Outlet Dia

Hi ,Derek,
Been doing a bit of research on this topic and,as it can be seen to be the start of the exhaust system design it`s logical
to begin here .
Not too much information is published about outlets per se , but that what is seems to draw to a firm conclution that there
is a correlation between exhaust port area and the out let. An SAE paper from 1996 suggests ratios of around 1.05 and is founded on exhaust
tests on a gp engine.
So on to cylinders, i have measured a Honda RS. a Yamaha TZ and my own water cooled Bantam.
Honda.
One big port with bridge and an area of 1177.5 sg mm, the outlet 41 dia giving an area ratio of 1.12
pipe volume is 3938cc and pipe to cylinder volume ratio is 31.55
Yamaha
Single port with two auxilliaries and a total area of 1156 sq mm , the outlet is 38mm giving an area ratio of .981
pipe volume is 4246cc and pipe to cylinder volume is 34
Bantam
area ratio of 1.1 pipe volume is 3447cc and pipe to cylinder volume is 27.9
So, we have three different barrels with different geometry providing comparable values and would provide a good starting point
to your pipe design, so get measuring!
Incidentally, if you can achieve the speeds you describe, you dont need any help from me!

well " where do I start really great info Trevor thank you.

I can not wait for the other answers!, These contribution I'm sure, will go down well for those looking in.

Also remeber there will be people reading this drooling over such information, AND we have a number of new people, engines /bikes being built, and new starters so try to keep it in layman's terms.

For instance i dont understand the reason why you have specified total volume of the exhaust pipe, does this include the down pipe defuser, center section and reverse cone. ? could you put a little more detail on this and why have you quoted ratio diameter to volume but not on your bantam.

But what you have said gives the suject you talk of background, really interesting stuff,! but please add the next level "like "if I were starting with a 175cc I would make it x big if a 125cc I would be around y big then the front pipe should be about xyz with a length between x&y, with a front pipe defuser taper/ inner outter diameters etc etc. Hope you get my drift, will be helpfull as we can then tabulate it to an information page on both 125 and 175 tuning over a period of the comming weeks. then do some comparison on different bikes.

what's your thoughts on tail pipe diameter to exhust port diameter/ ratio ?, I can not remember but think it was between 0.6/0.7 the exhaust port or front pipe start diameter??. mine exhaust port entry to the front pipe is 36mm, this is as big as i can make it without having a step, as its a std aircooled iron barrel, I,m currently working on.

Now - on the exhaust, my old front pipe had a diameter of 42mm, with a step as it joined the barrel, i'm sure I got this from you in about 1981/2 ish, I think, anyway the reason I ask, is most of the current front runners are using much smaller diameters, these days, I have been quoted as small as 32mm by one, this is why I asked you to start with exhausts with your contributions to our Forum, as it seem like a good subject to start with, As there appears to be a lot of difference to the way things were or bullshit on this subject.

Now on those old top speeds, I sold my bike and then purchased it back for my return last year, I dont have the old pipe it never came back with the bike, hence my questions, also pulling that tall gearing, always took too long to get there for me? I always found the rest of them had gone by the time I got there in top, I always noticed in 1st & 2nd I could hold them, but once in top they would jump away from me.

regards Derek

I just lost this so this one will be shorter

now measure exhaust port AREA - ureka RS 25% BIGGER area THAN a formula bantam

mine measure at 864.6sqmm now working on a 1.1 ratio to front pipe diameter = 38.5mm diameter

now my port is only 33mm cordal width taking it out as I can to say 68% (reasonable ring life) and to 37mm means my port area goes up almost 10% and front down tube to 41.5mm. but look what it does to volume working on the same stats as trevor suggest's

Ureka the RS and YAM, has the same if not smaller port opening than my formula bantam, but the area is at least 25/28% bigger forgetting the heat issue for the moment (very big impact), all I need do now is find my high speed grinder open my port width to 37mm and make the new pipe watch out you front runners. ?

anyone disagree! please feel free to make a comment on this, can you see any error in my sums James your the man for this.

I wrote this post for a second time and lost it, three times bigger with lots more information, so this is all I got time for now .

please feel free to post a reply and if you diagree lets hear why. !

regards Derek

Hi, Derek,
Don`t be too concearned about pipe volumes etc, a bit of fun , there are so many numbers on the forum right now
i thought i`d throw in a few of my own . Also, don`t take the area ratios as absolute, other combinations may give good results.
It is interesting to note that the most powerful engine doesn`t have the largest volume pipe.
Seeing as you asked, the volume measurements came from piston face to tail pipe entry.
Tail pipes, one golden rule, go too small and you`ll hit trouble , absolutely ! You really answered your own question
with a low cr and still detonating It is absolutely crucial to empty the pipe of exhaust gas during each complete cycle
or the returning " plugging pulse" will always contain high levels of contaminants and the combustion process will be compromised.
Also a choked pipe will seriously affect the scavenge process and so will lower power and torque values. Another point to considder ,
how much exhaust port is exposed during blow down, just before transfer release, not much i guess, that could mean a smaller out let.
A higher cr with a larger tailpipe will always give better perfomance than the reverse ,as i`m sure you`ve found out by now !
As for my engine, 22/23 works just fine !

Hi, Derek ,
Sorry, i missed a few lines from my last post , concentrating more on test results from the Doc than the job in hand!
It`s not always realised that the exhaust duct is the only one required to flow in both directions , and that there are areas
within the duct that gas moves at varying speeds and with different degrees of gas purity. The port geometry at the interface
with the cylinder creates and adds to the turbulence levels of incoming gas and ,thereby, increases overall turbulence in
the combustion chamber and aids the burn rate and overall efficiency !

Hi Trevor

so we have established that the diameter of the header sounds to be far more critical, and hence some considereation should be given to this, "and" why we start at this point first for our exhaust design" ? I always stated at the tuned length before.

also to your point about turbulance helping mixing in the front pipe is this why a lot of High performance bikes have steps or funny odd shapes in the area of exhaust to header intersection, ? RS 125 has a step, I'm told if you gring it out to smoth the port, the power drops by 4BHP? ANYWAY WE ARE TALKING OF BANTAMS SO sounds to me like the joint between header entry and port exit needs to have a step as long as its bigger than the port, but must not be smaller ? restriction.

really interesting stuff Trevor gas temp is at its highest here, so wave speed should be too ? or does this depend on rate of opening and port shape "please clarify".

In laymans Terms:- as I understand it
we have a front pipe for a 125mm short stroke of around 38mm internal with a standardish exahaust port iron barrel, but if you have bridged the port and the area is larger then the header pipe entry should be larger to.
aprox 44mm for a 175cc based on a 65% port size to bore diameter (ring life) -
now on the tail pipe a 125 should be approx 22.5mm minimum.
on 175cc should be 28.5mm minimum

can we move to the next stage - we need to fill in, the in-between bits of our designs,!

those of you building a 175cc please feel free to ask Trevor a question, if your not following this, I'm sure he will not bite.

incedently we are constructing another Iron barrel, working on the transfers at the moment and will be taking the exhaust port out to 37mm with eliptical shape, also intend to cut off the exhaust stub and wel on a new piece to lesten the s"bend, then shrink on some alum fins top three fins only, same as current set up, to tryout Trevors design, by going elipptical of the port shape dad recons we should be able to make the port 37mm width and could achieve 1a bigger volume of, 1104.8sqmm this would require a larger header entry. and tail pipe at 23mm so we may make this first with this in mind.


Thanks again Trevor, great background, lets keep going.

PS. what's "DOC" mean.

kind regards DEREK

trevor, great posts very interesting topic , especially as a good functioning exhaust can make all the difference if you can get it right.as you mentioned in your earlier posts with regard to the 130% more volume in the honda engine. would i be right in saying that you need a larger first section of pipe to help extract the gases quickly and to help draw in as large a fresh charge as is possible and not be too consernd if alot of this charge is drawn into this section of pipe as were hoping to return as much of this as possible(hence a pertentionally bigger volume of charge) with the retuning wave and would a step down in the port size help speed up the gases being retuned insay the last 50mm into the cylinder which is lost by this section of pipe being larger and losing its wave strength, or am i barking up the wrong tree?. had hoped to send you a small exhaust design program by pm to have a gander at , but even if the pm facility was able to do this my e-mail provider doesnt allow this kind of file to sent anyway. its been developed using the info provided by blair 2 stroke tuning thoery and had hoped to see if you felt it was relevent. as you probably know theory and personal experiance can be worlds apart. still thanks for your posts the more info the better

interesting theory Nobby !

Trevor, sounds like he has some logic here, what do you think, Nobby can I suggest you give trevor, some back ground on who you are and the type of bike you have just finish building but not raced yet, Trevor whats your take on his theory about the first section, are you working off pure maths or results orientated.

looking forwards to your next post on the next part of the development on how to design a Formula pipe design for 125 or 175 with back ground to be applied on future tuning topics.

hope your well!


kind regards Derek

Hi ,Nobby 1 , Derek,
Thanks for the comments, good to see you posting, we all learn that way, keep at it !
What is required of a pipe is to get rid of combustion gas as quickly as possible, and retain as
much of the purer gas from scavenging as it can. A large pipe just after the duct will allow the gas to slow,
cool and in doing so lose energy. Don`t forget you only have a finite amount of energy to play with , and
compared, say, to the Honda, it`s not a lot.
Any step at the duct/pipe junction will impede plugging flow and create spurious reflections which could lead to shock waves
that destroy energy levels and produce a temperature spike.
So , on to published info , i like to include this as it gives context , and, if there is sufficient correlation between makes
and over a long enough developement period you can take guidance from it . If that info is then supported by dyno
testing and /or simulation so much the better.
Honda RS .
Both pre94 and later revamped engines use front pipe angles of around 1.5 degrees , 3 degrees included.

Yamaha TZ .
I can`t confirm exact date of engine, but has a front pipe of 1.7 degrees , 3.4 included .
Quoting from the paper "The major difference between pipes A2 and B2 is that A2 has a1.67 degree
header pipe and B2 a 2.5 degree header. The measured bmep for A2 is greater than that for B2 over
most of the speed range and particularly at high speed".
Bantam.
1.5 degree front pipe ,3 included , with a stepless junction from duct to pipe.

I would have enjoyed the software , i don`t know of one piece of serious twostroke software that does`nt
have his or QUB influence in it somewhere so be assured if he says it`s so , it`s good advice !

Hi Trevor, you seen my pictures of my new exhaust"! what do you think" (be gentle with me she is my pride and joy), my welding is getting better, by the time I have this finished, this will be as it should,

The front pipe is actually 36mm I/d with a little over 1. deg tapering header, before entry to the box that tapers start 5-deg moving to 7 deg on three length changes, tail pipe is as adviced but 6 inches long, with a 25mm silencer on the end, revers cone had some considerable work on it, anyway "its a big improvment on the dyno" to what it was and much better looking curve so the operator says, mind you this was not apparent until we swapped the carb to the mikuni as it was so over jetted, also the mikuni has a smaller bore at 30.8mm as opposed to the 34mm amal.

Although the overall top figures are not much difference the power band is wider but more importantly we have developped a flat line in on the top of the curve, so spread is much better, but much more importantly the engine temperature has come right down.

Also its become extremly loud, almost as if there is no silencer, we have an "Alan" plan it fix this.

as a result of the last test we believe the ignition timing and cylinder head volume can be worked on. as we had reduced the head volume because of overheating, to 8.606 to 1 (approx 16.5cc) engine size 125.5cc!

it would be good to get other people's comments please feel free to make a comment on here good or bad, we noticed with a lazer temp spot probe on the dyno that the pipe temp, from about a point 25mm in front of the exp-box start, to a point where the reverse cone starts, its the pipe exterior temp -and this drops drastically between these two point copared to the reast of the header and revers cone /tail pipe, I was going to try and rap the exhaust to see if this made any difference on the dyno, not sure what significanse this would have ? but we will report back the difference.

anyway we will be making a new head, looking for a 125CC MZ we weld the stud holes up and re drill - these have a big fin area, compared to the bantam, besides we have no more GT cylinder heads that will do the job.

we want a smaller combustion chamber, any suggestions on shape anyone ?
we are now looking at a different ignition ? - (the dyno operator put a strobe on our motoplate and sid it was very varied /moving a lot compared modern ignitions we are not sure on what to look at or for, idea's anyone, ?
we would like a programmable one, but we don't know anything about them, or cure shapes we will not be going the full monty with a mappthat changes , we just want to be able to change the advance retard point say every 100 revs. ideas suggestions anyone?

I was expecting trouble at some point with reliability with the bike, as there are so many issues with this engine when building the crank seems to be holding up if you look back at my posts this was plugged on the crankpins, so far so good,

and it starts great, so easy with the new carb, we have some more work to do on the jetting as it appears a little lean in the midrange, normally would just lift the needle but now "I THINK" we must move the main jet. one way and the power jet the other.? we will see.

great post Trevor.

trevor could you look at the post from DPKING on gears could you possible put some history to this I'm prity sure you were involved.

kind regards Derek

Hi Derek/Trevor,

Hold my hands up, all confusing to me but great to read. Derek. With regards to you saying its loud is that an spl reading or just from experience? Does that mean you are going to try mesh and accoustic packing in the silencer? (pure speculation from a novice).




Eddie

trevor thanks for answering my questions... ive read most of the jennings blair and bell info on 2 strokes, had read previously that the internal surfaces of the exhaust should be smooth and the link between outlet and pipe be stepless, just thought that maybe things had changed, thought that perhaps it had been found that gas flows and turbulance around the exhaust port could maybe help the stuffing phase... obviously not... i notice that the exhaust data mentions b m e p. From this formula it has a figure for displacement, is this the stroke x bore displacement or this displacement plus that displacement caused by the extra charge being stuffed back into the cylinder? as otherwise only increasing torque could improve bmep.From the P L A N formula it seems that only increases in bmep and rpm can be infuenced to produce greater bhp as both the stroke and bore are fixed by the engine rules for the 175. also doesnt an increase in bmep also have the effect of having an engine with a very narrow power band? or can this be fixed by exhaust design its areal shame that 2 stroke tuning is in the decline what with this emissions thing. ive found it quite frustrating over the years trying to find info for myself and it even got me emailing to australia (itw/au) as i read they started karting with the bantam engine way back, asked for port timings , no chance...
anyway more info please like i say theory is ok but no real replacement for years of experiance

nobby" techko - show off "stop it" -

but an interesting question for trevor - as I see it your saying BMEP as it increses it also increases the potential for a narrow power band,? not sure I agree with this one, (only if it increases in a small range - normally) ?? what do you say Trevor.

regds Derek

Hi, all,
Bmep is a measure of specific engine torque per unit of displacement volume , and its maximum value
is obtained when engine torque reaches a maximum . It is also independent of powerband width, this being established
by the breathing characteristics of the engine, values of bmep only reflect what the engine produces and does not control it
Hope this helps a little , keep the posts coming , great stuff from you all , i`m learning!

10.5ftlbs at 9.850 rpm - volume 125.5cc - so how do I work out whats the BMEP is.

can we move to the next part of questions for expansion chamber design
exhaust port / front pipe dia taper / tail pipe /

what about the
1) centre section diameter /volume
2) header pipe length is it 6 or 8 x diameter for length or somthing like this.
3) expansion box first taper
4) reverse cone taper and centre section.

what your thoughts on the tuned length - "measured from point piston face to revers cone reflection point. ?

so you think all these posts are great "I agree" - I see you say your leaning loads too, "what "who knows" or don'nt ?

Kind regards Derek

BMEP= 75.4 X TORQUE (LB-FT) DIVIDED BY DISPLACEMENT (ci)............
. trevor did you manage to see the exhaust software download? if you did, what do you think. is it any good. if you havnt no problem. anyway can you answer dereks previous post? would your exhaust dimentions come with port timings to match and cylinder head spec? or am i being too cheeky

Good Morning,

Well im trying to keep up with this thread as it is interesting so hace been reading my roy bacon exhaust system copy.



Eddie

nobby1 wrote:

BMEP= 75.4 X TORQUE (LB-FT) DIVIDED BY DISPLACEMENT (ci)............
. Trevor did you manage to see the exhaust software download? if you did, what do you think. is it any good. if you havnt no problem. anyway can you answer dereks previous post? would your exhaust dimentions come with port timings to match and cylinder head spec? or am i being too cheeky

Hi Trevor did you answer Nobby's post on this port timings I have always worked around 94 to 96 deg or 188 to 192 total open period, I usually start with 94 to start then increase at a later date.

what's your theory on Ports Trevor ? - did you see the article posted in the old mags by John Sawyer (just loaded this week) on pressure points in the cylinder and hence the ability to check presure difference druing the scavenging above the piston, the time this would take to do this is mind boggling, and it can have so many variables?, I am stressed thinking about this surely it will be different at any point on the live stroke - mind boggling some one actually did this level of work on a Bantam, surely this is not a way to get the best power / scavenging system/ working /breathing motor ? .

A long time ago we put smoke pellets into our crankcase via a bag connected to the inlet port, with a false piston and a vacuum on the tail pipe to finish off, so we could see with two pieces of glass placed on the head with plastercine so we could look at it when we turned over the crankcase, dad made lots of changes to the direction of the transfers and I modelled the rear transfer port on it but to check the pressure ?? -

The transfer at the rear is one of the reasons we have so many problems finding a suitable piston we can use, with having to re-peg the rings, a little hit and miss.

Have we covered port timings already - so many numbers on here in the past two months I'm having trouble following them all, I just read the article done by Peter Tibbitts back in the day in the 80's just after he did his 80mph lap, of snet very interesting reading, he talks about many topics a number of really interesting items, - its all getting a bit confusing lately.

I am going into information overload I think.

James Could I ask you to construct an exel formula for creating the cone dimentions for cone construction from the static dimentions we have been creating with Trevors Formula. Please I know you know how to do this simply so we can just pump in the numbers, and it should create a patern we can then print off and use to cut out our bits of metal,

Alan has a really good std formula pipe too, it would be good to start by just cretaing the cone sizes for this while we are working on Trevors design. we can then take a pick between the two or three options we have as a base for starting a project racer, and cut out all these headlifter calculations and theory thinking.

Trevor
I think we have the reverse cone and front pipe length to work out,? or actually we all ready have the front pipe 38mm to 49mm 1/5 or 2 degrees taper. This will also tell us the reverse cone too we have tuned length, tailpipe diameter, some extrapolation and we should have whats left - no no we need a revese cone taper - yes yes" we need a taper

again Trevor !:- is this half the defuser taper ? how do we do this with multi stage diffuser ? or are we working on a multi taper revese cone in which case I'm lost to know what to do next. ?

kind Regards Derek

Hi ,Derek,
The reply to your reverse cone questions will be posted on Pipe Design , best not to juxtapose too much
or the thread gets confused . Be patient, everything takes time.