Pipe Design

Hi, Derek and all ,
You established " stars " will know all of this , or should do , but a lot of newcomers looking in perhaps don`t,
and are uncertain as to where, up to date, information can be found. In light of this , i thougt the following might help
unravel some of the mystery .
The main aim of the design process is to phase a plugging pulse to arrive at the exhaust port , correctly timed, at
the engine speed chosen . Typically this will be 30- 40 degrees before the exhaust port closes.
The next important function is to size the tail pipe to empty the system efficiently , but not over restrict , before
the next exhaust pulse arrives .
Thirdly. the end of the diffuser needs to be placed , in conjunction with the tail pipe , so that the suction and plugging
events are correctly phased. There are secondary and tertiary reflections bouncing around , but for our purpose , at the moment ,
they can be ignored .
The time, and this means real time in milli seconds , for the plugging pulse to return to the exhaust port is proportional to the
pipe length and inversely proportional to the exhaust temperature and gas composition .
Exhaust system design is a complex combination of Physics , Chemistry and a big dose of " cut and try", however , modern testing
under laboratory conditions using dynos and sophisticated simulation packages are pushing back the barriers .
To find our tuned length - t/l , we need to find the average speed of sound in the pipe.
The following will provide for that .

( (y.r.t)/m)^(1/2)

Where
y= specific ratio of heat- 1.33 in exhaust gas
r= universal gas constant - 8.314 j/mol k
t= temperature in degrees k
m=molecular weight of exhaust gas- 0,029kg/mol

Example.
527 degrees c pipe temp - 800k gives exhaust gas speed of sound of -
552.3 m/sec

Tuned length is given by
83.3x speed of sound x exhaust duation/rpm

Where
83.3 = a constant
552.3= speed of sound
190= exhaust duration
10,000= rpm

Example
874mm t/l



Hope this can help , the exhaust duation and rpm are just random numbers, plenty more if anyone is interested !

Hi Trevor

another great post.

"yes please" all the information / background is fine, I have to say this last post is some much esier for the newby to follow as its in a tabulated format, with the actual end result. Means I do'nt have to get out the books.

so to date we have 125-175
!) front pipe diameter 38-44
2) front pipe taper 1.5-2.0
4) tail pipe diameter 22/23 - 28/30
5) tunded length piston to return point in the revese cone. for max HP10,000 874mm

IS IT MAX HP AT 10,000 REVS REVS OR MAX TORQUE AT 10,000 RPM ?

what about the in between bits 6) FIRST FRONT TAPER on the expantion box 7) revers cone taper
maths or ratio's based on theory / practice -or maths.

Thank you Trevor

regards Derek

Great information Trevor ,I don,t reckonise that formula I,ve always used the Bell version,is this the Blair version?
I,m going to give it a work out later and see how it compares,could be very good ,the bell version tends to longer than required which is okay as a starting point.
Dereek nice work on the summary so far it always good reading your humorous posts.
Regards Dave

hi Dave

Thanks for those kind words, I feel its an important part shame the others dont comment as often as we would like them too.

glad you like trevor's posts, Trevor was one of the first guys to help me in the paddock a very long time ago, his reply to me when me and dad asked him how we could repay his help was simple "help someone else" he said - one day you will be in a position to help a bantam rider, you can repay me by continuing the tradition, when you get the opportunity"

I think its important for all the new guys walking round the paddock to remember, most racers enjoy talking about their bikes or subjects on racing/tuning, the guys who are the open guys, are not afraid of giving info out as they know nothing to lose as they really know its not always down to being secretive, its always a combination of things.

Most "if not all the guys racing to day" in and around the paddock are the same, and only too pleased to help you, especially new /STARTERS, as they ant no danger, the information trail slows as you get quicker, but then its to be expected, just some simple rules, to follow, never ask for money! or when they are busy /during a mod or working on the bike, don't ask before the first race.

I always look forwards to dinner time and enjoy wondering round during dinner break, and enjoy catching up, or just being nosy and gleeming what info was available and there is always plenty, the secret here is don't be afraid of asking, "they wont be to say no" when you get a reply don't be a smart arse by disagreeing with it, just accept and ask another.! you can be sure when you do ask a question that is key and they don't want you to know the answer, they will usually reply, very politly "I don't know" or "not sure why we did that" can'not remember" is another classic.

or if your really pissing them off - an all time quote to me was "its a very technical subject, you need to know a lot about this to understand the reasons" ? (private dig here).

anyway moving on - keep posting I'M looking forwards to seeing some more pictures of your bike, and interested to understand how/what you have done with the 4 speed std road cluster to make it work, what are the mods.?


kind regards Derek

Have to say that was some good info and i think what trevor said to you about helping someone else, im sure you have on many occasions and what you post certainly helps.

Cheers Derek


Eddie

Hi Trevor - we notice that the new front pipe is getting very very hot, this normal? burns the oil off the inside of the front pipe in seconds its smoking, but it does cstop smoking after a while, seems very sticky on the inside when cold - this normal ?,

Also my tail pipe is 6" inches long is this ok, it does run ok but boy is it getting hot too.

We went back up on the compression ratio looking for a little more power under peek RPM but HAD TO came back down again straight away. said getting incredibly hot so quickly.

Dad thinks the ignition is a source for concern, the coil stopped working, we had to change it to another, seems fine now. but we have been running moto-plat with advance and retard on it, I'm not sure the wires are the correct way round though?, anyone help here what wire goes where.

on the one we have on the bike the wires are blue and black, but on another moto-plat they are blue and brown ?

any one help with this ? we timed the ignition to align the marks align at approx 80thou BTDC.

Upping your compression does nothing for peak power,only widening your midrange power which is handy with the 3speed at the risk of higher temperature,carburation becomes critical,no room for error.Also timing needs to be perfect not about 2mm btdc at peak revs but exactly.To check timing run up motor and run a strobe light (beware cheap car ones don,t read up to 10,000rpm check specs) .add an external pointer to check timing marks and watch as timing mark retards towards tdc as the revs increase to peak revs.
regards Dave

Derek,

We have very different engines but I have never run my ignition at more than 1.5mm (60thou) BTC on pump fuel and a little further on avgas.

At one time I was running at 1.2mm as I kept burning holes in pistons, on the dyno with a particular carburettor and exhaust the output did not change dramatically reducing the ignition advance but the piston temperature did.

James

hi Trevor ,
Does the use of methanol change the input constants different heat value and also does that change the heat factor in the exhaust.
thanks Dave

Hi, Derek , Dave, James .
Great posts, both of you are spot on , good sound advice from people who have the wrecked pistons
as evidence , and now know what not to do , there are some very clever people out there helping !
Derek , is your timing , of .080 thou btdc set for peak rpm ? From my, off the cuff arithmetic , thats 20 degrees
btdc on a 54 stroke, and, at that advance ,for an iron barrel, you will produce a lot of heat . Daves advice
is the way to go, so, check the system is indeed retarding , any doubt, get it properly checked out .
Peter is an ex Motoplat man i'm sure he can advise you as well.
As to header temps , when you considder that it recieves combustion gas that has decayed from around
2200 degrees c to around 1000 degrees c at exhaust port opening it will be damned hot!
Pump fuel is ok but its distilled to run in cars at 6000 rpm ,so , go to Avgas , it has the octane and lead values
to provide safety and your engine will run cooler. Its very far from perfect, but a more consistent alternative .
Using silencers has taken the critical element out of tail pipe length , the pressure pulses that came from
it and their relections are now damped out by the silencer , after all , thats what its there for. RS Hondas
use a very long pipe of 24/25 dia but the rear cone has a very precise insert of 21 dia at the tail pipe entry
to create the pressure needed, stinger length is relegated to a side issue.
Remember , there are very few heat producing events in an engine , friction and combustion are the two prime
movers , friction is just about the same for every one , so look to combustion and all the events of the closed cycle
for the source of your problems
Hope that helps a little, this is developing into a good thread so keep posting !

Derek ,
Has this motor been pressure tested recently,if it hasn,t spend the hour required to make sure all is well.No major or minor air leaks it is a cause of overheating as well as timing.
regards Dave

Hi All especially Derek!
I still have all my inserts from testing loads of exhausts, and my experiments found that 22 mm was just slightly too small and created some overheating both on the dyno and at long circuits. I then tried 23mm with only very small deteriation in power output, but the engine temperature plummeted and never gave any trouble ever again!
I now use this as a standard in all the exhausts that I do, and so far so good!
You are welcome to all my old "Point S" inserts to experiment with! as will not be doing that one again. as my engine now run at safe temperatures.
Hope this helps
Alan

Hi All

Trevor " I'm timing it static with DTi, aligning the marks BTDC -
DAVE aproximatly - means "me eye" sight of alighnemt static marks ON THE MOTOPLAT rotor and outter stator, not sure how accurate the marks are Im sure they are not as accurate as my DTI - anyone suggest an appropriate strobe ? a catalogue/ or type, would be helpful for me to check out.

Alan "Great- I will double check this, I'm prity sure I started with 23mm as I machined the end piece as detailed by Trevor that Honda do, I believe they call this the "S" Point is this correct Trevor, do you know why its called the "s" point ? maks a big diff on high output GP bikes, I want to keep the thing from distroying itself, so all your help is very much appreaciated.

Even if I know some of them they are propting me in the right direction, as when your building a bike its someitmes easy to overlook the back to basics or obviouse. so please keep them comming
.
I'm planning to try it at the practice day that I make it "Friday Not Thursday"? did someone on here say it was Thursday, this week?.

what you doing this Friday Trevor "My dad lives not far from you, he's planning to make it - I could arrange for you to be collected. and spend a day in the paddock helping - talking - " and I'M sure answering lots of questions", ? how about it.

kind regards

Derek

Hi,Dave,
Sorry it has taken a while to comment on your methanol post, in short , yes on both counts.
To start with, the stoichiometric a/f ratio is 6.25-1 ,whereas for gas its 15-1 so you`ll need a huge
mainjet to pass masses of fuel. The latent heat of evaporation is many times that of gas and this is
reflected in a cool engine . Potentially more power can be extracted from from an engine by using a higher
c/r as methanol wont detonate . A friend of mine used to race an old Greeves Silverstone on methanol ,
and immediately after a race you could almost bear a naked hand on the cylinder !
I have to be honest and say i cant suggest any relevent temps , to do so could be misleading, no advice
is better than bad advice . It should follow that a cool running engine will produce lower temp exhaust gas ,
i seem to remember the the gas is also much denser, so that must be factored in as well .
Not much i`m afraid but i hope it helps a little . Oh, and remember , methanol is very nasty stuff so
be careful , full health and safety precautions ! Let me know how you get on .

Hi all - you may all find this intersteting!.

here we go again tested stripped checked found the following:-

1) we new the piston was uncovering the btn of the exhaust port at top dead center ( do anyof you think I may be getting some mixing of the exhaust gases in the crancase -pre heating of the charge) I have run like this before by mistake but it never made any difference though.
I will have this welded at some point but no time now so will run with it.

2) pressure tested seem ok - I think it was very good ? a slight leek on inlet block and head but nothing to shout about, it holds 10psi for 4.5 mins no problems, before dropping 1 psi

3) exhaust pressure test - front pipe presure tested "wow this its like a cullinder" ? I never knew!! , anyway rewelded all the spots 11 in total just pin pricks. ? I have never pressure tested an exhaust before ?.

4) ignition timing had moved????? (your probably right dave it was about 0.080 thou) was 2.1mm BTDC ? - now Retimed to 1.6MM BTDC and repeating spot on Dave - (still waiting for suggestions on that strobe in the uk anyone?)

5) Hi Alan your not going to impressed with me for this one, (I could kick myself for this) I had bored/measured the tail pipe to 22.58mm i/d with the intention of opening it up after welding by way of,making a small taper piece up to at 23mm o/d then to knocking it into the end of the tail pipe to open it up, so its accurate after welding ( Alans Idea but I'm taking all the credit for this boo boo) - rechecked this beauty and hey - presto "its 21.9mm I/D "WHAT "?? diameter - it must be my eye sight not so good, AND THE WELDING ? (see age catching up on me), well thats my excuse, I can not believe this- I actually turned this ? piece, makes me wonder what the piston clearance is ?, I remember it was tight ?.
all checked ok
anyway all checked and sorted now - tail pipe is 23mm I/D cut it off up on reverse cone away from tail pipe welds to reverse cone then opened it up till correct then rewelded it - , so this has now been rectified.

and its all in the process of reassembly.

I hope to disturb the couple next door tomorrow night !!.

answers and suggestions on post cards please ! I will anounce the winners soon.

Kind regards Derek

Thanks for the reply Tevor,no misleading advice is great advice I appreciate the frankness .I have been using methanol for about 15 years but I,m always used the standard formula,s to good effect.Has anyone played with two stroke computer tuning programs ,because that may be my only other source of alternative information on metanol variables.
regards Dave

Derek ,
Sorry to hear about your shocking discovery.A Longer piston would cure your problem yam ds7 or drop the sleeve out and put a insert in missing spot . If you don,t have a sleeve it,s about time you drop one in
.
regards Dave

Hi Dave, thanks for that.
I had some great fun today at the practice day, I feel this issues may be causing wave issues for the carb ? I had an unusual rich ? /- flat spot, at 6/6500 not really sure what it was, but it did clean up once I had come right down on the jetting, main, power and needle - but we are just guessing.

Testing today - when I did get it right, it seemed fine and much more potent than the other engine, we did about 20 laps in total, so run the engine in nice. no major issues.

I had to stop at the end as the clutch push rod seized on both ball bearings, I'm now currently in the process of changing the clutch thrust bearings to need rollers, also got to put one on the end of the mushroom, this should stop this.

i have entered Mallory 25th even though I know I'm on holiday in cornwall with the family,anyway such is life, I will be taking the bike ON HOLIDAY too, and so will have a very long drive there and back, Her in doors is not impressed" TOUGH. ?

Is anyone driving from cornwall, to the mallory meeting, THE TIME SPENT AT WHEEL I RECON IT WILL TAKE 6 TO 7 HRS ? i don't fancy this, but i have entered anyway, I WOULD BE HAPPY TO share diesel/transport / LIFT ?? WITH ANYONE WHO IS COMING FROM CORNWALL, if anyone to doing this meeting PM me or if you know some one who is let then know this please.

Dave finishing off on the barrel
I am considering having it welded to build up 2/3mm fillets on the base of the exhaust port, any comnets on this, should be ok with a specialist welder.

If I am going to get this done need to get it done this week.

keep you posted.

Derek ,
I don,t get it how can you seize a clutch push rod between the two ball bearings there should always free play (maybe I ,ve always adjusting it wrong) but I am going to in corporate the mushroom bearing in next rebuild).
Man, I am really impressed with your commitment to racing,on a family holiday and going racing .....legend.
Cast craft 100 or 200 is the rods for cast iron welding I personally think there will be too much distortion even with ample and controlled preheating and post heating after welding .Grind measure and maybe a possible rebore if your distortion is too great ,good luck.
regards Dave

holidays are for wimps who like sitting on the beaches, I always find them boring we prefer walking with the dogs, anyway##

welding :- yes I am realy concerned about distortion, it was not my intention to rebore it as I'm only intending to put on a few spots
I have a very specialist company who have looked after me in the past, for this type of thing, I,m hoping they can do it at a very low temp. ? .

what do you mean mushroom incorporated in the pushrod, can you send us a sketch to show us what you mean.

regards Derek

Hi ,all,
Another pipe design section- the diffuser.
When the exhaust port opens a large , positive , pressure wave travels down the pipe. The size of this pulse is determined by the
mass of trapped gas burned during combustion. The values of resulting pressure and temperature then drive the pulse into the
exhaust pipe, in a really efficient Bantam engine this pulse could be as high as 2atm. Upon reaching the start of the diffuser , a
suction wave travells back to the exhaust port to help draw out spent gas into the header ,and ,fresh charge into the cylinder.
The original positive wave continues its way to the reverse cone and is reflected , still as a positive wave ,back to the exhaust port
as a plugging wave .The tuned length determines that , at the correct engine speed , the wave hits the exhaust port just before
closure and rams fresh charge back into the cylinder thus improving charging efficiency , and, power and torque .
As can be seen , where as the plugging pulse is influential over a brief 30/40 degree period , the diffuser works for us over the duration
from transfer opening to about 20 degrees after bdc . It must be concluded that the correctly positioned and proportioned diffuser is of huge
importance.
The important question here is then , what is the the correct position and proportion! The short answer is that every engine is different
and so will be diffuser demands. Over , about, 60 years of of pipe developement a few ground rules have been established that have stood
the test of time and provide for initial guidance.
Yamaha, back in 1971, offered a paper to the SAE that layed down a few specifics based on their experince of the time. They suggest that
the length of the diffuser should be about .3-.4 times the total length of the exhaust system!
QUB , in the late 60s suggest that the effective end of the diffuser should be about .6-.7 of the tuned length .
Accepting these aproximate values , and i have never seen any contradiction here in any published work, we can fix a position for both the
header and diffuser lengths. At the same time diffuser angles of 4-9 degrees are typical for high speed engines of less than GP rating.
My own w/c engine has diff values within this range.
Proposed here is a three stage diff with included angles of 7.5- 12- 16 degrees, as wave energy begins to fade these angles will lift it without
absorbing too much overall to diminish the all important plugging pulse. Afterall there`s not much point in pulling alot of charge into the header
pipe if you havent the energy to stuff it back into the cylinder !
Experimenting has shown that center section diameters much over about 3x the exhaust duct diameter ,for modest output engines, provide little
gain , and, a big fat pipe will conduct and convect a lot of heat away ,slowing wave activity and throwing effective lengths askew.
The cone details are as follows.

1. inlet 49 dia - 72 outlet by 182 long

2. inlet 72 dia - 91 dia outlet by 89 long

3. inlet 91 dia- 108 dia outlet by 62 long

Three cones make an exponential profile for efficiency , energy recovery and have little chance of wave of wave separation from the chamber
wall. sharper angles give more intensity but over a shorter duration. With a threespeed box we need as wide a power band as can be provided
for.
A bit of a ramble here but i hope it helps someone , center section, reverse cone and tailpipe to come , if any one is interested by now !

Regards Trevor.

Hi ,Trevor- no doubt about it for sure great post. -

Im sure people will find it really interesting reading, for all those looking who dont know.

one question is 2atm 2x atmospheric 14.5psi x2 ?? (=28 psi wow)

we must try to keeping these posts somewhere under technical, exhaust design theory/experience

I use to work on Gordon Jennings maths - calulated! but it was so complex. proper job for James

also Trevor see you finished it off in layman's terms too this time, - "perfect" thank you.
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I'm banking on producing another pipe for later in the year, as long as the reliability continues.
I am half way through the one I started but stopped while trying a reliability thing with gear box.
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Now for my Mallory Report:- please feel free to comment "james light hearted PLEASE"

my first meeting in 2011 went ok - great even better when I found out I got some points too. thanks al /james / nick.!

we spent practice playing with the jetting I new at this point I was well over geared, (too tall to pull top gear) you could tell this off the line that it was sluggish. it was a little better in the second after we went down a tooth on the front, we should have gone down two.
practice race one - I was pulling too high a top-gear, would not pull top properly -max 9000 in top, even worse down the back straight but sorted this for the second race, just gearing -

sadly we wanted to look at the crown to plug/ head colour as we have swapped carbs and not really know where we are (excuses excuses) also running avgas, we are finding the plugs difficult to read, anyway - the colour looked dry very light purple/brown, but very clean patch wet round squish area and a small amount of Austrialia in the centrale shape, all looked good except I did miss a very important issue, while putting it back together and it nipped up during the second race as a result. tut tut.

" another excuse " I was disturbed while doing a number of jobs, so next time please if you see me working on the bike or putting it back together, please, please, please, dont come and talk to me" I will only forget something, it could cost me or anyone a lot more than a "nip up". or I may resort to teaching forren languages.

iron barrel - Dad feels we need to warm it up more and for longer before we go out, anyone got any thoughts on this, warm up time for longer we think, I was pushing it to the collecting area starting it when they opened the gate took it steady round siting lap then the usual R.I.F.N.

Fuel economy is unberleavable UNLESS WE HAVE THIS WRONG, we used a total of 1.8 ltrs while i did not go the full distance in the second race - we still did three practices / worked it out to be well over 100miles per gallon, ? and its still a little Rich.

please feel free to comment.

Great to see Tedd out and about, he's OK "he must be" ! "he's still full of it" - and mick - LOL

Having great fun:- kind regards Derek

Hi, Derek and all,
Another pipe design installment.
The reverse cone will reflect an infinite number of positive waves with the value of duration related to
angle and length of the cone. The duration of this reflected wave needs to be engineered to cover the
30/40 degree period just before exhaust closure .Hopefully then , we will have stuffed a good quantity
of reasonably combustable gas back into the cylinder , thus improving trapping efficiency. Remember
also thay as revs rise so the " real time" available for this phenomenon to occur is reducing. Every gas
exchange event in any engine takes time to complete , one reason why 125 two stroke engines don`t
rev to 20,000 rpm.
Exhaust gas flows into the pipe but can only exit from the tailpipe. All these flows are a function of the
driving pressure level , temperature, cross sectional area, length and many other features such as
skin friction , boundary layers and bends . Flow rates will increase with pressure but shear resistance
increases with gas speed . Pressure rises in the whole pipe untill the tail pipe flows the same ammount as
the engine is producing , when the mass flow capability of the tail pipe is exceeded by engine supply
trouble ensues. Temps will go up, flow starts to choke and if the unburned gas in the combustion chamber
reaches around 875 degrees c, detonation is a certainty . Dont go too small on diameter !

A couple of examples from the Honda and Yamaha. All angle are included.
Honda
26 degree cone 195 long from 120 dia
Yamaha
31.5 degree cone 190 long from 130 dia


For our pipe,
23 degree cone 210 long from 108 dia reducing to a 23 dia tail pipe.

Working backwards from the t/l the remaining "gap " is the middle section, which should be long enough
to separate the two main wave events.
As a design exercise, this is a fairly neutral pipe that will get the job done with nothing radical or extreme.
It will enable the new comer, the inexperienced or just plain confused to get on with making their engine
run well and reliably. But do remember, that the pipe is only one feature in the complex world of Bantam tuning!

Hope someone gains a little from this ramble!

Regards to you all , Trevor

looking back at these posts on the exhaust subject, I have found it very enjoyable, its taken 2/3 months from start to where we are now, to be taken through the correct process of designing an exhaust pipe, but its been hopefully a good exercise for all and put some background into why . and what does what job.

So my question now is "what area of the engine development do we ? or should we ? be looking at next" - Trevor ?


thanks again and Kind regards Derek

so trevor, ive been reading a thread on another web site with regards to exhaust design which quoted figures for the cc capacity of the exhaust system and also a ratio of this against the engine cc value...can you tell me what significants these values have and why they are quoted.. thanks