One size fits all ?

A considerable amount of time seems to have passed since I last felt the inclination, or indeed enthusiasm, to contribute again to the Bantam website. It would seem that I am not alone in feeling the whole exercise to be a waste of time judging by the almost total lack of new posts. With the notable exception of Les and Jimmie, who between them have kept the site going, there has been nothing of consequence!

I discovered the following short article a few months ago on another 2T site and it struck a chord with me, so I have presented it here.
At first glance there would seem to be a series contradictions, who would graft an RS Honda pipe to a modest home tuned 125 engine with a non-race pedigree and expect the same pipe to produce the goods with more than a 50% increase in power and rpm?
Perhaps you guys might be intrigued and then inspired enough to play around with the numerous performance variables of your own engines, along your favourite exhaust pipe formulas and see what happens, you may be surprised.
                                                                 ------------------------
“The RS exhaust has proved to be a very effective pipe, very tolerant of my different engines.
My motors started at 16hp at 8,000 rpm and now 30 at 12,000 rpm.
It`s not too surprising the RS has worked so well on all of them. Because it has a good shape and a suitable length for lower rpm, power and exhaust temperatures. A length that is much the same as required for higher rpm, power and consequently higher exhaust temperatures. So basically the RS pipe has the right length for all my engines as they have developed increased power and rpm.

It works like this:-

A) Lower rpm = longer pipe.
B) Lower exhaust gas temp = shorter pipe.
C) Higher rpm = shorter pipe.
D) Higher exhaust gas temp = longer pipe.

A+B = C+D

I have tried to simulate better designs in EngMod but could not develop anything that was superior “
                                                          _________________________

It might be nice if someone could post a comment on this and maybe get a conversation going?

Trevor.

Hello Trevor, hope your fit and well
just looked in to see what’s going on. im assuming that the exhaust has been fitted to this fellas engines with no other changes. if i remember correctly the tuned length is dependant on the rpm at which the engine has been designed to make best power exhaust port timings and the exhaust gas temp which has a bearing on the speed at which the pulse moves within the exhaust. it would seem that he has stumbled upon a sort of rule of thumb configuration which works for a number of applications. forgetting any change of cone configuration it would seem the gas temp is the factor which has the main effect, allowing this pipe to be so successful for him.

One of the most important of the almost infinite number of variables associated with a two-stroke race engine`s performance, is the relationship between exhaust wave speed and pipe length. That wave propagates along the pipe at the speed of sound which is proportional to the square root of the local temperature. The gas temperature is not constant throughout the engines rev range nor along the length of the pipe, being hottest adjacent to the engine, coolest at the largest diameter and spikes up again at the junction between rear cone and tail pipe. Other major influences on temps in the pipe are ignition timing, carburettor jetting to achieve the stoichiometric ideal, compression ratio and so it goes on. Every change to exhaust port timing demands a different pipe length along with rpm changes and so it all goes into the mix. Taking compression ratio as an example, the higher the ratio, the greater the burnt gasses expand in the cylinder before the exhaust event is actually underway. This means cooler gas flowing through the exhaust duct, and that may be great for keeping the cylinder cool on an air-cooled barrel. However the wave speed suffers from lower temps and consequently this is felt throughout the effective power range.
So it will be the combination of all of the changing variables acting on the engines in these two seemingly extreme engines that work in unison to enable the RS pipe to function well on both. I do suspect that the 30hp engine will derive a greater benefit than the lower output one does.
It`s a fascinating exercise trying to factor in all of the variables to come to a definitive conclusion, to answer what at face value is a contradictory proposition. But then it is just this that makes two-stroke race engines so satisfying a challenge!
Good to hear from you again Nigel, have you got another special enterprise coming together in you workshop?

Trevor

Trevor, ive just had my 4 speed engine bottom end back. I had some gear change problems which hopefully have been fixed. As the engine was made up from bits it had some tolerance issues

nothing really going on other than that. actually had my first full day off work in a month today ( and i feel guilty!) but as im knackerd ive been lying on the sofa all-day watching films. i think im falling into the trap of living to work again instead of working to live.

Seems that this site is very much on the decline as you say, but i look in from time to time to see if i have missed anything.

all the best , nigel.

From the article that started this thread we have a couple of examples of the apparent suitability of a Honda RS 125 exhaust pipe to other quite differing engines with contrasting power and rpm levels. Puzzlingly, for me at least, what was also shown was that the most efficient, class leader of the available software packages that aid the 2T tuner failed to improve on the original Honda pipe?
So what does the standard factory fitted pipe do for the Honda race engine; an interesting and revealing investigation!
The original customer pipe comes with a tuned length of around 825mm and worked with an exhaust port duration of 195* and a centre section diameter of 120mm. In a well set up engine that combination will provide a decent spread of power and an rpm peak of 13,000 in the over-rev, working with Steve`s RS more or less confirms this.
The `factory` tuning schedules suggest extending the exhaust timing to 200* with a corresponding shortening of the pipe`s tuned length to 800mm; the drawback being that this set up will really work only with electronic carb power jet switching operation and a totally different ignition curve package. The aim is to get the engine to rev on to 14,000. A further development of all of this is the old 125 KTM setup, which is really little more than a clone of the Honda with a power valve thrown in for good measure. This in turn is a tweaking of the so called Honda `A` spec. which at the time was revealed to only the selected few! The pipe for these engines was a fair bit different from the customer pipe, with a centre section diameter up at 128mm and a three stage diffuser with the steepest and longest angle between the two shorter and shallower cones. This was designed to create the deepest and widest depression effect from about half open to half closing through BDC, the point of maximum efficiency of transfer port open area phase. The position and dimensions of the mid diffuser cone is important to making good top end torque and over rev power in these engines
So where is all of this leading us I hear you say, well, just putting the later, more advanced pipe on an customer type Honda engine just doesn`t work. You still have to have the advanced exhaust timing plus carb switching and ignition system to get the thing to work as intended. Fitting an RS pipe on to a far lesser engine, as in our example just shouldn`t work, but `working` is a relative and subjective concept, a straight parallel piece of exhaust port will `work` but an expansion chamber of sorts works a bit better and so on. A suitable conclusion to this puzzle might lie in the juggling of port timing durations, rpm values, relative temperatures and all of the other remaining engine specification variables such as compression ratio, jetting, ignition timing, etc, thrown into the mix. On the other hand getting 30hp from a home tuned street bike 125 is no mean feat, so this guy might just know something the rest of us don`t?

Trevor

Hi Trevor!
Your comment of Aug 4th taken and swallowed..
You are quite correct -- we oldies have nothing to contribute relative modern two-stroke tuning on this site and all we have to offer is much, too much, of the past that we become boring after a few attempts to say anything worthwhile.. There weren't many with dynos -- or had the availability of a dyno. I only ever had contact-breaker ignition on our Bantam and I spent hours of Suck-it-&-See at Brands to get the pipe length right -- among other things. . It was fun though: especially when the Big Bike (Norton & Triumph) lads were wondering what passed them going into Clearways and then never got any closer than a couple of lengths. I can bring those exciting moments back at the flick of a MC Mag page and warming of the grea matter with a glass of good wine.. Most boring for others of the modern era -- naturally!

Of course, the reason there aren't many subscribers on this site is because most are on Facebook*** but I doubt whether any there would get into technical revelation unless you were holding them naked over a blazing fire. Having only 8400revs max (of course, I have said all this before ...Yawn -- Yawn) max revs meant much envy of the 11 to 12000rpm mptors some Bantam stars were said to have but a wide torque band meant much clutch slip with higher gears which made racing a hard time to get a good result, yet it was all worth it. When I got onto the ABS 250 I tended to despise the Bantam but I righted that later when I came to appreciate how fast the backyard tuners could make them.

*** As said before (Aww yawn again!) I tried to get on Facebook but even after giving me new passwords Facebook reckoned I was not me.

Good luck to the 4 of you -- keeping this site alive -- more power to your elbows -- as the saying goes -- for writers as well as racers.

JayBee -- A Hasbeen with only memories....

PS -- and a few Pots to prove it....

Long gone John....

John, believe me your not missing anything on face book. Nobody does much up front talking about engine design or what their up to. so dont feel left out there mate.If you’ve got something on your mind post, dont feel that you are in anyway unable to contribute.

Thanks Nigel!
For the kind words. I thought that might be the case with Facebook.
 In my previous post I (purposely) left out any reference to Icarus-2 which I "Tuned" myself. The tuning was not all disaster but it was something I'd not shout loudly  about because the reliability went to pot in a big way. Icarus-2 revved to 11000rpm  mainly because I made everything "loose" -- I think,  and experimented with crankshaft seals and that sort of thing to reduce friction drag losses.
 . . Having experimented with "Centre-Point" steering -- centre point under the trail bisection of wheel radius -- it had weird undrersteering. I lent Icarus-2 to riders who'd broke down and were'dying' for a ride, as the saying goes:  warning them of course that it was hellishly unreliable ( had standard BSA pistons whose gudgeon-pin holes lengthened or broke up...)  and rode it myself just for the heck of frightening myself which didn't happen too often with Icarus-1. .
   I had this theory that clutch slip acted as a torque-converter (increasing torque by reason of the clutch inserts centre radius being at a greater radius than gear teeth to mainshaft centre) and I remember Micky Scutt was the only one to agree that I might have been right. Well, with Icarus-1 it was very necessary: the hill at Cadwell up to Charlies was a nightmare and my left hand was always in a bad way after a Cadwell meeting.

Sorry about the -- yawwwwn-- yaawwwn -- boring ancient dribble but it is all I've got to offer.  

All the best,

Dribble -- dribble, yawn yawn, John....

Hello,
                  Always good to read anything you comment on John and I can see you are still on top form! Nigel is spot on with his assessment on this site and Facebook, however judging by the numbers of people curious enough to log on and read this whole post, things might be looking up? I guess it is up to just a handful of enthusiasts, keen enough to make the effort, to contribute something in order to keep the Bantam site ticking over.

Some time ago I posted dyno graphs of a triple pipe test session using Steve`s Honda RS 125. I only mention it again now because it fits well with the subject of Honda pipes suiting other engines. In a quote from the original article…. ”The poorest performing pipe is the original Honda item, as can be seen it gives good power up to around 11,750rpm and 40.84hp but then fades badly”. The best performing pipe went on to produce an additional 1.6hp at 12,300rpm but still matched the Honda pipe`s maximum at over 13,000rpm. Conversely, at 13,000rpm the Honda pipe was down to just 30hp, however, at 8,500rpm, the original Honda spec pipe had a massive 4.5hp advantage over the other two pipes tested that day, a very handy advantage on the track in the right circumstances.
On the basis of these tests it shouldn`t then come as too much of a surprise to imagine the spec of the Honda pipe suiting different engines of a lesser output providing good results.
Honda aren`t stupid, they are fully aware of all of this but the over the counter RS was a world bike being sold in a dozen different countries that all have hugely varying weather conditions and fuel quality plus the average European rider is heavier and taller than, say, Asian competitors. So, the pipe comes as a compromise that keeps the engine reliable when hauling along a sixteen stone six footer or a six stone five footer in hugely varying temperatures and humidity levels but still needs to perform well and reliably under all racing conditions.
Would such a pipe be viable for a Bantam race engine, probably not, but Bantams are unique in the race engine world so require a more specific approach to overcome the unique limits 3 speed transmission impose on us! What can be concluded is that a longer pipe than perhaps the exhaust `formulae` might predict will produce such results. Indeed, on my water cooled, reed valve engine bike Mark always preferred the “long pipe” as it gave much better performance around the whole circuit and out of each corner but lost a touch on a long straight. He always maintained that to be passed he had first to be caught!

Trevor

Hi Trevor!
An odd thing -- I had trouble logging on here. There was a blocking box which said, This site is not secure -- or something like that and I had to try 3 times....

Thanks for your comments...

I thought there was a good point in your last paragraph because a famous rider was said to be able to ride anything and often had won on what other riders would not touch: Barry Sheene for one on "The DucK".... Not to down your bike by that I'd say Mark was a true winner on a Bantam. I saw him at Cadwell and he was very impressive. Point I was trying to get to was his weight, the gearing and how much he used the slipping of the clutch because his acceleration off the line was phenomonel. It was -- and is -- how the rider 'Fits' the machine in terms of what sort of 'best' he can get out of the bike. I was scrambles, grass-tarck and Speedway practice orientat4ed (experienced perhaps better?) and road-racing was far more sophistacted than any sort of riding i had done before.....

Your 'thing' about pipes reminds how much tesing I had to do at Brands to get Icarus-1 competitive and from the above to get myself orientated into the right form of mind to be precise in everything. Fact was I was earning my living by dyno-testing (diesels-- dare I say the word) at Ford Dunton and having to get a pipe that worked Half-well at Brands by suck and see practice ..... We started with a Dr Erlich pipe which we kept the same lengths and fattened and thinned until it appeared to be optimum and then lengthened and shortened the parallel section until it felt "right". I always thought it didn't fele exactly right but the torqure band was wide (wider than several others who'd be willing to telll...) and learned to slip the clutch to get some decdent acceleartion. Almost a year was spent going frequently to Brands to get the feel of Icraus-1 being competitive and whilst I had always looked on road-racing as too specialised for me I got around to feeling I could not miss the opportunity. I've said all this before but I'll repeat it for here: We tthree in the Icarus-1 build team had decided to advertise for a jockey but by that time I had decided it was my job. Glad I did -- now.

It then became a matter of body weight (15stone stone 7) reduction which was hard. But I've said it all before and I have said enough for now.

More power to your elbow!
Cheers!
John-Boy -- eh...?

Continuing and to an extent, expanding the theme of relative exhaust pipe length influencing the positon and magnitude of useful torque within an engine`s power range, gas temperature hence pipe wall heat has a huge role to play.
In the early 90s QUB experimented with water injection into the header pipe of a 125 test engine and achieved an almost 25% improvement in the lower to midrange torque. Now that sort of torque gain would be transformational on a racing Bantam when bearing in mind it`s inherent design deficiencies.
The numbers associated with these experiments are very revealing, recorded temperatures in the diffuser were 425*c at 9,000rpm rising to 510*at 10,800rpm, they seem very typically Bantam values? With the water injection system functioning, the respective temperatures were reduced to 150*c at 9,000rpm and 420*c at 10,800rpm.
The reduction and contrast in temperatures recorded had the effect making the exhaust pipe tuned length much longer. Pipe calculations were based on an initial wave speed of 518mtrs/sec (1700ft/sec), with the water spray injecting into the header wave speed was reduced to 436mtrs/sec (1430mtrs/sec). Inserting those numbers into your favourite pipe formula will produce starkly differing results, and perhaps asking a few questions of one`s hither to assumptions!
It is therefore no surprise that the standard Honda RS125 pipe with its longer tuned length showed a stronger lower rpm performance to the admittedly stronger top end of the shorter home produced ones. This theme is confirmed in published test results for Yamaha TZ engines and other Honda RS racers, and in my own experience with the w/c, reed valve Bantam engine where Mark always favoured the longer pipe.
All is not sweetness and light though, as the works NSR 500cc Honda bosses found out when Mick Doohan tested just such a system at Philip Island. Massive extra power off the bottom end was gained but lost a full 1,000+ revs at the top end. They fiddled around with water volume but the pipes remained too cold to achieve full power down the straights. In typical Aussie fashion Mick told the mechanics to “shut the whole friggin thing off”, two laps into the next practice session he set pole time and went on to win the race, lesson learned!
Clearly pipe and port combinations play a significant role in balancing the type of power range that suits the average Bantam race engine performance in getting around the circuit in the shortest possible time, which at the end of the day is the name of the game!

Trevor

Interesting Trevor!
Your mentioning water addition reminded of a project I got involved in with at a Toronto university who were sponsored by an US mining company to do the reaerch. The interest of the latter was to reduce -- dare I mention the words -- Diesel exhaust particulates from underground shuttle vehicles. This university team actually achieved success witha Deutz engine of cleaner exhaust by 25% with an added advantage of better fuel consumption by 20%. Of course the addition of several bits which included the univeristy's invention: a water-shearer which enabled complete mixing of water and diesel made it too expensive, which was a shame.
During WW2 some of the Messerschmidt 109s had petrol injection and the company introduced water injection in a number some of their aircraft engines. I think some of the British planes had water injection into the intake manifold used only for take-off (-- I think?)...
I also heard about an Indian engineering graduate who got himself a Fellowship by writing a paper on water inection in a petrol engine where the results showed a cleaner exhaust & 20% improvement in fuel consumption.

I just wonder why such innovation in IC engine development has been ignored. Is it politics or is it too difficult to repeat in production? or just cost?

In terms of add-on-bits weight -- for racing -- better, of course, to do it all with the pipe....

Just chomping the gums -- not much use for any Bantam racer or tuner except to help him nod off...

Cheers!

JayBee............

Had a guy PM me recently asking, amongst other things, if I would provide him with dimensional details of the “long pipe” we used so effectively; “long” being a purely relative description of course!
Normally I like to help people all I can, but in this instance, after pointing out that his engine`s spec would in all probability mean that my pipe would be unsuitable for his engine, I recommended he use one of the many, freely available pipe software packages to formulate his own pipe whilst examining realistically the true potential of his own engine. Incidentally, the overwhelming majority of these software packages are re-hashes of Blair`s original, seminal work from years back, so don`t be too enthralled with colourful bells and whistles that claim to augment the fundamental workings. The laws of physics remain just that. I also suggested he would learn a whole lot more using that approach which can only be a very good thing?                                                                                                                                                                                                             It must surely be the ambition of every Bantam competitor to improve the performance of their engine,
but how can this be achieved if one`s individual, overall capabilities and knowledge of the subject are limited. Copying a fellow racer`s engine spec is the easiest way to go, so that perfect copy of that perfect example, will work perfectly? Only trouble is, you will have learned nothing other than finding that plagiarism is the way to go!
I would suggest that you have to make your own mistakes and victories along the way, invaluable experience can then be accrued, try to learn as well from other people`s mistakes that is a credible way to succeed; look, listen, ask and read! In short, get an education and as in all things undertaken, the picture become a whole lot clearer, problems always dissipate with accumulating experience.
If you want to know what is going on inside an engine read all the papers and articles you can lay your hands on, however, don`t slavishly believe everything you read, question their contents and react to it. Some of the popular, long revered books from last century that are available on two stroke engine tuning contain gems of wisdom and total rubbish even on the same page and then go on to fully contradict themselves on subsequent pages!
At the last Midlands meeting in Gloucester there was a very good, in depth discussion that examined a number of these books, SAE papers and web articles on just this topic where the general consensus was, that most are now fundamentally misleading in many key specifics.
All of this may be dismissed as being perhaps the benefit of hind-sight, I prefer the alternative that it is knowledge and experience gained making an informed judgement.

Trevor.

Very interesting thread.
I have a RS125 pipe for my short stroke watercooled 125.
My engineer mates son was making hydraulically blown chambers for brothers RS.
A scrap one with a calculated header length was thrown on and increased power from around 14 to 17.5 hP with 500 rpm greater top end.
No software, only a tuned length calculation

Seems we have more endorsement for the efficacy of a Honda RS style of exhaust pipe, this time fitted to a Bantam, the improvement in output from 14 to 17.5 hp, is an impressive 25%, well done John, tell us more!
In the past I would have posted up some pictures of pipes but since Photobucket now demand $400 for the privilege of hosting my stuff it just won`t happen. However, if anyone is interested and wants to PM me their email address I will endeavour to provide such pictures. The last `official` pipe with a triple cone diffuser, which was also cloned for the KTM, looks quite promising for a good 125 Bantam engine!

Trevor

Honestly I picked up some early experiments in hydraulically blown pipes from mates scrap bin. We fitted one to my old Blue Un (The ex Chas James long stroke 125) that had the big hP gain. Another better shape one was fitted to my Red Un (ex Lionel Howell short stroke water cooled 125) id increased peak power a bit and more torque at higher revs. I will see if I can find the dyno graphs. I will PM you my e mail address and send you some pictures

Trevor Amos wrote:

Trevor

Hi there:
I don't normally comment, but wish to say that "I do enjoy your technical posts, both from you and john!, please keep them coming.

" ref:- your above post, I would like to ask a question as having recently attended a dyno rig in Surrey, both an RS Honda and a Bantam were being run together. I have attended quite a few Dyno sessions over the years with 2 stroke's, I'm an avid 2 stroke fan.

afterwards I then invited myself to attend the next test session, this one held at a Wolverhampton location.  

I have not seen many RS' make more than 38.0 hp, on rolling road's, yet most are claimed to make 44/45.0 hp!.

But I never seen a Bantam racer on one.  "they were just finishing the session in Surrey so we missed it, but on arrival at Wolverhampton, the Bantam was on the rig, and already in full flow. I took with me a colleague fellow Bantam enthusiast, anyway,  the owner was disappointed on the dyno in Wolverhampton as it could not manage much more than 26.0 hp, so struggled to get near what it achieved in Surrey.?  

They then run his 125 Honda, this gave instantly over 43.0 hp at 12,500. while the bantam matched the Honda initially, only below 10,000, it gave better HP and torque, but it could not get near the Honda above 10,000.  

Your name was mentioned more than a few times, "I understand a lot of work goes into building these little bikes, he explained and about his association with you.

So my question is simple:- "why do you not just advise to run std Honda spec pipe on Bantam's. ?

At best the Bantam was 17.0hp shy of the RS.?  

regards AL.

Hello Alan, it`s good to hear from you again,
That is an interesting question you pose, backed up by first hand experience. However, as I am just about, this morning, to embark on the pre-Christmas visit to family and friends, and won`t return until sometime on Monday, sadly a reply will have to wait until then!

Cheers for now, Trevor

Hi JB glad you are still with us mate!! The way my abused body is feeling lately at my er youngish? age (71) suppose I should be grateful!

So agree with you all, all of my experimenting with Bantam engines revolved around getting gas flow right with no short circuiting of the gas out of the exhaust port, and good even and fast flow carb to cylinder. All my pipes were made using Mick Scutts formulae and provided good power with fair torque, and was pretty quick in it's final guise, before at Debden creeping past Micks RD250 along the straight it went bang. Just goes to show long stroke motors doing 11000 rpm plus doesn't really work!! Hence my abandoning the bantam and buying a TZ. Trouble was the TZ was so badly made I'm suprised they even survived the first few years as a 54 square motor at similar rpm, as the cranks were badly made, machined and wobbly. Keith

Happy xmas all, especialy our illustrious ex chairman JB. Cheers man!

Hi Alan,
            Why indeed just advise, even recommend, the use of a Honda produced exhaust system for a bantam race engine? To judge from some of the correspondence within this topic it would seem to be a positive way forward. However, it is not something I would personally encourage, and there are compelling reasons not to do so!
I`m pretty sure I know the identity of the Bantam/Honda owner you highlight and can attest to the competitive performance of both machines. On the figures you have provided and assuming an rpm level for the Bantam of 11,000, the BMEP numbers attained are 8.5 and 12.3 respectively, a 43% difference for the same swept volume, both are very respectable achievements. But within those two figures are the reasons why a common pipe will not favour the Bantam. It is the BMEP that initially determines the energy level that is available for the pipe to work with, and in both engines there is only a finite amount of energy available at exhaust port opening, the Honda clearly has far more to work with so the same pipe cannot respond equally for both engines particularly when the one is so obviously inferior. Intrinsically the pipe works with the cylinder contents which in turn is a function of the breathing capacity of the whole system.
The crankcase and cylinder are connected by the transfer ducts, in the Honda cylinder they are well defined and promote good flow, but the case of a Bantam, which is essentially a homemade effort, in the main they are very far from being adequate so consequently, flow poorly. Utilising the outer wall of a liner for the inner duct wall is bound to create massive turbulence so consequently is poor practice. A large inner radius to the duct wall with an increasing radius at the approach to the port outlet helps enormously to avoid flow separation. This `Coanda` effect limits chaotic turbulence, promotes flow velocity and maintains charge density, so that the coherence of the flow streams going into the cylinder do so in the direction intended. Turbulence is a real flow killer and as such must be avoided, that is with exception of the combustion chamber, where turbulence is absolutely essential to promote a rapid and complete charge burn.
 Exhaust pipes are a hugely complex combination of lengths and diameters, far more so than most books and software packages will have one believe. For example, if you enlarge the belly diameter of the pipe, the suction created by energy conversion by the diffuser will increase. The downsides to this is that the remaining energy will be diminished and the returning plugging pulse reflected from the rear cone is weaker and has slower wave action, so will return less charge back to the cylinder in a less than optimal time. Too aggressive a diffuser action will very probably cause over scavenging of the cylinder and pull excessive fresh charge into the pipe to be lost and so play no part in the subsequent combustion process. Both of these adverse actions are almost certain to occur in a Bantam scenario when fitted with a 120mm diameter diffuser as typically used on the Honda.
So far I haven`t mentioned reduction of charge density due to expansion within a large volume pipe which will rob power, and of heat loss to the large surface area which also reduces density and slows everything down. Nothing slows energy waves quicker than rapid volume expansion and a reduction in temperature.
The exhaust gas flowing from a Honda at full chat will be a lot hotter than that in a Bantam so unless the tuned length is adjusted to compensate, the phasing of the pipe cone functions will be way out sync. The speed of sound used for pipe length calculations has one major variable, temperature, a higher temp increases the wave velocity and so requires a different tuned length to obtain an optimally timed return wave.  As the length has been changed so will all of the angles of the various cones change, the length proportions of the header, diffuser and rear cone will also alter. In the end you no longer have a Honda pipe!  
Most of the points raised here are worthy of lengthy articles to themselves but I hope to have gone some way in answering the questions asked?

Cheers for now,  Trevor

Trevor

  your problem posting pictures might be overcome if you look at postimage.org. I readily admit it's beyond my ability to use it sucessfully, but others who suggested it to me use it with ease.

Re the engine mentioned in the RS125 exhaust tests, it's very difficult to make any worthwhile comment unless the engine details are available. I'm assuming it's a 125, but what bore and stroke, and is it a water cooled unit ?

Your recalling your rider who preferred the longer exhaust reminded me of a friend in South Africa who built himself a special 250 racer, using a Honda 250 MX engine which he installed in an Aprilla chassis. His first race was an open event, and he took his place on the grid among the likes of Honda Fireblades and similar large capacity machines. They all outsped him into the first corner but he passed so many he came out third. Being repassed down the next straight, but only by a couple of machines, he repeated the manoever, and from there on he was never headed.

There is a lot to be said for raceability, and not getting carried away with big horsepower numbers !

Hello again `Prorad` always good to hear from you, keeping well I trust?
That’s an interesting anecdote of your friend`s exploits down in South Africa, getting good results from unlikely ingredients is always a satisfying achievement, I bet you could write a whole book of such stories of your experience of personalities and performances from around the world`s race venues!
 The exhaust that Mark used so effectively wasn`t the only pipe design compromise we used to `pull out` the limited, effective power range, the lack of which is so crippling for a 3 speed transmission.
The pipe`s tailpipe used was of a larger diameter than convention might dictate for the bmep achieved by the engine at peak torque rpm. Within reason, the larger diameter restriction to flow from the tailpipe provides a lower average pressure, greater gas expansion and lower temperature of the exhaust gas, thus a lower velocity of sound and resonance frequency, all of which is of useful help at lower rpm levels where the returning pressure wave arrives too early. Most Bantam engines are blowdown restricted, too high a pipe pressure at the wrong engine rpm can promote flow reversal from the cylinder into the transfer ducts preheating the fresh charge and delaying the transfer phase and thus reducing power. Until the revs are high enough to overcome this the engine just stumbles along, none of which is a problem with a 6 speed gearbox having multiple interchangeable internal ratios. Losing the odd hp at peak rpm will be more than compensated for by gaining a couple in the effective power band so will certainly improve lap times, worth thinking about!
   Thanks for bringing Postimage to my attention, I`m grateful to you and will explore it when I can give it some concentration, it all seems, at least to me, a bit intimidating!
  Incidentally, how did the 350 Brigestone investigation end up?

Cheers for now, Trevor

I'm afraid I very seldom get to visit the forum, being virtually completely occupied with four stroke work, but on this occasion the thing that caught my eye was the difference in claimed output between one exhaust and the RS replacement. Allowing for the fact that to make the 30 odd rwhp with the RS exhaust the engine's potential must have been there in the first place, then the first exhaust must have been a pretty poor effort, by comparison.

Regarding getting an engine to run on at peak rpm, some years ago I was involved in working to try and make this happen with an engine that was used in go kart racing, and was restricted by various rules, one of which was to use a mandated exhaust system, which could be made up to fit any particular installation, but had to adhere to the mandated dimensions re section lengths and diameters. Eventually a way was found to obtain the over run by adjusting the mean squish velocity, and for best effect this became quite critical to get right.
How this would be of use for a bantam engine I don't know, by all the posts I've read most racers seem to be getting more than enough revs, the issue seems to be how to get a bit more power throughout the existing rev range, and if an RS exhaust is beneficial all well and good, but it is most enlightening to eventually understand just how the improvement was effected.

Trevor, in the past you have so well explained gas dynamics, and most specifically as they apply to a bantam racing engine, but the response to your posts appears to be coming from just a few members which leaves me to wonder whether the difficulty of understanding gas dynamics and thermodynamics in all their complexity is of less interest to many racers who would much prefer to hear how they may make a different port shape to get a worthwhile boost.

Having found such a gain from installing an RS125 exhaust, the next obvious (to me) task would be to try and understand just how the pipe made possible the performance increase, and from there look at how it might enable the next step forward to be made.
Unfortunately it keeps coming back to gas and thermo dynamics !

If anyone should think designing a two stroke exhaust system is difficult enough, try an exhaust system for a multi cylinder four stroke with a number of interconnected pipes and the accompanying pipe junctions.

Thank you for the encouraging words Prorad, means a lot to me!
You may well be correct in suggesting that trying to achieve extra performance from an engine is a difficult challenge for those with little or no experience of the complexities involved. I have always tried to offer to the Bantam fraternity, a simplified but entirely relevant version of the “science” involved, with the encouragement that, if I can do it, so can you! To this end I continually quote the mantra……. one should never stop learning, for knowledge is power!

 Back in the day, Erv Kanemoto tried every way to get the big TZ 750cc four cylinder Yamahas to run a pipe diameter greater than 110mm, when he did so power went down. The primary reason was the greater diffuser suction of a bigger diameter destroyed what little directional control that the dead straight transfer ducts gave to the scavenge streams, so that the whole lot of over scavenged mixture short circuited out the exhaust ports. Some years later Mike Sinclair, of Team Roberts was able to confirm exactly what Erv had experienced. So, can we realistically expect a Bantam with a similar transfer duct regime and a lower bmep to fair better with a 120mm diameter RS pipe than the more sophisticated cylinder of the TZ750 did? My Bantam “long” pipe has a belly diameter of 108mm along with a triple cone diffuser, going bigger proved no better, some of the very powerful Kart 125s pushing 50 hp go to 130+, the average Bantam is half of that power, doing some sums around those numbers may prove very enlightening.  
Sometimes a modification provides a positive effect which in turn may lead to a false conclusion. To take as an example, if you were to enlarge just one side of the cylindrical belly of a pipe the engine may run better. You may then think the revised shape has produced that result, but in reality it may be that the changed pipe volume, along with the attendant Helmholtz frequency, has produced the improvement?

Cheers for now, Trevor