expansion pipe anomoly

those are just examples I mentionned. I use it mostly for a Honda Sky moped with a single exhaust.
it's all about time*area. with a single port you have less area, but with lower rev's you have more time.

the state of tuning is determinned by the choice of BMEP, just use it to see if your timings, area's and rpm's all match-up . if not, lower your goal (BMEP), raise area (more timing or wider ports) or lower rpm (off course, with lower rpm the number might be correct, but the power drop's).

Frits Overmars a écrit:

aljaxon a écrit:

ill be honest and admit i havent worked out my angle.areas  i was hoping that would take care of itself...
looks like i will have to spend some time working it out. but when ive done that if my cylinder ...

Al, I will be glad to help you but you need to spend some of your time before you ask me to spend more of mine.

i have used janbros excel to work out my sta and blowdown and transfer angle. area and i did a little porting and now have 189 and 128
and have reduced peak power to 9500 so X comes out at 27.1 i run out of breath at 9841 exhaust and 9239 transfers.
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my exhaust port effective area is equivalent to 28.1mm

and your pipe says D1 should be 28.3 but the cylinder will only allow 26mm unless i butcher it which i would like to avoid if possible.

i would like the bike to rev as high as the first project motor (176 124) which revs happily past 10500 and on to 12rpm. but the higher my peak revs the larger x becomes. and if i have to start pipe diameter at flange at 26mm im guessing this could spoil things.

can you tell me what you think frits please?

the motor at the moment revs as high as the first motor but stutters at 5-7000 rpm.

if you raise the exhaust timing and want peak power at the same rev's as before, you need a slightly longer pipe, as the time between exhaust open and exh close is also a bit longer .

aljaxon a écrit:

can you tell me what you think frits please? the motor at the moment revs as high as the first motor but stutters at 5-7000 rpm.

Even if carburation, blowdown, transfer, ignition and combustion are perfect, a two-stroke depending on exhaust resonance for its power will have a torque dip at 2/3 the rpm of maximum torque. You can try to tackle it with a rich mixture and a lot of ignition advance (careful !) but you won't get rid of it without an exhaust power valve or, ideally, a trombone pipe.

Frits Overmars a écrit:

aljaxon a écrit:

can you tell me what you think frits please? the motor at the moment revs as high as the first motor but stutters at 5-7000 rpm.

Even if carburation, blowdown, transfer, ignition and combustion are perfect, a two-stroke depending on exhaust resonance for its power will have a torque dip at 2/3 the rpm of maximum torque. You can try to tackle it with a rich mixture and a lot of ignition advance (careful !) but you won't get rid of it without an exhaust power valve or, ideally, a trombone pipe.

a trombone pipe? is that a 3 stage diffuser? thats the only part of janbros excel sheet i cant get to grips with. i might have to ask him for advice. do i have to have a multi stage baffle as well?

to be honest im not too concerned about the dip.
all i want to do now is find out if i can start the pipes header at a reduced diameter 26mm due to the limitations of my cylinder instead of 28mm as per your calcs on the fos pipe.
i know the header will flare out at a sharper angle from 26 to 34.7mm as opposed to the 28-34.7 as per calcs. iirc.
i have some free time coming up in a weeks time inbetween jobs i can spend making a couple of pipes. if it will cause a big loss in power i can hack my cylinder but i wanted to be able to swap pipes on the dyno easily

JanBros a écrit:

if you raise the exhaust timing and want peak power at the same rev's as before, you need a slightly longer pipe, as the time between exhaust open and exh close is also a bit longer .

i only raised the exhaust timing to give me more blowdown as per frits recommendations. because 176 124 was only good for less than 4000rpm !!!

i am using your excel sheet. i do find if i try the 3 stage it comes out the wrong shape no baffle and i realised i can move the percentages. but i havent a clue what they should be?

aljaxon a écrit:

a trombone pipe? is that a 3 stage diffuser? .....
i have some free time  coming up in a weeks time inbetween jobs i can spend making a couple of pipes.

If you want my honest advice, I think your free time would be better spent doing some reading.
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There are several options in lengthening an exhaust pipe. You can move the end cone, or you can lengthen the header, like a trombone.
The gas pressure generates a force that is proportional to the cross section area of the moving part and proportional to the pressure difference at either side of that area. For a moving end cone this force can be up to 6 times larger than for a sliding header. That is one reason to go for the trombone system rather than the moving cone system.

The second reason: sealing. The circumferential gap that has to be sealed, is three times shorter for the trombone system. That means three times less leakage and three times less friction.

The third reason: say you wish to lengthen the total length of the pipe by 10 %. If you do it by moving the end cone, you will also enlarge the pipe volume by a little over 10 %.
But in a good pipe configuration the header length is about 1/3 of total pipe length, so in the trombone system, lengthening the pipe by 10 % will be done by lengthening the header by about 30 %. That gives a far greater variation in the pipe's Helmholtz frequency than a 10 % volume change.

It is true that the length percentages of all pipe components should be in a rather fixed relation to each other. Varying the lengths of all components by the same percentage would be the theoretical optimum, but that is not feasible.
Lengthening the belly will disturb the optimum relations, as will lengthening the header. So the pipe in its lengthened version will not be the optimum for the low resonance rpm dictated by the length. But it will be a hell of a lot better than using an exhaust power valve that spoils the 180° effective exhaust timing, necessary for true resonance.
And a pipe shortened beyond its optimum may not show the optimum length relations between its components either, but it will be a lot more effective in overrev than artificially raising the exhaust gas temperature by retarding the ignition, or by weakening the mixture strength through closing a power jet, which has the disadvantage that not all inhaled air is used for combustion.

Video courtesy of Rimar Motors:
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There is a whole lot more where this came from. You may want to take a  look here:
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and here:
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and here of course:
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thanks fritz, i didnt realise you meant trombone as in the sliding instrument. i used to play a trumpet and thought you meant an exponential type diffuser.
my motor swings backwards as it revs and i have often wondered if it would be worth utilising that to get the pipe shortening. and by using 3 sets of springs of differing tensions i could get all three components shortening in the right proportions.
but i think that is for the next life.

i read a lot of that ESE tuner thread last weekend as i was chasing up info on Luc Foekema to see what he recommended re pipes and if he had any formula or software out there.
and i downloaded your stuff and read it a few times including last again weekend. thanks for putting it out there.

for now i just want to know which is the best way to go re 28mm or 25mm as the start to my header.
start at 28mm as per calcs and the rest of the pipe follows the specs on your fos pipe.
or start at 25mm but the rest of the pipe conforms as if i had started with 28mm.
what do you think?
i could make the two pipes and dyno them. to see whats what.
i am already set on making one that a pal designed for me using torqsoft software. reverse engineering it its pretty much similar to yours and blairs recommendations. belly length 9.5%, yours is 8% and blairs 11% and so on.

also the two pipes i made and fitted to the 176 124 motor it would be interesting to see how they perform on the mk2 motor with proper blowdown. they both gave power 500 rpm lower than design so my temps are lower. exhaust wrap?