tapered front exhaust pipe

Hi All.
I've been racking my little brain trying to work out how to fabricate a tapered header ( American) for JS2, I can make a straight cone cut the bits and reverse the sections but that will only disrupt the flow and shorten the tuned length what I would like to know is how to work out the individual sections for example, 38mm to 51mm with a taper of say 2deg over say 9".
Any help will be welcome as I've run out of cardboard.

Thanks, Les2012

Les,
Have a look at the "Exhausts" topic on this page, Nigel had the same problem and devised a very good way to produce a curved, tapered header. Have a word with him he`s been there and done it!
Before you embark upon this exercise be sure you are using an appropriate taper/tapers of the correct proportion to the remainder of the pipe?

Trevor

Les, Just got the calculator out, check your various numbers they don`t quite match.

Also if this pipe is for a bantam then 38mm duct outlet is right on the maximum, I wouldn`t go that big my self, even if it is for a 175/186.

Trevor amos

Hi Trevor, thank you for your input I'll check out the page re exhaust. The figures I gave were right off the top of my head and not those I intend to use, I need to do some calculations before hand but thanks anyway, incidentally I have used 38mm straight headers since I built JS1 way back with good results so it will be an interesting project to start at 38mm and depending on the angle and major diameter I decide on will determine it's length. I'll keep you posted.

Les2012

Les,
Way back in my younger days, of all knowing ignorance, I also used headers starting at 38mm. Had I used a smaller volume duct with a smaller diameter header the bike would have performed so much better for it.
38mm diameter has an area of 1134 sq/mm, if your proposed exhaust port has its Total area less than this, as I suspect it will, then you don`t need a 38mm outlet. The outlet need be no larger than 90% of the exhaust port Total area.
For the moment we will ignore the Effective area or indeed the Working area, a modern engine deserves a modern approach!
As an example the RS Honda exhaust duct outlet extrapolates to an area equivalent of 36.5mm diameter, and around 70% of exhaust port area, and that engine flows far more gas than any Bantam will ever do!

I hope that may provide some food for thought?

Trevor

Hi Les,

34mm id worked very well on my 50.6 x 56.0 motor. Single exh port, inverted trapezium shape. HTH

Pete

There you go Les, top notch information from a multi champion, you can`t get better than that!

Something else you might want to consider; tying the duct dimensions to cylinder blow-down may well be a far more realistic way to go?

Trevor

Hello Trevor and my old friend Peter. Thank you both for your input and what an interesting subject this has turned out to be. Interesting to read tuners idea's but at the same time confusing. Pete states his engine sported a 34mm pipe( a reed valve motor and dam quick it is but I do wonder what the results would have been with a 38mm pipe). In the Bantam Tuning Manual David Hunter states for his standard bore and stroke engine a 1.5" = 38.1mm exit. Michael Scutt states for his 54x54b/s 35-38mm and George Harris for his 56x50.5b/s a taper of 38mm out to 53.97mm in 11 inches.

I have nearly finished Brian Lane's Yamaha TZ125g fitted with a Honda CR125cc engine, it has an exhaust exit of 38mm. These guy's built very quick engines and won championships, now I'm being advised to reduce the exit, to what for JS2?. For the time being I'll stick to what I think I know, I'm no expert, all my experience has been achieved by calculated suck it and see and reading and following expert advice like Graham Bell's article on Exhaust. Maybe the Bantam Tuning Manual is out of date? if so who would rewrite it?. Now there is an opportunity for an expert on tuning the Bantam.

Thanks again, I'll keep you posted.
.
Les 2012

Hi Trevor, thanks for that endorsement.

I shall explain where the 34mm id came from. In approx 1982 a colleague of mine in Testing Operations, Ford Motor Company, finding a little spare time, started measuring up 125cc cylinders of particular engines that had performed well in top level road racing. The samples were coming from works RG500 Suzukis, works 250 tandem twin Kawasaki, and a certain colonial gentleman's works 250 Yamaha. Suffice it to say, these cylinders would turn up and be measured up and returned before the "owners" discovered they were missing. A great chart was made showing the dimensions and differences, and I think Angle-area was a key parameter under study.

To cut a long story short, it was the works RG500 which had this dimension, and it was tied into angle-area. I wish I could remember the theory.

In 1984 I made an exhaust with a fully tapering front pipe and dimensions provided by the above engineer. It replaced a 38mm one, and was much more successful. Subsequent design input on my engine from another source (1997 onwards) , kept the 34mm, as it matched the port area available. For a fee, in Euros, this designer can provide dimensions of your optimum expansion chamber.

Les.
The Bantam tuning manual has effectively been re-written. It is here on the Forum in all of the excellent technical articles submitted by knowledgeable engineers and tuners. These guys know more about Bantam race engines than anyone else.
Forget Bell and Jennings and confine them to their place in history. Information that is 30+ years old is just that, outmoded and in the most part obsolete. Back then 125s just about managed 30hp, today its 54 and with a bmep of 16bar, a figure that would have been utterly unimaginable back then!
Todays exhaust systems bear no strategic, proportional resemblance to their predecessors, headers included. Phrased very simply; use the smallest diameter and shortest duct that will not limit gas flow at peak torque rpm. At rpm lower than this it will not be required to handle that mass of exhaust flow, rendering the duct too large. Engines are all about creating energy, retaining it and then exploiting it, a large diameter duct will forfeit some!

Also, have a think about the reverse-flow mode, where the returning pressure pulse, reflected from the rear cone, arrives at the exhaust port just before it closes, a large duct can be a great liability?

Trevor

Courtesy Of Trevor Amos.

First learned concepts with their deep, ingraining influence can often prove difficult to totally eradicate from the mind, and can insidiously segue into absolute truths even though they may subsequently be utterly discredited or disproved. Similarly, there was a period when the speed potential of one racing Bantam engine set against another was judged almost exclusively by the timing of the exhaust port, and the size of the carburettor.

Numbers of 200* and more were common place, together with transfers way up over 130*, as were 38mm carbs and 13,000 mega revers, and were considered by many as an absolute pre-requisite for achieving a winning performance! Blowdown was thus relegated to a little discussed side issue. So intense was this scrutiny that it became an almost universal obsession with relative piston speeds also being ignored. Somewhat perversely, the consequences were that power bands diminished and only began at high rpm, giving a light switch delivery and not much when it did eventually appear. Going too radical with porting was always going to leave a big hole in mid-range torque delivery, but this important factor was largely ignored. All metal clutches that could survive endless slipping saved that embarrassment for such Bantams but of course did nothing to address the real cause of this predicament. Solace is sought in ever lower gearing and increasing revs. In so many instances there was a complete miss-match of transfer and exhaust port STAs with neither being near what was required. Paired together with all of this were poorly matched exhaust pipe dimensions that served only to compound these mistakes. Guessing at what is needed, never mind what is actually happening, only makes for acutely tricky power delivery characteristics, overheated clutches and poor lap times! Over stressing an engine, be it mechanical, thermal or both, will bring it ever closer to the inevitable, DNF!


Thankfully in main-land Europe, but alas not universally so within the Bantam world, the “New Age”, forward thinking engineers then took centre stage and today sanity prevails. These guys began challenging and analysing what they thought was factual, re-appraised, and came to a fresh set of conclusions. So much so, that in the final thirty years of the 125 GP class, engine power just about doubled with peak rpm rising by only a few percent. Actual timings are now reduced and time-area values predominate. Incremental improvements garnered from continued innovation from fertile minds being explored by hard work and extensive testing using the flow bench, dyno and track is the methodology by which this is achieved. During those last eight or nine years, timings remained fairly static, that fact alone should tell us something significant, and with all of this progress being made despite the constraints of the same count of 6 gears. Even my family car has now progressed to having six gears! It is by distilling out the relevant elements of this new thinking that Bantams can benefit, finding what can be applied or not is the challenge?

However, I have always found it a little puzzling that 50cc GP engines of the 70s produced as much power then as most Bantams of the 21stcentury, a gap of some 40 potential development years and an almost fourfold capacity increase in favour of the larger Bantam? How many enticing blind alleys were explored in those four decades, with performance trends remaining up the dead end cul-de-sac, determined by pre-conditioning through obsolescent beliefs?
Throughout this period, progressive development and testing has shown that a smaller exhaust duct is far more efficient not only in terms of removing burned combustion gas, but also in preventing excessive scavenge loss to the exhaust port. Retaining more of the good stuff, discharging the bad and not allowing the two to mix is then rewarded by more thorough, complete, and faster combustion. Increasing efficiency = increasing power, and right across an expanded torque range as well, now that can`t be bad, can it? Fundamentally, go for the smallest duct volume that doesn`t impede flow at peak torque.

The duct is where old gas and fresh charge can mix, keeping the duct volume small to maintain gas velocity even at low rpm helps to reduce this mixing from happening. Returning pressure waves reflected from the pipe`s rear cone coming into the cylinder is what is needed, not re-cycling the hot, incombustible gas lurking in the duct that can promote overheating of the piston crown. Keeping the duct as cool as possible by making it as short as is achievable will reduce the quantity of hot gas, and its residual heat from soaking the cylinder and piston by too much. The GT 186, with its straight out port had stacks of finning rooted symmetrically to the duct, so cooling airflow could access the hottest parts of the barrel thereby reducing thermal loading, just a shame about the, too large, outlet diameter. Bantam barrel traditional layout with its off-set port has minimal, and largely ineffective, fins at the one side effectively shielded from direct cooling by for-and-aft air flow. Strategic ducting would be of real advantage here in promoting cool air to get at this hotter area of the barrel! Having different temperature gradients across the barrel width inevitably introduces the very real possibility of distortion and possible seizure. The solution to which is usually power robbing mixture enrichment.

The actual exhaust port TOTAL area has now been relegated to one of the less critical elements of a race engine porting analysis. Matching the blow-down and transfer specific time- areas correctly has now become the predominant factor in race engine design! It is possible to have almost any reasonable exhaust timing, provided the necessary blow-down can be made available. The Aprilia manages a cool 33hp at 10,000 rpm with an effective ex. duration of 180*? Complications arise for us when it is realised that the requirements for Bantams of blow-down at 6,500rpm are vastly different from 10,500rpm, and that is down to the critical time element, and with the port working with a continually varying area, but static timing. Two stroke engines that rely upon a resonant exhaust pipe for power must have a transmission that allows them to keep within the rpm of that power band. If not, then for Bantams, the resonant frequency arrived at in the optimism of the design stage will not cover the short-comings of compromised performance inherent in having only three gears and an ineffectual power range.

Absolute exhaust bulk flow will only be able to occur at peak torque rpm where the most fuel is being consumed. This will be the maximum that the blow-down and duct volume will be required to handle. Flow- rate will be no greater than the values of pressure, temperature and mass particle velocity allows for. Engine speeds outside of this ideal will leave the duct volume too large, here the combination of excessive cross section and surface areas conspire to further slow-down and de-energise the now cooler running exhaust gas. Lower than designed in assumptions for pipe temps mess with the calculated tuned length and its effect inevitably diminishes.
For an example of what not to do, look no further than the D14/4. The D7 size duct dumping abruptly into that huge front pipe, is just a silly marketing stunt with style over substance producing a negative result. Bultaco was also guilty of this thinking with their TSS range of racers of the 60s. A vast duct led to an equally large front pipe, the philosophy of the day held that rapid expansion of exhaust gas cooled the cylinder. Not only did this not actually work to the benefit of the engine, but the remainder of the pipe was doubly robbed of the heat energy that enables it to function effectively. Similarly, one well respected (sic) British techno journalist/tuner from that era forcibly declared that ex. port area should never be any larger than 85% of the exhaust pipe?? What rationale determined the actual pipe diameter in the first place we were not told! I think he may also have been president of the flat earth society. Small wonder the Japanese took over!

For the conventional single exhaust port, the duct exit at the junction with the pipe need be no greater than 90% of the EFFECTIVE port area. The transition from port to pipe should be a smooth, regular, reducing area change. For our modestly powered Bantam engines that figure could be smaller still, for small power produces small residues. Providing that the smaller size copes with what the engine produces then big is certainly not beautiful. A smaller duct is also beneficial in the reverse, cylinder re-charging mode. The following analogy explains what is happening. Imagine trying to pump up your race bike tyre with an old style cycle hand pump. If the connector was of a large diameter (large duct) then all that will happen will be a pressure oscillation that is in time with the pumping action. The tyre will never inflate, and it is the same for an exhaust pipe trying to shove escaped charge back into the cylinder just before port closure with pressure is starting to rise. And if you still believe that header length and diffuser diameters are a working and mathematical function of exhaust duct outlet diameter you will certainly experience some serious problems. Another valid reason then for viewing with total scepticism tuning book instruction on ports and pipes emanating from the 70s and 80s!

The power available will only be what the weakest link is capable of producing! Emphasis now is diverted from ex. timing alone to blow-down time/area, the maximising of this will see benefits in mid-range to peak torque. The original Honda RS cylinder had its oval outlet at 40x35mm, the much later, and more powerful, A-kit was 40x32mm. Now I know that was for a bridged and profiled port, but it does serve very well to illustrate the general trend. Extrapolating a size for the outlet from port effective area would give a diameter of 36.5mm!
With the blow-down period set correctly for the anticipated rpm and power, then at transfer opening the optimum mass of scavenge inflow can, in turn, offer the highest cylinder pressure after combustion is complete, which then translates to the best achievable torque and revs, which in turn makes make best power! It is this mix of events, all of which interact around blow-down pressure that makes for the most efficient engine. Retention of more of the transfer mixture by imposing a physical barrier to short- circuiting out of the exhaust port will see improvements in power. First we had the power valve we are all now familiar with, but attention has now changed to include the lower third of the port and the floor to side corner radii. The modern consensus has it that in terms of high speed exhaust flow at peak rpm, the bottom third of the port is all but irrelevant, playing virtually no part in blow-down and scavenging. Cylinder scavenging decays so rapidly that it`s practically all over by time the piston opens the transfers and then on downwards to the subsequent events of diffuser influence around bdc. Critical to this is maximising exhaust port area for the duration of cylinder blow-down prior to the transfer phase, but residual pressure will still influence initial transfer flow. Maximising available blowdown area provides the optimum conditions for clearance of burned gas from cylinder and pipe. One notable GP entrant flow bench tested a cylinder with the ex. duct floor level with the top of the transfers, with very encouraging results. It was only the intervention of Dorna, in its banning of 2Ts for GP competitions which curtailed this line of progressive experimentation!

EveningTrevor, would you say then that its possible  for a standard cast iron 175 barrels exhaust port area , below the transfer port opening hieght,  does nothing but rob it of power due to excessive short circuiting? would this then make wider but shorter exhaust port hieghts( as for transfer ports) being the way to go to get the required/ extra blowdown area time? if this is so would the transfer ports still need to be wide and low or could they be larger in height if blow down is increased by exhaust port floor hieghts being raised? would longer stroked engines now be a better tuning option?

Many thanks Ed.

Nigel,
All transfer ports suffer from short circuiting, to a greater of lesser degree simply because of the pressure differences, high migrates to low, every time. This can be made a good deal worse by an incorrectly dimensioned and positioned pipe diffuser, excessive "suck" with compromised (D7) style transfers always does this.
Taking the floor of the exhaust port above the bdc and provides a small but effective dam, and if a generous blending rad is included this also helps to stifle leakage. This means the ex. duct can be made smaller to help maintain high gas velocity and reduce heat loss to the duct walls. The more heat energy that goes into the pipe the higher the power potential there is. It is up to the pipe designer to capitalise on this. The hot front edge of the piston also get a longer respite from exhaust gas. The dam will have no negative influence on ex flow but will have a positive one on short circuiting. It is always a matter of small incremental gains, but poor directional control of transfer flow is lose, lose situation!
As a general rule transfer ports should always be low and wide. That way sufficient blow-down can be achieved whilst still retaining a reasonable exhaust port height to hold torque levels over a wide band, 3 speeds demand this.

Trevor