ESE's works engine tuner [Archive] - Page 166

wobbly

10th January 2026, 11:03

Having done hundreds of sims for differing engine projects, one of the many " trends " I have noted is that in a reed engine, especially anything that runs to high rpm's
is that as you increase the case volume, the petals need to be made thinner, thus lowering their 1st Mode Natural Frequency.
This then reaches a point where, even with effective curved stopper plates, the petals loose control and go spastic - smashing into the stops and bouncing off them.
In a sim, running the Inlet dialogue , you can see the tip lift being erratic from one run iteration to the next.
When this happens power drops at that rpm.

The CCR for a reed seems to be limited to about 1.3 - that gives enough volume for the pipe to draw from, and also allows petals thick enough not to go out of control.
But many larger displacement cylinders like a value slightly larger than this ie a smaller case.

EDIT - I have just read the spec sheet above and the Rotary Valve timing is well different from what Frits has given us at 152.5/90. Closing at 88 is the lowest number I have ever seen quoted.

speedpro

10th January 2026, 20:25

Off topic but I had a bit of a chat with SS90 at the 2026 NZ Grand Prix meeting 3&4 Jan at Ruapuna. Not who I expected at all.

Rudex

10th January 2026, 22:13

Wob/Frits

Can you say me what is the sense of using Bushing Bearing Modena KZ kk1? and if it would be better to use plastic insert in whole case (around the crank too) or like kk1?.
Why didn't they just machine the crankcase?

https://www.mondokart.com/10015-square_large_default/bushing-bearing-52-x-56-x-147-modena-kz.jpg
https://i.ibb.co/mVKk32CZ/kk1-case.png (https://ibb.co/LXym3czj)

Regards

JanBros

10th January 2026, 22:42

if the bearring gets lose in the case, you don't have to buy a new expensive crankcase, but simply change the inexpensive bushing ?

Frits Overmars

11th January 2026, 00:20

You have wrote far more and we are greatfull for everpart thats just little what a quick search turned up.
That last translated part i had never seen before? I never had a chance to read it. My own german is limited to what was in comics.
looks like it was indeed translated from something your wrote in 2009 translated using google and posted by RN? 2019?
Ignition Curves Copyright 2009 Friits Overmars - (translated via google translate and some manual editing RN Jan-201999
here is another verson
looks closer to english
https://opensimspark.org/_media/fos-ignition-curves-eng.pdf
Yes, that last version, translated with Deepl, does look more like the king's English; not bad at all.
In case you wish to compare it to the original text and at the same time brush up your German a bit:

katinas

11th January 2026, 01:02

the petals need to be made thinner, thus lowering their 1st Mode Natural Frequency.
This then reaches a point where, even with effective curved stopper plates, the petals loose control and go spastic - smashing into the stops and bouncing off them.

Just add illustration of your words for more clear imagination of what might be happening with the reeds (free oscillating) .

flyonly

11th January 2026, 06:05

54 x 54 twin port cylinder underway. Rear disc valve this one, this is for Vinduro so is 125cc.
This same setup ( bottom end) with a smaller cylinder could well be my next Bucket engine, after what, thirty odd years.

What is the plan to drive the disc?

Sent from my iPhone using Tapatalk

husaberg

11th January 2026, 06:11

Yes, that last version, translated with Deepl, does look more like the king's English; not bad at all.
In case you wish to compare it to the original text and at the same time brush up your German a bit:
Cheers everyday is a school day. But i struggle enough with English.

The CCR for a reed seems to be limited to about 1.3 - that gives enough volume for the pipe to draw from, and also allows petals thick enough not to go out of control.
But many larger displacement cylinders like a value slightly larger than this ie a smaller case.
.

In my search the other day i had seen a reference Frits had made ot the size of he reed valve Derbi case or the Rumi (i was skimming sorry)
Frits referenced it as being i think (if i recall correctly) as being quite a bit larger than the RSA.
i will have a look later an put it in. here...
looks like i asked the question ....

Would that not relate only to a disk valve? I have often seen it mentioned the optimum Crankcase volume would be smaller on a Crankcase reed. but i have never seen a definitive figure.
If you or Jan recalls what were the volumes on the Rumi and the Derbi reed valve. I realise that you may not have the figures available......but a a rough guide would be helpful

Frits Overmars esquire The Aprilia RSA125 has a TDC crankcase volume of 675 cc. That gives a primary compression ratio of 675 / (675 - 125) = 1,23. Rather different from your value....

I remember Jan saying that the crankcase volume of the Derbi reed valve engine was even larger than that of the rotary engine. And after Jan had laid his hands on it, that reed valve engine was the strongest 125 cc reed valver ever.
You see, the volumes of the transfer ducts and the volumes between the crank wheels and inside the piston are largely equal, and the volume of the reed cavity is a lot larger than the volume of a rotary inlet duct.

but your post and looking though an old collection of 1990s stuff i seen an old Boyseen 2 stage reeds packet (i had kept an it listed the reed number on the back for all sorts of bikes so you could cross reference )
357705357706

i got me thinking did those old 2 stage reeds do anything and do they work in such a situation.
They were successful in separating my money from my wallet and the stickers impressing passers by....

did they work in the old days and could they wok on say a VFoorce set up as anyone tried them in a modern reed valve?

Flettner

11th January 2026, 07:16

What is the plan to drive the disc?

Sent from my iPhone using Tapatalk

Don't know yet. Initially I was looking at the twisted tooth belt system, but over Christmas I've had a massive clean up of my workshop and have found a nice little set of helical right angle gears. So maybe that with a drive gear running off the top of the clutch gear.

flyonly

11th January 2026, 07:20

Don't know yet. Initially I was looking at the twisted tooth belt system, but over Christmas I've had a massive clean up of my workshop and have found a nice little set of helical right angle gears. So maybe that with a drive gear running off the top of the clutch gear.

Have you decided on the diameter. I am interested in doing something similar but would need to be twisted belt

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Flettner

11th January 2026, 07:26

Have you decided on the diameter. I am interested in doing something similar but would need to be twisted belt

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I have a large disc, spare out of my 360cc engine, that Ill use. It is all going to fit into a TF Suzuki 125 engine (farmbike) , most probably have to cast a new case to accommodate this disc drive.

Flettner

11th January 2026, 08:06

140 dia, plus small angle drive, hardened teeth.
Disc still to be cut to the appropriate timing.

lodgernz

11th January 2026, 09:55

If I were to channel my inner "wobbly" and attempt to throw down a hand at 'long distance troubleshooting roullette'
My first question is, why so cold, Batman?
Targetting 470c with the pipe design???
That should be ~600c if you want to make real power.
Only achieving a derived (not directly measured with high speed EGT probe....) 340c?
Re-running the pipe design for 340c would NOT be my first advice.
I would first wonder why your pipe is so cold to begin with, assuming it truly is.
The most likely culprit from a general theory perspective would be the tailpipt outlet diameter. Often termed "D_restrictor" in some pipe formulas....
If you have the exact diameter in your pipe on the bike from the formula you used, and you are that far under the formulas target EGT, I would have questions about the engine actually meeting the input parameters that went into the pipe calculator, or the pipe calculator itself, is it the FOS formula?
You should be closer to 600c EGT for an incredibly general ideal.
Being cold either means you are not delivering the heat energy that the calculator assumes you would have been delivering to the pipe or the tailpipe diameter on your actual pipe is larger than the calculator actually spit out.
Taking the tailpipe restrictor diameter smaller WILL increase the pipes EGT, and if you are currently "too large", making it smaller WILL bring with it an increase in power up until the point where you go "too small" and you burn holes in your piston, so there is a balance point to be found, and the pipe formula wont give you the EXACT perfect number for your engine. It SHOULD give you a generally safe place to START, and you tune it from there.
Generally speaking shrinking the tailpipe diameter is going to increase the EGT.
Inreasing the EGT will increase the net temperature of the pipe.
Increasing the temperature of the pipe will make the speed of sound within it FASTER.
Faster speed of sound = faster wave travel.
Faster wave travel = pipes natural "resonant frequency" goes up.
Well look at that:
1. You pipe is too cold (not metting the formula number).
2. Your pipe is peaking at too low an RPM (resoanant frequency LOWER than formula number).
So, logic says:
Make the pipe hotter (decrease tailpipe restrictor diameter) and you will see an attendent increase in pipe resonant frequency, maybe even landing exactly where the formula said you would be once you actually achieve the 470c EGT you entered into the formula.

This pipe was the sixth I have made for this bike, and my temperature assumption for it was based on similar experiences with previous pipes.
The tailpipe de Laval nozzle ID in this pipe is 14.0mm. Frits's formula for the nozzle ID, based on a crank HP of 16, is 12.4mm. This seemed too small to me, but I agree that making it smaller than 14 is the first step.
I'll fit a 13.0mm nozzle and do some more dyno runs. I suspect that a higher EGT will show the Lt of this pipe to then be too short.

Wos has also suggested that EO 192⁰ is a bit conservative. Maybe I need to raise that a little as well.

Thank you both for your advice.

wobbly

11th January 2026, 10:29

F5 Dave

11th January 2026, 18:56

I bought a small electric weed eater for the wife. Only works on small stuff.

The Grass cutterer guy has progressed to doing our lawn with a weed eater as he's transferring from heavy lawn mowers as he's well past 80. He'll probably stop all together soon. Can't rely on the in-laws forever.

So where can I get a tuned weed eater? Sounds like it could make the mundane fun.
As long as it siezes occasionally and throws me to the turf. Roughly.

Jan Van Hamme

11th January 2026, 19:24

Also, if you are absolutely insistent on accelerated engine wear, and loosing Hp, carry on using Motul 800 @ 30:1, I wouldn't use that cats pee if it was free even @ 20:1 where you should be.

Yes, damn it, I've made the “right” choice again.
A few years ago, I bought a pack of 12 x 1 litre cans of Motul 800 road race oil because I thought it was very good oil.
It is unopened and waiting to be used when my motorbike is assembled.
Now, after reading that message, I won't be able to get rid of that “cat's piss” anymore. Does it really cause so much wear and tear that I'd better not use it?

Translated with DeepL.com (free version)

F5 Dave

11th January 2026, 19:28

Works great in my trials bike.

Jan Van Hamme

11th January 2026, 20:59

In case you wish to compare it to the original text and at the same time brush up your German a bit:

Hi Frits,
I had already saved and printed your text on ignition curves for later use.
Now, in order to find and compile an optimal ignition curve, with your text in mind, I ran a simulation (in turbulent mode) in Engmod2T for every half degree of advanced ignition across the entire rev range.
So 8° fully from 3000 rpm to 9700 rpm.
8.5° fully from 3000 rpm to 9700 rpm....
up to and including 28° fully from 3000 rpm to 9700 rpm.

When I combine all the highest horsepower values per half degree in a curve, I get a strange ignition curve.
Namely, a flat curve of 15.5° over a large part of the rev range.
Could that be useful in practice?

Regards,
Jan

Wos

11th January 2026, 20:59

LODGER - I see a few things that have set you off on a dead end tangent.
You say you used an EGT of 470* - this is completely erroneous.
Any engine with that number in the header is about as well tuned as a weed eater.

All of the pipe compute programs are using the BULK AVERAGE PIPE TEMP, including the cooler duct, and this is in the range of 550* to around 625* dependent upon the fuel quality.
This gives an average speed of sound of 560 to 610m/s, thus a pipe average wall temp of around 425*C to 450*C at peak, and the EGT as measured in the header well into the 600*'s.
Quit working in the dark, even after a ton of carrots, IMHO it is absolutely impossible to do any meaningful engine design work, let alone accurate track/dyno tuning without using a data logged or at the very least a max recall EGT gauge.

Also, if you are absolutely insistent on accelerated engine wear, and loosing Hp, carry on using Motul 800 @ 30:1, I wouldn't use that cats pee if it was free even @ 20:1 where you should be.

The Wobbly Duct info sheet also states a recommended cooled duct length @ 1.5 bore for a square engine, with the header entry @ 2X bore - you stated a cooled duct out to 2X bore, this is NFG
as the A/F out at 2X bore is never going to make it back into the cylinder due to the return wave, as the port is closing.

EDIT - again, taking a suggestion the 192* " is a bit conservative ". What the hell does that mean, what is the actual Blowdown STA, and what are the transfers doing. JanBros has a free , really good port calculator.
Quit working in the dark - free technology is at your fingertips.

Dear Wobb,

For sure i am not experienced in diffrent 2 T Oils like you.
We run motul 800 in 8 h race liedolsheim where the best 50cc bikes have about 20 hp13000 and reving over 15000. Mix is 1:25

Which negative experience do you have with the french cats pee:wings: :laugh: :niceone:

Which oil would you adive for pump gas 98 mixture ? :niceone:

Thanks a lot

:niceone: every little step forward is a part of the puzzle ��

Grüße! Wolfgang

Frits Overmars

12th January 2026, 00:41

I ran a simulation (in turbulent mode) in Engmod2T for every half degree of advanced ignition across the entire rev range. So 8° fully from 3000 rpm to 9700 rpm. 8.5° fully from 3000 rpm to 9700 rpm.... up to and including 28° fully from 3000 rpm to 9700 rpm.
When I combine all the highest horsepower values per half degree in a curve, I get a strange ignition curve. Namely, a flat curve of 15.5° over a large part of the rev range.
Could that be useful in practice?
Jan, I dropped my crystal ball, so I don't know anything about your engine. I assume it's a two-stroke, but I'm not even sure about that. That's not much to base an opinion on...

Jan Van Hamme

12th January 2026, 01:38

Jan, I dropped my crystal ball, so I don't know anything about your engine. I assume it's a two-stroke, but I'm not even sure about that. That's not much to base an opinion on...

Sorry Frits,
I didn't phrase my question clearly enough.
The ignition curves for 2-stroke engines that I see here and there always start low in degrees, then rise steeply to +/-30 degrees and then drop back down at the start of the power band. Around max hp and rpm to 15° and from there further down.

What I mean is, an almost flat curve of 15.5° is very different from the usual curve shape I describe above. Would that still be useful in practice?
My engine... See attached screenshots.

wobbly

12th January 2026, 07:21

Wos, I made a post a few pages back about oils and the results I got in a quite extensive dyno test session.
The customer wanted to choose a replacement for R30, they had been running for years at 16:1.

The short story was that at high EGT numbers ( 650*C+) the oils I tested at 30:1, 25:1, 20:1 and 16:1 all made more power going down to 20:1 but no more at 16:1
Two oils made more power than the R30, these were HTX 909 and Maxima 927.
I tested several fully synthetic oils and none got close to the two oils above, and Motul 800 was the worst.
During the testing I decided to check what was occurring with the Motul and did some extended running - and discovered that the piston skirt wear was dramatically worse than the semi synthetic castor mix oils.
It was easy to measure as the wear was double, the clearance as new was exactly 0.002" ( the customer wanted Imperial numbers, as that is what he was honing to ) after1 Hr of hard running the Motul was 0.003"
where as the Maxima was 0.0025".

This result confirmed what I had read in the well known DynoTech publication ( Snowmobile guys with a real good Superflow dyno setup ).
They did alot of oil tests as well, and the technical issue is that when run at very high temps the Synthetic breaks down into its component chemicals, none of which are actual lubricants.
The Castor based oils when overheated, they break down at a lower oil film strength, but the component parts still lubricate very well.
Many 4T experts rubbish the Timken/Falex oil test rig, saying its not realistically representing what happens in a running engine.
But the rig results mirror exactly what happens in a 2T - Synthetics can have 10X the film strength of a Castor based oil, but the instant the film is broken the surfaces get physically torn up badly.
The Castor film fails much earlyer, but the wear pattern on the test drum is dead smooth.

Thus since that result I have been using Vrooam on track, and Maxima 927 on the dyno in the KZ engines , both semi synthetics, where we see 685*C EGT when running 110 octane.
The Vrooam is very clean, and hardly gives piston maps at all, but that is irrelevant when using EGT to get within one jet size of deto every run session.

HTX 909 will separate out of AvGas in about 1/2 Hr, but it and the others have no issues in 98 pump gas.

F5 - of course Motul 800 is just fine in a trials bike, not much more high temp loading then a weed eater.

EDIT, the other pointer I have is that I rebuild many KZ cranks, all after the same logged run time. The cranks run on full synthetic oils, have much of the silver coating worn off the cage and washer wear surfaces.
The Vrooam cranks still look like brand new.

lodgernz

12th January 2026, 08:32

LODGER - I see a few things that have set you off on a dead end tangent.
You say you used an EGT of 470* - this is completely erroneous.
Any engine with that number in the header is about as well tuned as a weed eater.

All of the pipe compute programs are using the BULK AVERAGE PIPE TEMP, including the cooler duct, and this is in the range of 550* to around 625* dependent upon the fuel quality.
This gives an average speed of sound of 560 to 610m/s, thus a pipe average wall temp of around 425*C to 450*C at peak, and the EGT as measured in the header well into the 600*'s.
Quit working in the dark, even after a ton of carrots, IMHO it is absolutely impossible to do any meaningful engine design work, let alone accurate track/dyno tuning without using a data logged or at the very least a max recall EGT gauge.

Also, if you are absolutely insistent on accelerated engine wear, and loosing Hp, carry on using Motul 800 @ 30:1, I wouldn't use that cats pee if it was free even @ 20:1 where you should be.

The Wobbly Duct info sheet also states a recommended cooled duct length @ 1.5 bore for a square engine, with the header entry @ 2X bore - you stated a cooled duct out to 2X bore, this is NFG
as the A/F out at 2X bore is never going to make it back into the cylinder due to the return wave, as the port is closing.

EDIT - again, taking a suggestion the 192* " is a bit conservative ". What the hell does that mean, what is the actual Blowdown STA, and what are the transfers doing. JanBros has a free , really good port calculator.
Quit working in the dark - free technology is at your fingertips.

Thank you again Wobbly. Let me address your points please:

1. I used the term EGT erroneously, I really meant the bulk average pipe temp

2. I take your point re the Motul 800. I used to run 25:1 but changed to 30:1 recently. I'll go to 20:1, or change to a better oil. I remember you said the Motul Kart oil is better?

3. I missed the advice to cool the port only to 1.5 x bore, so will fix that. The engine is close to square at 39.5 x 41.4

4. EO is 192°, blowdown angle-area is 2932⁰-mm2. This is 45% of the optimal angle-area recommended by Frits for 50cc and 13500RPM, based on the Aprilia RS125.

5. Transfers are as close as I can get to all of yours and Frits's axial and radial specifications, with angle-area of 28241⁰-mm2. A transfers duration 126°

6. I'll think about getting an EGT probe from Exhaust Gas Technologies. Which of their probes is best for bikes?

wobbly

12th January 2026, 09:34

Lodger - glad you get the point about the pipe temps.
The Motul Kart GP is a full synthetic, and suffers the same big end cage/washers wear as the others.
Depending upon who will give you a good price , any of the castor based oils I mentioned work just fine.
HTX 909 was used by Aprilia, we use Vrooam in KZ as its CIK approved and is very clean as I said.
But I use Maxima 927 on the dyno as it is one of the few oils that does not suffer from chrome bore streaking when running in.

I would still get JanBros port calculator as it works really well if you dont want the expense and drama of learning EngMod 2T.

The Stinger exposed tip probes are the best, and have a 2 year guarantee - I've sold and used literally a hundred of the things, never had a failure.

Gauge wise the best bet is to talk to the Kart shop guys, there are plenty of Mychron 4 - 2T( two temp inputs ) around as everyone has gone for the 5 version, that does the same logging but uses GPS
and Bluetooth, not " better " just easier.

diesel pig

12th January 2026, 11:14

Very interesting about the 2T oil's.
So just to make sure I understood what you are saying.
Use either
Maxima 927
HTX 909
Vrooam Caster based
Use whichever one is easier for one to get a hold of?

I would just like to back what was said about Exhaust Gas Technologies exposed tip probes. My one has ever given no problems since I have had it.

lodgernz

12th January 2026, 12:07

Lodger - glad you get the point about the pipe temps.
The Motul Kart GP is a full synthetic, and suffers the same big end cage/washers wear as the others.
Depending upon who will give you a good price , any of the castor based oils I mentioned work just fine.
HTX 909 was used by Aprilia, we use Vrooam in KZ as its CIK approved and is very clean as I said.
But I use Maxima 927 on the dyno as it is one of the few oils that does not suffer from chrome bore streaking when running in.

Thanks again Wobbly. I seem to be saying that a lot lately.

From your oil related post above, I gather that Vrooam Factory Racing 2T Kart oil is fine with 98, with no separation or carb deposits.
Would you still recommend 20:1 for this oil in a Bucket??

reefmuncher

12th January 2026, 14:18

Not sure if my comment was missed, was wondering if anyone had experience with the effect of axial and radial of the additional "boost" ports have around the C port on many smaller capacity Spanish and Italian race cylinders? The axial angles can be quite different between brands as seen in the image with DEA, Bidalot and Voca. 357724

Appreciate any thoughts on the subject.

diesel pig

12th January 2026, 16:10

That picture of the water cooled DEA cylinder fed reed. Got me all excited hoping it might be for a bored and stroked 50cc that would perfect to revitalize the Honda H100 engines I have in the rafters of my shed. But no they are for a 150cc vespa.

wobbly

12th January 2026, 17:31

So just to reiterate, yea all of the castor based oils work fine in pump gas, and yea they all make more power at 20:1.
Buckets , whatever, are no different.

Mike Fisher

12th January 2026, 22:11

I've just looked at Vrooam, is it the Factory or the Castor Blend you recommend, both seem to be descibed as having castor and synthetic components? I presently run Rock Oil Kart 100 which is a castor synthetic blend formulated for air cooled motors at 20:1 in a 370 Bultaco Astro used for short track, mainly 300m ovals. https://rockoil.co.uk/products/castor-kart-100/ I aim to gear to 9500rpm and to keep CHT under the plug washer to 215C.
Going right back to Gordon Jennings, in 1978, he ran tests on a PE250 that showed a power increse upto 15:1 with R40 thta is available here: https://vintagesleds.com/library/manuals/misc/jennings/oilpremix.pdf

Frits Overmars

12th January 2026, 23:32

The ignition curves for 2-stroke engines that I see here and there always start low in degrees, then rise steeply to +/-30 degrees and then drop back down at the start of the power band. Around max hp and rpm to 15° and from there further down. An almost flat curve of 15.5° is very different from the usual curve shape I describe above. Would that still be useful in practice?Indeed, most ignition curves start with their timing close to TDC to avoid kickbacks or even running backwards when starting. Then they switch to as much as 30° before TDC in order to have a lengthy expansion phase between end of combustion and exhaust opening, so the exhaust gases have lost most of their energy by the time they enter the pipe. That way the pipe cannot mess up the scavenging quite so much at revs that are too low for its layout.
But there are other options. Yes, at 2/3 the rpm of max.torque all pipe pulses arrive too early, messing up the scavenging. But what happens even lower in the rpm band?
At 1/2 the rpm of max.torque both the suction pulse and the stuffing pulse arrive back at the cylinder before BDC. But after this, the exhaust port is still open for a long time, so a secondary suction pulse and a secondary stuffing pulse can still manage to arrive at the cylinder before the exhaust port closes. And these secondary pulses can do some good, so it may not hurt to keep some energy in the exhaust gases at these low revs, i.e. start combustion closer to the 15° before TDC-region.
This still does not answer the question about your flat-line ignition curve. I did notice one thing about it though: the burn duration in your EngMod screenshots is extremely short....

Jan Van Hamme

13th January 2026, 00:21

Indeed, most ignition curves start with their timing close to TDC to avoid kickbacks or even running backwards when starting. Then they switch to as much as 30° before TDC in order to have a lengthy expansion phase between end of combustion and exhaust opening, so the exhaust gases have lost most of their energy by the time they enter the pipe. That way the pipe cannot mess up the scavenging quite so much at revs that are too low for its layout.
But there are other options. Yes, at 2/3 the rpm of max.torque all pipe pulses arrive too early, messing up the scavenging. But what happens even lower in the rpm band?
At 1/2 the rpm of max.torque both the suction pulse and the stuffing pulse arrive back at the cylinder before BDC. But after this, the exhaust port is still open for a long time, so a secondary suction pulse and a secondary stuffing pulse can still manage to arrive at the cylinder before the exhaust port closes. And these secondary pulses can do some good, so it may not hurt to keep some energy in the exhaust gases at these low revs, i.e. start combustion closer to the 15° before TDC-region.
This still does not answer the question about your flat-line ignition curve. I did notice one thing about it though: the burn duration in your EngMod screenshots is extremely short....

Those duration values are my fault.
I will have engmod generate the vibe and combustion parameters itself and then enter all these calculated values and repeat the entire process of searching for max hp per +0.5 degrees.

Wos

13th January 2026, 04:17

So just to reiterate, yea all of the castor based oils work fine in pump gas, and yea they all make more power at 20:1.
Buckets , whatever, are no different.

Wobb!!

Soory... christmas days i read not verry carefully

Thank you so much for repeading!!!
Sounds the parts will lif very much longer:cool::2thumbsup

What are the diffreces in reading the spark plugs?
Castor blends readable?

Recogniced that motul variates from black to light grey colors when others, for example ravenol fullsynth ester do variations in brown...

Merci!! ��

Wolfgang

wobbly

13th January 2026, 08:01

Mike F - its the Factory version of Vrooam, I have not tested the other type as its not available here.
I really just don't get why you, with plenty of experience and intellect , tell me you are attempting to tune with a CHT, that tells you absolutely nothing except how hot the plug washer actually is.
As I have said a 1000 times EGT tells you exactly the state of combustion, and allows incredibly simple tuning for peak power with any Density Altitude to within 1 jet of deto, in any engine.
In the tests I did I started with a baseline of R40 at 16:1 as per Jennings, but the modern castor based mixes made more power with less oil at 20:1.

Re the ignition curve testing - when off the pipe the scavenging efficiency is crap so the combustible mixture is full of residuals thus the burn speed is much lower as is power.
With a bunch of advance this gives time to at least use what energy is in the fuel that is available, and the biggest upside is that throttle response is hugely improved.
Further up into the powerband a similar but reduced version of the same effect is in play, but then we have the downside of increasing losses due to compressing the rising pressure from the increasing burn speed,
thus less advance is needed.

The whole idea of advance manipulation is as Frits described, plus as the combustion " cleans up " then the point of maximum cylinder pressure needs to trend toward 15* ATDC for peak effective use
of the rising pressure pushing on the piston.
The other side of that coin is the optimum crossover of burn duration Vs compressing losses is 15* BTDC that we all know is where most well tuned ( ie the correct compression ratio for the fuel ) engines end up at peak power.

We have the exact scenario being asked about in the KZ engines, with a flat line ignition set at about 17* BTDC in stock form.
This requires a very short pipe ( 760mm ), as there is way to much advance past peak power.
When these engines are retuned for road racing say, then the pipe reverts to the normal 800 mm TL, and the ignition , that is now adjustable, does the normal 30* dropping to 15* at peak, then down to 10 or so in the overev.
This gives a huge increase in power everywhere ( peak power much the same ) - except if a bigger carb is added.

Wos, see my remarks to Mike above, I refuse to answer any questions re tuning by looking at plugs or pistons - except the burn line on fine wire ground electrodes, but even then the ignition timing
should have been well established on the dyno, long before looking at plugs at the track.

Jan Van Hamme

14th January 2026, 07:25

I did notice one thing about it though: the burn duration in your EngMod screenshots is extremely short....
I think that a fast combustion time achieved by the high squish speed, the 55% squish band and the transfer scavenging angles used is not a bad thing?
When I let Engmod generate the combustion parameters, those duration values are back in the same range as before.
I hope that i can assume that my Zeeltronic PDCI-15V with 100 mJ spark energy en high energy Zeeltronic coil ZK3550 can still pull the spark in that "gust of wind"?

wobbly

14th January 2026, 08:57

Wos

14th January 2026, 08:59

Called Maxima !!!:niceone:
We are so exited wobb... how maxima:scratch::lol: smells:drool::lol:

Jan Van Hamme

14th January 2026, 09:21

JVH, the issue with " high" MSV is that this is analogous to adding advance due to the increase in flame speed, but to prevent deto you have to pull timing out.
What is " high " - and I am suspicious of any build that has 55% SAR.
Wobbly,
How I arrived at that 55%:
I set the ignition to 15° at maximum HP engine speed.
I selected the stinger diameter at which the MACH value in the nozzle is around 0.9.
Then I set the compression so that there is no detonation.
And to get msv to 40m/s, I had to go up to 55% squishband.

I don't know how strong that "high" energy coil is myself. The term ‘high energy’ is on the packaging, but that doesn't really say much.
Perhaps those of you with extensive knowledge of materials and product experience know what that type is capable of.

husaberg

14th January 2026, 21:07

Hi is logging in issue with Neels (vannik) (https://vannik.co.za/About.htm) sorted?

wobbly

15th January 2026, 08:56

Just sent an email to Neels.

wobbly

15th January 2026, 11:00

The answer is NO, wtf is going on - how hard can this be.

wobbly

15th January 2026, 11:05

Wos

15th January 2026, 11:55

JVH, the 40m/s is perfect, but can you not achieve this with 50% and less gap ?
Mach 0.9 is a little too high, ie stinger is too small, I believe there would be more power with 0.8 Mach and more compression or added timing.

So my conclusion is

If AF Ratio is ok/Sound, squish area set to 50%, gap for 40 squish velocity at peak power, lengt of pipe fits to peak, restrictor too...and so on:sweatdrop... did i forget something...i am sure:brick: ��

Then ....The only correct advance should be 15 degree at (about ) peak power??? ��

Experts? ;)

Let us continue fellows :drinkup:

Thanks! Wolfgang

wobbly

15th January 2026, 13:25

WOS, 15* BTDC is an optimum advance number that occurs when the static compression plus the dynamic compression created by the Delivery Ratio and the Trapping Efficiency combine
to reduce the pumping losses of compressing that retained charge, and if it all works together the peak cylinder pressure occurs near 15* ATDC.
If you can get away with more advance then one or both of the compression cycles is sub optimum.
Having to retard the ignition, leads to the opposite conclusion.

husaberg

15th January 2026, 17:29

Just sent an email to Neels.

The answer is NO, wtf is going on - how hard can this be.

might i suggest as many users as possible on ask the same question (respectfully) in this thread) worth a crack?https://www.kiwibiker.co.nz/forums/showthread.php/190561-IP-issue?p=1131243035#post1131243035

Wos

16th January 2026, 00:30

Jan Van Hamme

16th January 2026, 05:22

Deleted post

husaberg

16th January 2026, 15:57

Just sent an email to Neels.
KB's top dog has replied for Neels to try now.

Vannik

16th January 2026, 16:53

might i suggest as many users as possible on ask the same question (respectfully) in this thread) worth a crack?https://www.kiwibiker.co.nz/forums/showthread.php/190561-IP-issue?p=1131243035#post1131243035

After more than a month I am back! Wow, talk about serious withdrawal symptoms.

Jan Van Hamme

16th January 2026, 18:01

After more than a month I am back! Wow, talk about serious withdrawal symptoms.
Welcome back, Neels. I am delighted to be able to read your contribution once again.

@Wobbly, Mach 0.8 and 51% gives less power when I simulate it. What I have also found is that when Engmod generates a fospipe on my engine, it results in a stinger diameter that causes a Mach of 1.
And that Mach 1 does not cause any detonation in the fos exhaust or in my own exhaust.
I think something is wrong and will send a pack file to you and Neels.

F5 Dave

16th January 2026, 18:02

Thanks for making the effort. And for Spank for sorting the issue. It must be hard for amateurs to cope with odd issues.

Peter1962

17th January 2026, 07:13

After more than a month I am back! Wow, talk about serious withdrawal symptoms.

Great news ! Welcome back, Vannik. :wings:

wobbly

17th January 2026, 08:03

JVH, having Mach in the stinger nozzle means that tube is sonic choked. ie it is impossible for more flow to happen - talk to GPT about this.
If its not getting into deto, then that simply means the TubMax is very safely low.
IMHO, with a bigger stinger @ 0.8 Mach ( plus with MSV @ 40m/s ) and way more compression to push the TubMax up near the deto limit , it will make alot more power.
The only question to ask is will the new squish configuration cause piston clipping - I doubt it, as for example Jan has said the RSA didnt hit at 0.4mm/14000 rpm.

I did ask sarcastically " how hard can it be " - what was the actual problem with Neels access here?

Jan Van Hamme

17th January 2026, 08:30

Frits Overmars

18th January 2026, 02:06

having Mach in the stinger nozzle means that tube is sonic choked. ie it is impossible for more flow to happen...In sonic choked flow, lowering the pressure downstream of the throat will not increase the flow velocity past Mach 1, and neither will raising the pressure upstream of the throat.
But raising the upstream pressure will increase the gas density and thus the mass flow, even if the throat velocity remains Mach 1.
One more pedantic point: Mach 1 flow in the stinger may not be Mach 1 all of the time. The practical limit of Mach 0,8 may be an average value derived from practical experience and your Mach 0,8 flow guideline may contain instants of Mach 1. But we must proceed with caution there; Mach 1 all of the time will be risky.

wobbly

18th January 2026, 08:04

Here is a stinger nozzle Mach trace at peak power.
In this and all the other cases I have simulated if 0.8 Mach is exceeded it means ignition or compression has to be reduced to some unexpectedly low value
as the retained pipe pressure is overly excessive.
The peak velocity is occurring as the main wave descends down the rear cone and enters the stinger - this plot is logged 1/2 way along a 25mm long nozzle.
Subsequent peaks and troughs, are reflections and superpositions off atmosphere, the nozzle entry and the step up to the main stinger tube, I assume.
I do not know if going sonic creates a shock wave at the nozzle entry - maybe Neels can enlighten me/us.

ApolloMotoMoto

22nd January 2026, 14:08

After more than a month I am back! Wow, talk about serious withdrawal symptoms.

Welcome back, Neels!

:D

ApolloMotoMoto

22nd January 2026, 14:22

Wobbly, Msv 40m/s
Squishband 50%
Compression 12.7
Stinger mach1
Squish 0,65mm
Stroke 57mm
Tubmax 960
Deto @ Tubmax 970
Thanks that you will look at it over the weekend.

0.65mm squish???

Come on man, what are you afraid of?

:D

0.01 x stroke is nearly guaranteed safe from clipping.

So, going larger than 0.57mm squish is just giving up MSV potential for no good reason at all.

Strong caveat that this number "40m/s MSV" is more of an imperical tuning symbiote that has been discovered over COUNTLESS engine examples...

But, from having attempting this same game myself, you are going to make sacrifices "trying to find" 40 MSV when you fully enforce all of the statoe of the art design guidance as Wobbly has clearly laid out.

Its NOT easy to get there, the CR being high enough to get your TubMax into the "making power" region is limiting the MSV somewhat, higher CR = lower MSV for all other geometries being equal; the compression is "resisting" the acceleration of the squish event.

So, maybe you have been limiting CR to sneak out some extra MSV, but as Wobbly has correctly pointed out, now your TubMax falls into the toilet and your doing weed-eater stuff.

You need to use ALL of your available "squish gap" potential, period. Or you WILL be leaving power on the table.

0.65mm squish with 57mm stroke is just screaming "but WHY though???" to my eye.

The aprilia RSA is ~54mm stroke.

Wayne just gave you the key reference;

"The only question to ask is will the new squish configuration cause piston clipping - I doubt it, as for example Jan has said the RSA didnt hit at 0.4mm/14000 rpm."

54mm stroke and they could safely use 0.4mm up to 14k.

To scale that to your 57mm;

0.4 / 54 = 0.0074 squish gap per mm stroke

57 x 0.0074 = 0.42mm

I suggest you plug 0.42 squish gap in and see how much easier it gets to achieve that 40 MSV with useful compression; and you shouldn't need near 55% SAR to do it ;)

Jan Van Hamme

22nd January 2026, 18:22

0.65mm squish???

Come on man, what are you afraid of?

:D

0.01 x stroke is nearly guaranteed safe from clipping.

So, going larger than 0.57mm squish is just giving up MSV potential for no good reason at all.

ApolloMotoMoto,
The reason why I thought not to go too small with the squish is the following:

A 125RSA has a squish gap of 0.7mm
It could well be that the crankshaft is made of 15NiCr13.
The bearings in which it rotates are BC1-1442-B cylindrical roller bearings (found in the datasheet by Jan Thiel & Frits Overmars)

My crankshaft is from a Yamaha DT200. (I don't know what material it is made of from the factory).
I have to press the crankshaft apart and back together.
The bearings are ball bearings.
I suspect that the distance between the two bearings (support points) on a DT200 crankshaft is greater than on a 125RSA.

Although the piston speed is much lower in my application than in an RSA, I thought that with much lower crankshaft rigidity, a lower steel alloy, and less stable ball bearings... I should be cautious about the squish distance.
There is someone working behind the scenes for me, for which I am extremely grateful, who has a wealth of experience that could fill an Olympic swimming pool...I have now also heard that it may be less and that there would be no contact between the piston and the head. I am waiting patiently.

Thank you for your insights and help. I am just a private individual, not a competitive rider or anything... but I am a 2-stroke enthusiast at heart.
Jan

ApolloMotoMoto

22nd January 2026, 18:56

ApolloMotoMoto,
The reason why I thought not to go too small with the squish is the following:

A 125RSA has a squish gap of 0.7mm

Jan

You super sure about that? ;)

It was a MotoGP engine, and the squish gap IS a "tunable feature" of the engine. Are you super confident that everytime the RSA125's rolled off the paddock they did so set to 0.7mm squish gap???

I will repeat that Wayne shared a rather large clue, and if you do the deep dive (husaberg where are you!) this has been discussed probably 10-20 times over the course of this thread; what squish gaps WERE actually used in racing, what was tested on the track but never really "raced" and what was tested on the dyno. I can assure you it was not a static single numer over the entire racing pedigree of the engine.

I can absolutely assure you that no matter how "inadequate" your crankshafts are, they PALE in comparison to what I am playing with.

I build and tune "80cc Motorized Bicycle Engine Kit" engines from such fine makers as Jilin-Zeda Technology Corporation, Runtime Products, and Zong Dong Freight Corportation (not joking).

It is made with Chinese "cough" equivalent to something like SCM440, called CR40.

The videos you see on facebook of the fine manufacturing conditions that you will find in a shop with dirt floors, you can start to understand what I am working with.

I am certainly not running them as they come "out of the box" :D, and with correct preperation, mostly a good press fit and well trued, I am running my 42mm stroke at 0.30mm squish gap without clipping at 15k RPM's.

I totally understand your hesitation in theory, but in practice your concern is actually unfounded (especially if your piston speed is significantly lower than the RSA), and you are leaving a key piece of the Rosetta Stone un-leveraged because of it.

Jan Van Hamme

22nd January 2026, 19:19

You super sure about that? ;)
.
No, I'm not sure about that at all. That's just what I read and found in a datasheet.
I also have no idea how a racing engine is tuned when it goes on the track or on the test bench.
There are people with tons of experience and knowledge that far exceed mine, and there are also probably a lot of falsehoods to be found here and there on forums.
It's not easy to always make the right synthesis that would then be the correct one.
At the end of the day, I really value the practical experience and approach of those who have already proven themselves.
After all, the world doesn't revolve around just pressing the run button on a 5-axis CNC machine.

Niels Abildgaard

22nd January 2026, 22:16

You super sure about that? ;)

I
I can absolutely assure you that no matter how "inadequate" your crankshafts are, they PALE in comparison to what I am playing with.

I build and tune "80cc Motorized Bicycle Engine Kit" engines from such fine makers as Jilin-Zeda Technology Corporation, Runtime Products, and Zong Dong Freight Corportation (not joking).

It is made with Chinese "cough" equivalent to something like SCM440, called CR40.

The videos you see on facebook of the fine manufacturing conditions that you will find in a shop with dirt floors, you can start to understand what I am working with.

I am certainly not running them as they come "out of the box" :D, and with correct preperation, mostly a good press fit and well trued, I am running my 42mm stroke at 0.30mm squish gap without clipping at 15k RPM's.

Sounds interesting.Have You put pictures somewhere?

ApolloMotoMoto

23rd January 2026, 07:36

Sounds interesting.Have You put pictures somewhere?

Yea I think I can find a few ;)

Niels Abildgaard

23rd January 2026, 08:10

Yea I think I can find a few ;)

Very interesting.
How much is each engine ab China and how much modification is allowed in this class?
Can we have a picture of bike with engine?

wobbly

23rd January 2026, 08:28

Here is my understanding of the issue - mostly from the usual on line conjecture.
Jan stated he had tested the RSA to 14,000 with 0.4mm gap and had no problem.
The engine, as delivered, was at 0,6.
Many team engineers at the track increased that to 0.7 as it was easier to tune and rev'd slightly harder.

Even with a crap crank and bearings, a 57 stroke at 9500 is a million miles away from clipping.
Here are the numbers I have 14:1, on E10 Unleaded, 0.6mm/50% = 40.8m/s , 9500 = 18m/s - safe as hell.

ApolloMotoMoto

23rd January 2026, 08:38

Very interesting.
How much is each engine ab China and how much modification is allowed in this class?
Can we have a picture of bike with engine?

I'll see what I can dig up...

The "motorized bicycle" engine kits are ~$200 a piece to start with ;)

There are many classes that allow various levels of modification;

There is the ChinaGirl "Series":

ChinaGirl SuperStock class for unmodified engines.

ChinaGirl SuperSport class allowing intake/ exhaust/ head/ ignition modification, by no internal engine mods.

ChinaGirl Superbike class allowing essentially ANY modification a person can dream up within a given displacement limit of 90cc, but it must utilize the stock Chinese cast carnkcases and cylinders (modification of these parts is allowed, just no "custom" cast/CNC crankcases or cylinders).

And, there is the "Formula" Series:

Formula 2 allowing for any SINGLE SPEED/DIRECT DRIVE configuration of air cooled 2-stroke engine design to compete with a displacement limit of 100cc.

Formula 1 allowing for any SINGLE SPEED/DIRECT DRIVE configuration of water or air cooled 2 stroke engine design to compete with a displacement limit of 125cc.

CNC crankcases allowed, ANY cylinder from any source that meets the requirements allowed.

There are NO fuel rules.

As of yet there are no "elecronics restrictions", anything you can dream up through electronic/digital controls is allowed until something shows up that requires regulation.

Frits Overmars

23rd January 2026, 13:36

Jan stated he had tested the RSA to 14,000 with 0.4mm gap and had no problem.
Many team engineers at the track increased that to 0.7 as it was easier to tune and rev'd slightly harder.46 years ago I was investigating the lower squish gap limit for the grandfather of the RSA, the Rotax 124. I ran it to 14.000 rpm with 0,4 mm squish gap and had no problem either (a well-set up RSA even revs to 14.500 rpm without problems).
Increasing the squish gap reduces the squish velocity and the combustion speed, raises the EGT and increases overrev, which in some circumstances can make life easier for the rider, even if it costs power. So that is what these team mechanics did.

We may need to define 'crap bearings'. The ball bearings in the Rotax 124 and the roller bearings in the Aprilia RSW and RSA are fine. The Aprilias with their 54,5 mm stroke ran a piston speed of 26,3 m/s at 14.500 rpm with 0,4 mm squish gap without problems.
But when I was searching the squish gap limit for the Rotax 124, I also had a 50cc Kreidler with its typical angular contact crankshaft ball bearings on the dyno.
The Kreidler with its 39,7 mm stroke, a much lighter piston and a piston speed of only 21,2 m/s at 16.000 rpm required a squish gap of at least 0,7 mm because those bearings allowed much more crankshaft flex. Crap bearings? They ran very light but shimming them was a pain and you cannot get away with the squish that roller bearings allow.

F5 Dave

23rd January 2026, 14:51

Ahh thanks, my Suzuki 50 i set up as 0.5mm. I was running 2 base gasket and all was well. But if I reused the base gaskets twice the piston would tickle the head. 15mm pin from memory. I started running bigger pin and yamaha rod. Never thought to see if a stronger crank assembly could take closer squish.

Vannik

23rd January 2026, 17:14

Thanks for making the effort. And for Spank for sorting the issue. It must be hard for amateurs to cope with odd issues.

Seems I am only halfway back, I can get access from work but still not from home. And work has nothing to do with 2T engines so not really good to access from here.

Who do I contact about sorting it out all the way?

husaberg

23rd January 2026, 17:18

Seems I am only halfway back, I can get access from work but still not from home. And work has nothing to do with 2T engines so not really good to access from here.

Who do I contact about sorting it out all the way?

Ask here.
https://www.kiwibiker.co.nz/forums/showthread.php/190561-IP-issue?p=1131243035#post1131243035
preferably quote Spankme. (thats not a joke)
https://www.kiwibiker.co.nz/forums/showthread.php/190561-IP-issue?p=1131243055#post1131243055

Niels Abildgaard

23rd January 2026, 18:45

I'll see what I can dig up...

The "motorized bicycle" engine kits are ~$200 a piece to start with ;)

There are many classes that allow various levels of modification;

There is the ChinaGirl "Series":

ChinaGirl SuperStock class for unmodified engines.

ChinaGirl SuperSport class allowing intake/ exhaust/ head/ ignition modification, by no internal engine mods.

ChinaGirl Superbike class allowing essentially ANY modification a person can dream up within a given displacement limit of 90cc, but it must utilize the stock Chinese cast carnkcases and cylinders (modification of these parts is allowed, just no "custom" cast/CNC crankcases or cylinders).

And, there is the "Formula" Series:

Formula 2 allowing for any SINGLE SPEED/DIRECT DRIVE configuration of air cooled 2-stroke engine design to compete with a displacement limit of 100cc.

Formula 1 allowing for any SINGLE SPEED/DIRECT DRIVE configuration of water or air cooled 2 stroke engine design to compete with a displacement limit of 125cc.

CNC crankcases allowed, ANY cylinder from any source that meets the requirements allowed.

There are NO fuel rules.

As of yet there are no "elecronics restrictions", anything you can dream up through electronic/digital controls is allowed until something shows up that requires regulation.

Thank You for pictures and explanation.
Sounds lot of fun for reasonable funds.