, merely derivatives of Gordon Blair`s work of the70s and 80s that are now tending towards obsolescence!
In this section we look at the pipe sketch that Rex posted on social media and has been used for successful, early season competition, the inset on the alternative pipe drawing is that same drawing.
The first dimension to consider is the tuned length, and that is shown to be 1064mm, we are also provided with values for exhaust port timing and the rpm. By inserting these into the formula for tuned length we can re-arrange the numbers to discover the speed of sound in mtrs/sec.
t/l = speed of sound x exhaust timing x 88 / rpm 1064 = S of S x 178 x 88 / 7845
Re-arranging for S of S gives….. 533mtrs/sec
It is also useful to know the temperature value used in these calculations, and this can be found by re-arranging the equation for speed of sound.
533 = sq root…( YR(temp + 273)). YR is a constant at 392.85, temp is in Kelvin, hence +273
Re-arranging for temp….533^2/392.85 = 723, minus 273 = 450*c
Working through the pipe from the piston face there is the 67mm length of the exhaust duct and the header pipe itself that begins at a diameter of 34mm then terminates at a diameter of 50.2mm producing an angle of 1.63*.
There is a finite quantity of wave energy available to be exploited, but by the time the exhaust gas has reached the first, larger diameter area it has expanded a lot, with the consequence that gas density and thermal energy density are much lower. The surface area of that cone is 37220 sq/mm and volume is 392cc. If we reduce that angle to 1.5*those numbers reduce to 36544sq/mm and volume of 381cc. The relationship between flow area and pipe wall temperatures means that the 1.5* header surface area has a smaller percentage of gas touching the wall and so the centre core remains hotter, maintains velocity and the rest of the pipe retains more energy with which to do good things. For any one that has seen those terrific thermal images of the pipe on the Bonneville Boy`s dyno tests will see confirmation of the centre core heat, whereby the mid-section is cool and the tailpipe entry glows with heat!
Header length, and that includes the ex. duct, should be in the range 30-33% of tuned length, so at around 348mm Rex`s pipe is on the money at 32.7% and straight into the ball game! The header serves ostensibly to time the beginning of diffuser action, as such we don`t want any nasty reflections travelling back towards the cylinder to mess things up and is the main reason for its shallow taper.
Reference was made earlier of the header volume, in itself is not an overly significant number but short circuited gas and some of the over scavenged mixture lurk in the header awaiting return to the cylinder, curtesy of the later plugging pulse, to augment the following compression process . With the smaller area of the 1.5* header the higher velocity will encourage combusted gas to evacuate the scene and the washed through fresh mixture will be a little cooler and thereby denser, and that is of benefit to a thermally stressed air cooled engine.
It is with the diffuser section that this alternative design is conceptually at variance with Rex`s pipe.
In an ideal world we would want the transfer flow to actually start at transfer port opening and come to a halt at transfer closing. The big problem with this is that the pressures at transfer duct entry in the crankcase and their exit into the cylinder are constantly in a state change, with one short exception. This occurs when the piston is stationary at BDC and where the transfer port area is also at its greatest! It is at this point in crank rotation that the diffuser should be able to draw the maximum amount of mixture into the cylinder for the longest period of time.
What is important is the way that exhaust gas energy is converted into suction by diffuser action, and whilst doing so retaining some of that energy to create a later, positive return plugging pulse. For Bantam race pipes, it`s all about finding a balance. There is absolutely no benefit in having an aggressively angled diffuser that sucks a litre of fresh mixture into the header if the retuning pulse is so weak only half a litre is stuffed back in. Equally it is of no help when the returning plugging pulse not only pushes the washed through mixture but also a large amount of hot, spent gas back into the cylinder that dilutes and pre-heats the following combustion phase!
At this point it might be helpful to establish some aspects of what a diffuser actually is and what it does?
A diffuser is a gradually expanding passage in which gas flow speed decreases and pressure rises, with recovery of pressure from kinetic energy in a time dependent process. An efficient diffuser is one which converts the highest possible percentage of kinetic energy into pressure energy within the given length and expansion ratio of entry to exit. Gas expansion occurs in both the flow direction and out to the passage walls, here the velocity is almost zero and highest about its centre line core.
The diffuser on the Rex pipe has a two stages with a combined length of some 291.1mm, with the respective cone`s relationship of 2/3 and 1/3, this length represents 27.36% of the tuned length. From piston face to the end of the diffuser represents 60% of the tuned length. Almost every two stroke race engine in our current era, 2000+, uses an exhaust system in which the end of the diffuser is in the general proportion of 66% of t/l, accepting this then the diffuser is some 6% short of ideal, or almost 64mm, a significant number in exhaust pipe terms!
The alternative diffuser has 3 stages, with the steepest angle in the middle with equal angles either side, these were not necessarily desirable, but keeping the 106mm main diameter forces compromises. The general layout considers the wave activity throughout the power range right up to overrev. The first cone is longer, partly because because the transfer flow is delayed from initial port opening by flow reversal; but it also positions the start of the middle cone that makes the mid-top end power, however when referencing to bdc the cycle become asymmetric in nature. If we have the correct individual diffuser lengths, the effective part of the depression will always move with rpm from close to transfer opening, then centre on bdc at peak power and move towards transfer closing in the over rev area. The third short, shallow cone helps to get the long steep main cone leading to the belly section, again a compromise on maximum diameter. It makes no sense to have the steep angle right up to the belly and so cut its influence short, which can then only rely on the inertia of the transfer gas column to maintain flow at a point where case pressure is falling with a rising piston and cylinder pressure is increasing enough to oppose in-flow! If this should happen before transfer port closure scavenging efficiency is inevitably reduced.
The belly section almost maintains the Rex`s pipe diameter, it is never profitable to use big diameter pipes which have steep cones on engines with less than optimised transfer duct profiles. The outcome is that in-cylinder directional control of the high velocity scavenge streams is hugely compromised and large amounts of mixture short circuits out of the exhaust port.
Timing of the rear cone is performed by the belly length and as such has no impact on wave activity in that parallel sections won`t reflect pressure waves. The tiny taper out to 108mm diameter merely enables the rear cone to have a slightly steeper angle, up to 9.4* from a lowly 7.88*, accommodated in the same length.
By its very nature, an excessively long convergent cone will supress peak value and also has a heavily detrimental effect either side of the tuned length; the return pulse is of long duration but low in amplitude and incorrectly timed at low rpm and arrives far too late towards peak rpm. As such the plugging effect so essential in augmenting the combustion charge is heavily compromised. The steeper cone of the alternative pipe helps in this respect but is still shallow by contemporary standards. The highest I have used was 14*(28*inc) but that was on pipe we made for Steve`s RS, with ignition timing control and electronic carb mixture control, both keeping power up when pushing over rev to 13,500rpm where things start to drop off and power fades.
As the diameter of the tail pipe is determined largely by the amount of exhausted gas it has to handle, and a less than 8 bar bmep engine doesn`t produce too much; then the returning pressure wave from the rear cone can be enhanced a touch more with a smaller pipe/insert diameter. Care has to be taken here to prevent overheating from going too small and preventing full evacuation of the cylinder/pipe combination before the next cycle begins.
All of the forgoing is merely a rejigging of the original design, and in its self can be altered. But I have tried to give credence to this alternative design with cogent and I think, well-reasoned explanations.
Of course we are very keen to share and promote this knowledge so we all can learn lots more about the exciting Bantam Opportunity!
Have a Great Week!..................Rex