expansion pipe anomoly

i am a hobbyist playing with small mopeds just for fun.
i  have just built two pipes and on the FOS inspired pipe i noticed a strange dip in the power delivery curve just after peak power and have seen this on another pipe made by a well known uk pipe maker on the same dyno fitted to another bike. fitting 2 different pipes to my motor with same porting and carb ignition etc dont display this strange dip.

i realised due to changes in attaching to the cylinder ie making it longer by 23mm that the pipe deviated from the FOS single diffuser layout.
i wonder if this 23mm is enough to cause my problem.
my percentages were as follows
header plus duct 0.37
diffuser   0.31
belly .077
baffle 0.255

but by simply deducting 23mm from the start of the header i am now back at  header=0.34 diff 0.32 belly 0.08 and baffle 0.26 ie the FOS recommendations.
i am yet to dyno to see if coming back to the FOS percentages will remedy this.
is 23mm enough to throw the power delivery out that much?
the pipe curve is the blue one.
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i cut 23mm off the header so that all the dimensions were in the FOS percentages exactly and the pipe performed worse on the dyno and still had the dip.
we dont know what to do to get rid of the dip?

Aljaxon, don't you think some more info about the engine might help? At the moment we know nothing, not even if it's a two-stroke.
And while you're at it, a short introduction would be appreciated as well.

Hi Frits, i am a hobbyist working out of his shed with a foredom porting tool and a tig welder and a calculator and lots of enthusiasm but little experience. based in the uk aged 61.

the bike is a 1974 mobylette 50V fitted with an AV10 motor with crankcase reeds as std. fitted with a parmakit 50mm bore cylinder which with a std stoke performance crank gives 82cc. the durations are 176 126. the port layout is single exhaust port with 2 main transfers 2 auxilliary and 2 boost transfers.

i was aiming for peak power at 9250 rpm. timing is at 1.1mm static. 19mm dellorto phbg clone.

with an off the shelf doppler expansion chamber that with my porting has a tuned length to suit 10800rpm i get max power of 10.8hp at 9250rpm. so i thought i would design a single stage exhaust pipe using your percentages and also do a copy of the doppler pipe but lengthen everything by a small percentage to get the desired tuned length to suit my motor and my desired peak of 9250.

the FOS pipe is in blue on the graph and the doppler modified copy is in green. notice the peak is at 8k. i used 500m's as speed of gas. i am positive temps in the pipe are all over the place but wondered if this will alter the pulse speeds enough to warrant a change in the percentages.

since that dyno session i shortened the header by 23mm to get the percentages exact with your recommendations but the dyno showed a slight decrease in power everywhere and the dip at 8500 remained.
the 23mm error was due to a miscalculation in what i thought my connection was going to be. removing 23mm from the header brings everything back into spec.

my headers are segmented and im wondering if this is robbing power. i am in the process of making some new headers with 15 segments as opposed to 8 and also some that a pal is going to hydroform for me.

i see some experts -Blair? suggested a short parallel length from the duct at 10% or so of tuned length before any taper? im wondering why.


to me the power curve looks like their is something amiss. i dont get this dip with the off the shelf doppler. but the green curve = my doppler mod shows a little bit of an anomoly after the peak. so it could be my welding/rolling.

i was hoping altering the header length or belly length might bring the two "peaks" closer together or get rid of the dip..

ive seen this dip before on a couple of pipes made by known pipe makers local to me.
we tried a gas analyser and found there was a richness coinciding with the dip as shown on the graph.
a few runs later we got the fuelling better but the dip still remained.

the bike with the off the shelf doppler will ride for 10 miles at a time at 3/4 full throttle and temps are fine 125-135 at the spark plug. cruises at 90kmh and top speed is just over 100 kmh on the flat gps no wind.

your pipe specs gave a lot more power below the peak but then i get that dip.
i did not work out my exact port size and just used the duct diameter as the basis for my pipe design.
because i cannot easily change that. but i can if i must.
i have just bought some vinamold and am going to get my porting more symetrical. the piston wash isnt perfect but looks fairly ok to my untrained eye.

Aljaxon, thank you for the engine details and for your introduction.
Your previous post lead me to think that there is more to an engine than just the pipe; I had some suspicions about the inlet side - reed fluctuation and all that. That may still be the case; I have no way of calculating it.
At the moment, I can only do a rough calculation of the blowdown angle.area and of the X-value in the FOS exhaust concept.
You wrote that you used the exhaust duct diameter as a basis for the pipe design but more often than not the original duct diameter is already off.
If I encounter any probable causes for your camel-like power curve, I'll get back to you.

thanks for taking the time to reply Frits. like i say ive seen this pattern on other pipes.

and with other pipes i do not get this dip.

cheers.

Aljaxon, I still had a fair amount of guessing to do. Your 50 mm bore and 82cc cylinder capacity should have a stroke of about 41,8 mm and a con rod length of about 85 mm. I also had to gues all port dimensions and the combustion chamber volume. But don't bother to send me those values; I would still need a lot more info.

The disadvantage of a short-stroke engine really shows up: 126° transfer timing may be good for maximum power at 10.000 rpm, but the blowdown angle.area from the 176° exhaust timing is only good for 3400 rpm, so it's a miracle that your engine revved as high as it did, and shortening the pipe only forced it to rev even higher, which it obviously did not like.
Assuming you are stuck with the 126° transfer timing, the best thing to do would be to raise the exhaust timing to no less than 200°, good for blowdown at 9250 rpm. This would give an X-value of 25,4 mm and an Lmax-value of 951 mm for the pipe calculation.
A 82 cc engine could also do with a bigger carburetor: at least 26 mm, but your reed valve would have to be up to it as well. That's up to you; I have no idea what a 1974 Mobylette 50V fitted with an AV10 motor looks like.
In any case, you might advance the ignition timing right now to 1,6 mm; that's about 20° bTDC.

thanks for that. it is an 85mm conrod and 41.8mm stroke.
i have several engines and this particular one i wanted to cruise at around 8k rpm with a top speed of 10k rpm. and i thought 176 126 was a good combination.
i will have a look at the exhaust port to check the actual area and raise it a little bit. am too scared to go straight to 200 duration.
i do have a spare 21mm carb and will see if that improves as well.
i found with timing more than 1.3mm it got hot. it is air cooled and the fins are tiny. i have fitted legshields to get a ram air cooling effect.

aljaxon a écrit:

i wanted to cruise at around 8k rpm with a top speed of 10k rpm. and i thought 176 126 was a good combination

8000 rpm is clearly in excess of what your actual blowdown angle.area can handle, so hot combustion gases will enter not only the exhaust duct but also the transfer ducts, quickly heating up de cylinder. Raising the exhaust port will increase power and reduce the engine temperature.

Advancing the ignition timing tends to lower the exhaust gas temperature, which should be a good thing. But this also lowers the resonance frequency of the exhaust system, perhaps bringing it more into line with the revs you are using, thus increasing the cylinder filling, and the EGT....

Frits i am going to copy and paste the info about blowdown i used. its a bit long winded and the english is a bit poor. its aimed at mopeds with variators.

blowdown timing



Blowdown is best set up using a % of your transfer port timing as a base guide.

For single EX-port kits you want the top of your EX-port to be flat in the center of the top of the port. This flat area with shod not exceed ether 50% the combined with of the transfer ports or exceed 55% of the EX-port with. This will be close to 1/3rd=33% of the bore if you have good wide porting.

Blowdown tuning starting points for single EX-port kits.

16% to 17.5% of transfer timing with stock or weak hitting pipes up to 8,500RPMs with 154deg to 165deg exhaust.

17.5% to 18.5% Good timing for strong pipes working in the 7,000RPM's to 9,200RPM rang with 164deg to 170degs exhaust.

18.% to 19.% Hard hitting pipes working in the 7,500RPM to 10,500RPM rang with 168deg to 174degs exhaust.

18.5% to 19.5% This is near the max for most one speeds with clutches engaging in the 4,000 to 5,000RPM rang,

With hard hitting pipes working in the 8,500RPM to 11,000RPM rang with 172deg to 178degs exhaust.

This is the end of the road for 90% of the one speeds, now entering the variomatic zone.

19.5% to 20% Is for big pipes working the 9,000RPM to 12,500 with EX-port timings of 176deg to 180deg.

20% to 20.5% If you don't know what your doing just stop, this is for 178deg to 187 EX-port timings Spinning 10,000RPM to 13,500+RPM.



Exhaust timing:

138-150: Stock on most Mopeds to keep you under 30 MPH.

151-161: Low RPM powerband timing.Good for low & mid rang torque power, up to 7K-8K RPM rang.

162-168: Very usable timing. Good mid rang torque & power up to 8K-9K RPM range.

169-175: Best all around power band timing. Good upper mid rang torque with hi RPM power up to a 9K-10K RPM range.

176-182: Gives weak power under 5K, good power at 7K, and will rip from 8K up peaking from 9K up to a 10.2K-11.2K RPM range.

183-187: Gives no power under 5.5K very weak upper mid ranger, some power at 7K, good at 8K, starts to rip from 9+K, pulls to the 11.5K-13.5 RPM range.

188-198:Very limited use. Almost no torque with a narrow high high RPM power band at Pro use racing high revs.

Old single EX-port timings chart/notes.

Blowdown of 16 to 20: Stock on most Mopeds to keep you under 30 MPH with the best MPG.

Blowdown of 18 to 22: Low RPM powerband timing.Good for low & mid rang torque power, up to 8K-9.5K RPM rang.

Blowdown of 20 to 24: Very usable timing. Works best with weaker hitting pipes. Good mid rang torque & power up to 9K-10.5K RPM range.

Blowdown of 23 to 26: very good all around Wide power band timing. Will work with most pipes. Strong upper mid rang torque with hi RPM power up to a 9.5K-11K RPM range.

Blowdown of 25 to 27: A good pipe is a must. Best all around power band timing. Riper upper mid rang torque with hi RPM power Wide power band with a big power hit. Best power for everyday use up to 10K-11.5 RPM

Blowdown of 26 to 28: Low power under 5K, Good mid range. works best with pipes that peak hard in the 8K to 11.5K range fading/topping out at 12K This the max for most street set ups.

Blowdown of 28 to 30: Works best with a tuned variomatic or shifter engine, Very low power under 6K. Weak mid rang under 7.5K. The power band starts to get narrower and more peaky. needs a HIGH RPM pipe. gives the best power in the 9k to 12+K rang, will pull past 13.5K with some set ups.

Blowdown of 30: At this point 99.% of you will just have a door stop. Very narrow power band. No power at RPM's under 8K, higher power at high high revs. You will fry most one speed well tuned clutch's to be able to ride with a blowdown as high as this, You need a well tuned variomatic or shifter engine that can spin over 14K.

Blowdown of 32+: Way too high for any 1 speed or 2 speed automatic. This is only ride able at the race track on the very very best tuned variomatic shifter engine.

As a rule of thumb, a blowdown higher then 25deg isn't for everyday use on an automatic with full clutch engagement under 5,000RPM. Don't go higher than 22deg if you don't know what you're doing



Now for the split/twin and three port exhaust with these you can have the flat area of the top of the EX-ports that is two times wider in the flat zone. So now it's close to 66%=2/3rds of the bore instead of one 1/3rd with the single port exhaust that's 100% more area. With 100% more flow area with we need less blowdown timing to mach the wider area cross section.

Blowdown tuning starting points for split/twin and three port exhaust port kits.14.0% to 15.0% of transfer timing with stock or weak hitting pipes up to 8,500RPMs with 156deg to 165deg exhaust.

15.0% to 16.0% Good timing for strong pipes working in the 7,000RPM's to 9,200RPM rang with 164deg to 170degs exhaust.

15.5% to 16.5% Hard hitting pipes working in the 7,500RPM to 10,500RPM rang with 168deg to 174degs exhaust.

16.0% to 17.0% This is near the max for most one speeds with clutches engaging in the 4,000 to 5,000RPM rang,

With hard hitting pipes working in the 8,500RPM to 11,000RPM rang with 172deg to 178degs exhaust.

This is the end of the road for 90% of the one speeds, now entering the variomatic zone.

17.0% to 17.5% Is for big pipes working the 9,000RPM to 12,500rpm rang with EX-port timings of 176deg to 180deg.

17.5% to 18.0% If you don't know what your doing just stop, this is for 178deg to 187 EX-port timings Spinning 10,000RPM to 13,500+RPM.

----------------------------------------


me again
using the above info i went for 176 126 and my moped reaches 65 mph and has max power at 9250k rpm and revs to 12k
with the doppler original pipe i get the following hp curve.
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is any of the copy paste useful or is it wrong?

aljaxon a écrit:

Frits i am going to copy and paste the info about blowdown i used.
is any of the  copy paste useful or is it wrong?

Forget all about it. Burn it.
Where did you find this?

hahaha it was on an american moped forum called moped army. and is referred to on a few websites.

plus here he gives tables.

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i have seen you suggest the ex port duration should be 190. is this just for the aprilia 125? surely not for my moped that i want to rev only to 11kpm.? with a peak at 9k for cruising. thats why i went for 176. or should that be 190 too?

i have seen you mention blowdown angle.area where its best to work out the area of ex port open for each degree and add them up as you go along and do the same with transfer angle area. this will be very tedious for me to do but i am prepared to do it if it will give me a good idea of what to aim for.
i am worried your guidelines are for modern 3 port ex port cylinders and revs to 13000rpm.
mine is a single ex port. no way to fit aux ports due to studs. i could move them but there isnt much meat in the cylinder behind the liner. certainly not for the teapot arm transfers and so on.

i appreciate you taking the time to point me in the right way.

aljaxon a écrit:

hahaha it was on an american moped forum called moped army. and is referred to on a few websites. plus here he gives tables.
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Forget all about that too. I know where this comes from. It is hopelessly outdated and following that advice could cost you a cylinder.

aljaxon a écrit:

i have seen you suggest the ex port duration should be 190. is this just for the aprilia 125? surely not for my moped that i want to rev only to 11kpm.? with a peak at 9k for cruising.

The advice I gave was calculated specifically for your engine and your desires.

aljaxon a écrit:

i have seen you mention blowdown angle.area where its best to work out the area of ex port open for each degree and add them up as you go along and do the same with transfer angle area. this will be very tedious for me to do but i am prepared to do it if it will give me a good idea of what to aim for.

Yes it's tedious; that's why I did it for you.

aljaxon a écrit:

i am worried your guidelines are for modern 3 port ex port cylinders and revs to 13000rpm.

These guidelines are supposed to be universal, for any size and any purpose of two-stroke engine, from 800cc MX singles  to 36,000 rpm multi-world champion model engines

thanks for that frits.
im a bit of a coward to go straight to 190. part of me wants to go half way to 184.

i need some power at 6000 rpm because thats what my (auto) clutch bites at. will 190 still give me some?

i thought something was up with those recommendations because a pal has just gone 180 120, inspired by go kart tuners he says and it goes very well.

Why is everything relativised?

Hi Frits, from what I've read in the 125 RSA topic, I understood that you believed that a value close to 180° exhaust timing was the best compromise.
Above, you are recommending 200°, is this because the AV10 cylinder layout is far from what it is in the RSA, so the best way to improve blowdown is making the exhaust port higher instead of wider ? Or because it is an automatic transmission that does need less powerband width?

aljaxon a écrit:

i have seen you mention blowdown angle.area where its best to work out the area of ex port open for each degree and add them up as you go along and do the same with transfer angle area. this will be very tedious for me to do but i am prepared to do it if it will give me a good idea of what to aim for.

for free for everybody :

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carlovitch1 a écrit:

Hi Frits, from what I've read in the 125 RSA topic, I understood that you believed that a value close to 180° exhaust timing was the best compromise.
Above, you are recommending 200°, is this because the AV10 cylinder layout is far from what it is in the RSA, so the best way to improve blowdown is making the exhaust port higher instead of wider ? Or because it is an automatic transmission that does need less powerband width?

200° exhaust timing in the AV10 cylinder is far from ideal. It was forced by Aljaxons message that the wanted maximum power around 9000 rpm, combined with the 126° transfer timing that already was too high for these revs. His message that he could not add auxiliary exhaust ports, did the rest: it forced the exhaust timing way up in order to achieve the blowdown angle.area that was necessary to prevent the engine from severe overheating at the desired revs.

Making a powerband narrower will not improve maximum power. Increasing the blowdown angle.area beyond its optimum value or making the exhaust pipe reflector shorter and steeper will narrow the powerband alright, but it will only cost power, not enhance it.

Thanks for your precise reply, Frits, as always !

While we are at it, do you have a kind of rule of thumb, Frits, about best the transfer and exhaust timings versus the targeted max power rpm range?

carlovitch1 a écrit:

While we are at it, do you have a kind of rule of thumb, Frits, about best the transfer and exhaust timings versus the targeted max power rpm range?

Yes I do: forget anything that has been written about any relationship between max.power rpm and port timings.
Just try to get the time.areas right:
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Thanks for reminding with this, Frits, I obviously need to re-read this part.

frits i hope you see this, i see you once said the suction wave can travel as slow as 200m/sec. could the stuffing waves be killing the suction waves because my blowdown is wrong? and also do all the pipe making programs/ software just allow for suction and stuffing waves to be the same speed but really we muct use slower speeds for calculating the divergent cone dimensions and start point??

aljaxon a écrit:

frits i hope you see this, i see you once said the suction wave can travel as slow as 200m/sec. could the stuffing waves be killing the suction waves because my blowdown is wrong? and also do all the pipe making programs/ software  just allow for suction and stuffing waves to be the same speed but really we muct use slower speeds for calculating the divergent cone dimensions and start point??

Stuffing waves can kill suction waves (and commit suicide in the process) if the length percentages of the various pipe segments are wrong.
You question regarding 'all the pipe making programs' is impossible to answer; I don't know them all and I suppose nobody does..
But I do know that my favorite program, EngMod2T,  takes account of the different wave velocities.

Frits Overmars a écrit:

aljaxon a écrit:

frits i hope you see this, i see you once said the suction wave can travel as slow as 200m/sec. could the stuffing waves be killing the suction waves because my blowdown is wrong? and also do all the pipe making programs/ software  just allow for suction and stuffing waves to be the same speed but really we muct use slower speeds for calculating the divergent cone dimensions and start point??

Stuffing waves can kill suction waves (and commit suicide in the process) if the length percentages of the various pipe segments are wrong.
You question regarding 'all the pipe making programs' is impossible to answer; I don't know them all and I suppose nobody does..
But I do know that my favorite program, EngMod2T,  takes account of the different wave velocities.

thanks for that, im assuming your FOS pipe design percentages also account for it or could a bit of tweaking be required? i know that not all motors/pipes run at the same temp and i usually find pipes i design have the peak power lower than expected possibly due to the temp/speed being slower.
with me being merely a hobbyist i cannot justify spending a lot of money on what could prove to be complicated software. so im using yours and blairs recommendations which are only a few percent out in various areas.
thanks for putting your info out there for all to see btw.