Hi,
Can anyone give advice please? If you need to shorten a piston skirt, how do you rebalance the piston?
With the method in the left column or the right column?
Best regards Jan.
Scooter boy
Hi,
Can anyone give advice please? If you need to shorten a piston skirt, how do you rebalance the piston?
With the method in the left column or the right column?
Best regards Jan.
L-Plate Rider
Hello Jan. I would say balancing it like on the right side of your drawing makes more sense. Imbalance between right and left side of the wrist pin leads to a torque around the wristpin when Piston is accelerated. Would be interessting how high the resulting forces are.Originally Posted by Jan Van Hamme
Hi,
Can anyone give advice please? If you need to shorten a piston skirt, how do you rebalance the piston?
With the method in the left column or the right column?
Best regards Jan.
Moped rider
i missed this post. i did search to see if you can weld aluminium with dc and a youtube video showed a guy doing it. he reversed the polarity and the welds looked terrible but held fast. i tried it with a flux coated stick scrounged of a welder pal last year. he told me to scrape the flux off? i may have tried both ways i cant remember it was last year on a stud hole i'd gone through via the ex port. all i got was just a sparkly fizzing mess. some guy said you need helium?
ive only wanted an ac rig for almost the last 10 years and its nearly xmas....
as for pipes i know without a sim and just a dyno im not going to be able to test and refine and then repeat this noting improvements.
peeing into the wind trying to find the best pipe.
i will make two pipes for this project same percentages length wise but slightly different angles and just select the best performance.
it will be pot luck narrowed down by using recommendations on here etc.
i made 2 sets of pipes for my rd350 and settled with the second set and that was 12 years ago. im know they could easily be bettered but they seem good enough.
this was the constrictor on my mobylette that i didnt want to change. i did bore it out so it was wafer thin. and on another cylinder i binned that mounting idea and used studs.
Moped rider
lol isnt that the case. just with tweaking an exhaust design never mind all the variables you have to contend with.
Moped rider
hi frits, just read your helmholtz lesson translation and i think im 11yrs old.
im struggling to understand how when the pressures in Pcyl and Pout (pipe) are equal, that gives you maximum speed of the mass. isnt it already slowing down at this point?
all ive ever read about with pipe design is to do with the sonic waves and very little to the actual gas flow which i found weird because at first glance to the uneducated you would think an exhaust was only about gas flow. that is until you learn about the sound waves helping hands.
L-Plate Rider
If I could help youHello fellow 2-stroke sufferers,
I want to do a report of my High Power 50cc project after some 2 years of trials and tribulations.
In 2021 I cast this 50cc watercooled cylinder from cast iron (the sewing machine, some may remember).
It was supposed to be a record breaker. I based the radial transfer angles on Frits' MB40 transfer angle picture: https://servimg.com/view/19023519/1#
The axial angles were 20*(A), 10*(B) and 55 for C. (Now that I look at this again, I realize that the axial angles in Frits' concept are much different...doh.)
The inner wall is a "generous" bulge without much thought put into it. The transfer entries are also not much thought, I just drew something that roughly resembled Frits' MB40 cylinder.
The cylinder is mounted to AM6 engine using a 15mm thick spacer plate which is probably very unideal. The transfer ducts are very short.
The exhaust duct is a normal 3-part, with wobbly's 1.5*bore length and restricted outlet area. The duct is water cooled on the top side completely. The auxiliary ducts are very long and quite slim, later I shortened the bridges quite a lot.
Main exhaust port floor is 3mm above BDC.
I made a lot of errors with the exhaust port design. At first I made it too low and the timing was only about 185 degrees. This was because I had thought that a properly designed auxiliary duct would flow so well that
the cylinder would easily surpass a typical power of a stud-compromised triple exhaust even with a low timing.
I made the auxiliary windows quite small, there was a lot of room to enlarge them. Then I made the outlet about 75% of the port effective area, but since the cylinder ports were already small, the outlet area ended up being equivalent to only a 20mm round port which was way too small.
On the first tests the cylinder was about as weak as a unported stock AM6 cylinder with a general powerpipe. My pipe is made with Blair's formulas, designed power peak at 14000, belly diameter 82mm, tuned length 710mm. At first only about 8hp at the rear wheel and not much revs.
First major improvement came from raising the exhaust port to 198 degrees and enlarging the auxiliaries. Peak power didn't improve much but I got way more usable range on high rpm. Second major improvement was again from the exhaust area, by enlarging the duct and header diameter from 22mm to 25mm brought some 2.5hp more power. Record power is now 12.5hp at 12800 measured on rear wheel. With some fine adjustment and a new piston ring I think it could do close to 13. But still that is nothing. This cylinder design is ought to give 18+ rear wheel horses when you think of all the ideas that have been read from this forum and implemented.
In addition to those mods I have also tried almost everything else possible. I have tried compression ratios of 15:1 and 13.5:1 with maybe only a slight improvement on rpm range and less power. I have tried 53% and 40% squish ratio, squish gap from 0.4 to 0.65mm and different ignition timing without much improvement to speak of.
Porting the cases for a more direct reed flow path. Different reed valve. Much stiffer petals. Different manufacturer carburetor. Basically only loses power or there is only a very slight improvement. My last trial was the crankcase compression. Changed the compression from 1.19 to 1.23 with a 45cc stuffer. The crankcase volume at TDC is 266cc by the way. Crankcase compression did nothing, only lost power on powerband beginning.
Current tail restriction is 15mm. I also tried to make a completely different looking exhaust with a 20mm fatter belly and about 30mm shorter TL. Even this was a failure, there was noticeably less peak power but the rpm range was pretty much identical.
I still have some hope of getting a bit more by enlarging the exhaust duct further. Apparently the 75% rule doesn't seem to work in a 50cc cylinder. At least I can assure it doesn't work if the cylinder ports are even a little bit on the small side.
But I would be surprised if enlarging the duct a bit more would give more than 1hp. It may be the case that the FOS-MB40 transfer concept does not work in a 50cc cylinder, or then I have done something wrong. Many people think that it is the missing B duct hooks that cause it but I really doubt that. 2strokestuffing had already tried to add the hooks and it didn't seem to do much at all. (but he too had a badly restricted exhaust channel which probably hinders all other changes). I have also tried to choke the B ducts with 3D printed plugs but didn't get a breakthrough. Funnily enough, adding a plug which reduced the entry area to maybe 15% of the original, in only the LEFT B duct, didn't make any difference to the power curve. Adding plugs to both ducts reduced power a lot.
Description for pictures:
1. cores in the mould. The water flow is so that cold water enters the cylinder head from the C port side and flows out above the exhaust duct. The engine stays remarkably cool under driving conditions with a standard Derbi moped radiator, which I think may be an indication of the effectiveness of the "reversed" water flow heat transfer. Or then it is just due to low power.
2. transfer duct top view
3. B duct side profile. The duct is modeled in the cad as if the cylinder bore was 36mm for core making reasons. The 2mm straight part is bored out.
4. Cylinder with some machining done
5. Cylinder adapter plate in the cases. There is also a resin-silicone-mess crankcase stuffer there which failed miserably later.
6. Exhaust port after raising it to 198(main port, aux is a bit lower)
One friend has an AM6 engine with an original single exhaust port Minarelli cylinder, another guy ported that cylinder and the bike pushed out 12.4hp to the rear wheel, so my cylinder is clearly failed. I am desperate to get more power next year but haven't yet made up my mind what to do next. Maybe a new cylinder but what to change?
1 the water jacket is too far from the transfers and in my opinion you need a lot more water around the exhaust duct, for me it stays cool because of the lack of power
2 transfers are totally wrong you need to measure the port area times the sin (axial angle) and then increase 7.5mm2 every 7.5mm of duct until the end of the duct, max area at the bottom need to be around 1.25-1.28 time the effective port area.
About port cordal lenght I think a good start is A port 0.426*bore, B port 0.41*bore C port 0.33*bore, Main Ex 0.71*bore and Aux 0.35*bore.
Position of the transfer ports really important: the divider from A to B (on the A side) move toward C port 0.12*bore.
Radial angles
A port: ex side towards the center of C, B side 15° towards opposite B (0° would be parallel to the piston pin)
B port : A side 10° towards C, C side towards the opposite exhaust divider
Axial angles
A port 25° min
B port 15° min
C port 55° min
3 totally wrong you have to adjust the shape and the area
on an am6 50cc I think 195-196 main and 191-192 aux would be good. the area of the aux duct at the dividers needs to be around 50% of the effective area of the aux port. 78-80% of the total area on the exit it's good
do a big staggered configuration on transfers like 132.5 128 126
If you make that corrections I think you can achieve 320-330cv/l (real)
PS
first you design the ports the rest of the duct geometry is a consequence
Scooter boy
Thank you for explaining.
I have now calculated it at 11000rpm. I arrive at extra force of 9.275N perpendicular to the cylinder wall at the bottom of the skirt.
bore 57mm x stroke 50mm
Distance center wristpin to bottom skirt 33mm
Cutaway mass =4g
F=m.a
s=a.t2/2
v=a.t
M=F.s
That is 22000 times per minute 9.275N extra force on and off on the skirt at max rpm.
Forum whore
..........
Forum whore
.
Not sure if this is any help. Modified piston at the top. Suzuki RG50 Gamma 13 RWHP @ 14,000 rpm.
We cut the skirts on our Suzuki RG50's right back to get the inlet timing we wanted.
The exhaust side was left with enough to cover the exhaust port at BDC and the inlet side was cut back to the piston pin bosses.
Never had any problems with the pistons themselves.
L-Plate Rider
This were only my thougts about your problem. As I am only a amateur tuner i dont know if balancing piston plays any role at all. There are so many forces while a piston engine is rumning. I am not sure if 9N makes any difference.
Around two years ago I asked some questions in this thread about getting more power out of Honda MBX/MTXNS1 80cc engine.
When time allows I am still playing around. A few days ago I tried shorter intake. I followed Frits advice of making manifold as short as possible. Frits idea is to mount the carb via pinged O ring. I have to thank Frits for this idea.
Compared the shorty intake against an intake made of rubber, which is about 10mm longer. shorty gives more power across the whole power band. I am not smart enough to explain what exactly happens. But seems like reed Petals are open for longer time.
I think most people here already know about this effect. But maybe this is of interesst for some tuners. For this small amount of effort it is a good power gain. Around 0.4HP out of 20HP.
Forum whore
To answer why intake length ( from bellmouth end + end correction to the reed tips ) is important and is capable of adding considerable Hp.
There are two pressure ratios that affect reed opening and the duration of the this lift period.
First is depression within the case , and second is the wave bouncing up and down the intake tuned length , off the atmospheric end.
The optimum length for a specific rpm is when the two effects are operating together , but of opposite sign.
That is , the case goes negative at the same time as the intake has a positive pressure ratio at the reeds - this opens them quickly and holds them open.
The effect has an effective range of around 2000 rpm and is most usefully tuned to be near peak torque.
Here is an example of where the intake needed lengthening by 20mm to get the red trace ( case pressure ratio ) crossing the zero line at the same time as the
intake yellow line crosses zero ( intake pressure ratio ).
Thus we get the most efficient sonic " supercharging " of intake flow into the case as shown by the reed tip lift rising dramatically as the two opposing pressure ratios act in concert.
The effectiveness of this is also shown in KZ engines , where with peak power around 13500 shortening the intake length by counterboring the reed flange face and inserting the carb
4mm closer is worth close to 1 Hp in 50.
Ive got a thing thats unique and new.To prove it I'll have the last laugh on you.Cause instead of one head I got two.And you know two heads are better than one.
Scooter boy
I read on forums when people adjust piston skirts (enlarge windows, extra ports, shorten) this often leads to breakage. The explanation on forums about this is that you weaken the piston and that you disrupt the balance of the piston..
Not sure if this is any help. Modified piston at the top. Suzuki RG50 Gamma 13 RWHP @ 14,000 rpm.
We cut the skirts on our Suzuki RG50's right back to get the inlet timing we wanted.
The exhaust side was left with enough to cover the exhaust port at BDC and the inlet side was cut back to the piston pin bosses.
Never had any problems with the pistons themselves.
That's why I asked myself "how do you rebalance a piston?".
To achieve the desired STA in Engmod2T I need an inlet area of 1277mm2.
The existing inlet in the cylinder must be enlarged. The inlet consists of 3 slotted holes. See picture.
There are 2 windows in the piston skirt and these are not the same shape as the cylinder inlet. There is obstruction of gas flow.
I would either make 3 appropriate holes in a new piston with a full skirt, or make a large cutout so that the cylinder inlet is free. But that quickly causes wear and rattling of the piston.
I would like to simulate this in Engmod2T, but there is no option to model 3 cylinder intake slots. There is also no option to model a piston with 3 slotted holes. A cutout can be modeled. Maybe it cant be modeled because it is so unusual or because it is not right to do it that way?
Machining piston skirts in a cast piston leads to microcracks and ultimately to breakage. A forged piston can handle this better, I read on forums.
So to try to prevent breakage I would adapt a forged piston to cylinder inlet shape, balance it (but 9N extra force seems completely negligible to me) , make the cylinder inlet slots 1277mm2 and try it.
No8 Wire Extraordinaire
My several iterations ago RG50 was reliable with full skirt (I got more than enough area by grinding std twin inlets downward) until the bore wore in that area. Then it would crack skirts.
The remedy (warning: Dad joke) was boring.
Don't you look at my accountant.
He's the only one I've got.
Scooter boy
Hello Frits,..........
I don't know if your quote was intended for me, but I do feel it applies to me.
I give my quote in a general form.
I understand English, French and a little German.
However, writing and speaking in those languages is a disaster. That's why I translate from Dutch to English. Sorry everyone for the grammar and language errors.
My native language is Dutch, I live in East Flanders, 100m from the Dutch border with Zeeland.
I am an amateur, a modest novice who realizes that he still has to learn everything that others have already forgotten.
There is the Master of 2stroke, Frits, Neels, Wobbly,... and others.
All people with an amount of knowledge and experience that could fill an entire galaxy.
I have deep respect for you and them.
I still have to learn how to roll a cigarette that looks like an expansion pipe. They and you have made countless of them.
I have not yet seen the needles of a needle bearing fly out through the carburetor. They do.
It almost makes me scared in these first posts to ask about something that could be ridiculous.
I read forums, Frits's columns and all his tips and tricks, I try to use/understand a little bit of Engmod2t, I read Jan's masterpiece book in one go.
There is so much info on forums that I have a hard time understanding some things.
2 examples:
Jan Thiel wrote that he has never understood why people try to make pistons lighter. With a heavier piston he could do many test runs without losing power, he said. I believe every word he says.
Frits advises and motivates to round off the piston edge instead of the transfers. Better piston head cooling and flow and behaves like a delaval nozzle on the exhaust side. I would therefore like to provide my piston with such a rounding. Do I also have to make the same rounding in the combustion chamber? If not, am I not creating a detonation in that "lost space" if this zone does not also have a rounding? Is this only recommended for racing? And do you just stay away from it for street use?
A combination lock of 10? I only have 1 of the 10 and I don't even know if it is correct. Let alone the other 9.
That's why I'm asking this novice questions.
Groeten Jan
regards Jan
No8 Wire Extraordinaire
It amuses me that English as 2nd or 3rd language apologise for language, but invariably post more legibly than us native speakers.
Whatever you are using, continue with confidence,
Welcome.
Don't you look at my accountant.
He's the only one I've got.
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