ESE's works engine tuner

[wobbly]

Re the 50cc carb deal - yes there are plenty of very hotrod 50 racers using 30mm carbs in Europe.
As far as I am aware they are mostly using Lectrons, the HV version of these has a nice radiused venturi behind the slide and the standard dimensions
of this is 4mm smaller than the exit diameter.
Thus a 30 is actually a 26 venturi at the slide.
This correlates real well with the port areas of those engines that run up past 14,000.
I will be doing the same with the 50 engine in the sim I posted - the SmartCarb is a 30 to match the reed inlet, but the egg shaped venturi is a 25.
The SmartCarbs do work real well, its just that they dont have the well developed range of flat sided needles like Lectron do.
So you do have to datalog the engine and adjust the needle by moving it up/down and rubbing it on a sand stone and measuring with a digital.
I asked them right at the beginning to incorporate a jet adjustable powerjet to fine tune, especially on a twin, but they know everything and ignored me.

[husaberg]

wobbly, strange thing to me with that carb size issue.

48 sprocket hp from a 125 single cylinder engine with only a 30mm carb. that leads me to think that all those 125 mx bikes with their 38 mm carbs are totally wrong. with only about i guess 35 sprocket hp the 38mm carbs are way too big, aren`t they? so why did the factorys do that if you dont need a big carb like this and can have even more low and mid power with a smaller carb? the 35 hp mx bikes should be happy with a 28mm carb easilie!?

but on the other hand several other engines are running such big carb sizes. 50ccm engines with 32mm carbs are not a joke for example. Frits can confirm.

maybe there is a difference between rotary disc valve engines and reed valve engines. can one say that a reeder needs a much smaller carb than the rotary?

happy new year to all!

[Martin1981]

Yes Husa, that diagramm looks familar. it is in a dellorto manual i have.

but...... it recommends about a 40mm carb for 40hp. remember wobblys 48 hp kz2 engine with a 30mm carb....

and remember all the mx bikes with their 38mm carbs. and i guess they are nowhere near 48hp.

i am really confused that this was not tested to dead. when i have a lets say 35hp mx engine that will be happy with a lets say 28mm carb and will have an even better low and mid range pull with a smaller carb, why should i use a 38mm? i can not imagine that the factorys are stupid or way behind in development.

on the other hand, the 30mm carb of the 48hp kart engine is meant to be a restriction by the rulemakers. so without that restriction what carb size would one use to achieve max power with the kz2 engine?

i am asking so much because i think that calculating the right carbsize should be possible for guys like frits and wob for example and one of the easier things on a two stroke. if there is anything easy on a two stroke at all, apart of its simple mechanical structure.

from my logical thinking (if it is logical...) i would say that the carb area can not be bigger than the total area of all transfer ports. it even can be way smaller because the tranferports are not completely open all the time. the carb is when on full throttle.

[wobbly]

The 125 KZ2 engines are rule limited by the carb size, the straight line ignition, and no powervalve.
They have to pull hard out of the slowest hairpin corners at around 9000 rpm,and the best configurations make peak Hp at around 13000
and will spin to 15,000 with the drop off slope shallow enough to enable not having to change gear between close corners.
In this case the small carb only really just gets the job done down at 9000 with no advance and no powervalve.
Thus they are a special scenario, with huge development time put in by hundreds of us trying to get the best jet settings/port timings/pipe lengths
etc to operate with the highest peak, and widest powerband within the CIK rule limits.

125 MX engines all have digital ignitions with a proper advance curve, and all have powervalves.
Thus ripping out of dirt corners is made easy with the big 38mm carb matched to the ports for good top end and overev - any more mid from a small carb would simply light up the tyre and loose traction.
The 125 GP engines up over 50 Hp at the sprocket also have digital ignitions and powervalves, so they can utilise 41mm+ carbs to get max peak and overev to 14500.

[husaberg]

Yes Husa, that diagramm looks familar. it is in a dellorto manual i have.

but...... it recommends about a 40mm carb for 40hp. remember wobblys 48 hp kz2 engine with a 30mm carb....

and remember all the mx bikes with their 38mm carbs. and i guess they are nowhere near 48hp.

i am really confused that this was not tested to dead. when i have a lets say 35hp mx engine that will be happy with a lets say 28mm carb and will have an even better low and mid range pull with a smaller carb, why should i use a 38mm? i can not imagine that the factorys are stupid or way behind in development.

on the other hand, the 30mm carb of the 48hp kart engine is meant to be a restriction by the rulemakers. so without that restriction what carb size would one use to achieve max power with the kz2 engine?

i am asking so much because i think that calculating the right carbsize should be possible for guys like frits and wob for example and one of the easier things on a two stroke. if there is anything easy on a two stroke at all, apart of its simple mechanical structure.

from my logical thinking (if it is logical...) i would say that the carb area can not be bigger than the total area of all transfer ports. it even can be way smaller because the tranferports are not completely open all the time. the carb is when on full throttle.

I haven't read your answer but the original question was about the size the size of the carb, while the MX and GP bikes have huge ones compared to your kart the size depends on the rules, the gearbox ratios and the availability of a clutch
A single gear kart runs small carbs to enable them to work with a single gear, An mx bike has 6 ratio and a clutch, as well as the ability to use both in tandem and break traction at will.
Manufacturers spend huge amounts of time getting the big carbs on mx bikes to work over a wide rev range and to carburate cleanly. Maybe have a look through Camerons section on carbs in the sportbike performance handbook.
As do Kart people with restricted carb rules and single speed gearboxs. As always compromises are made and clever dodges are employed to work within a set of rules.
If you want to see whats the real best way to make the most hp look at the 250 superkarts, the carb sizes for the ultimate power answer is there.

[lodgernz]

My rule of thumb for determining appropriate carb size is very simplified, based on the unsubstantiated assumption that the flow speed through a carb should not exceed 30m/s. The flow volume rate in cc/min might be engine cc * RPM. Divide that by the CSA of the carb throat in cm3 and you have the flow speed in cm/min. Reduce that to m/s and you have a possible flow speed.

In reverse, carb throat diam D = SQRT(cc*peak RPM/(450*pi)) mm.
So a 50 at 12000 RPM needs at least 20.6mm throat, and a 91cc motor needs at least 27.8mm.

All this is approximate and subject to argument because the amount ingested by the engine will be dependent not only on its swept volume, but also on its reed/RV efficiency, scavenging efficiency and the effectiveness of the exhaust system and, I'm sure, a number of other factors I haven't considered.

It's just a starting point. Use it if you like.

[ken seeber]

FOS transfer port lowering/restriction thoughts
Possibly living in dreamworld, I thought it’d be neat to incorporate removable covers for the transfer passages. Given the liberty of doing some shitty pencil sketches that don’t have any semblance to accurate scaling, I have come up with the following:
A. General idea of covers. Probably possible for the front and rear passages, but could be a real task for those partially under the exhausts.
B. By using covers, this opens up the scope to be able to realistically manufacture and incorporate a range of valve types. This one is using the roller shutter principle where the valve is a thin spring steel strip (think tape measure) that follows the contour if the side slots
C. Thinking about covers, one would expect that the temps would not be too high that one could not use 3D ABS filament printed covers. However, if they were metal, eg cast aluminium, one might as well incorporate some finning
D. This got me thinking about regular rear exh facing cylinders, KZs being a case in point. I am sure that at any time, the external surfaces of the transfer passages (which are usually exposed in their contoured shape) are running much hotter than ambient. Given this, any form of additional air cooled finning must be beneficial. Note this is not like putting fins on the outside of a water jacket
E. The swing style valve could be incorporated by putting its spindle axis at the cover/cylinder interface. Actually I think this style of valve is not bad as it does provide an insulating layer for those passages under the exhaust passage.
F. Guillotine valve, with the top edge aimed at a level suitable for an appropriate step change in port height. Cover not really necessary for this.
G. Straddling valve
H. Sliding cylinder. In this case the head insert and crankcase are rigidly connected and the cylinder (inc exhaust & cooling) slide up and down. For a 54 stroke 120 rod engine at 190 exhaust and 130 transfers, if the cyl was dropped by 4 mm, these timings would become 173 and 108 respectively. Have no idea if this is enough. Certainly would be a monstrosity from the outside. The cylinder should be independent from any major axial forces.
Pretty much all these completely disobey the KISS principles with no consideration to any linkages & actuation mechanism etc. D is possibly the simplest.
However, there are many questions re what transfer duration reduction would be necessary and would partially blocking/restricting the passage somewhere upstream be also beneficial?
There you go. Back to work tomorrow.

[TZ350]

These are someone elses pictures of a RGV250 cylinder and port maps but I have a couple of these cylinders with three piece valves that I am building a motor with. Plenty of transfer STA straight off and the Exhaust can be modified to have the STA of a RS125's but better, as it will have a progressive power valve setup too.



I want the EFI for this air cooled RGV125 setup. I am hoping for high 30's with the RGV cylinder, larger rotary valve, 34mm Ball Valve inlet and 3L plenum chamber. The straight cut gear is out of a Chinese clone Honda monkey bike engine. It has the same C/C distance as my GP primary drive.

[seattle smitty]

Ken, what's "dreamworld" about that? Does a person have to apologise for thinking out of the box? But I guess I do that, too.

For starters, the idea of putting the "long-turn" or "roof" or whatever term we are using to describe the outside curve of the transfers into a removable cover has been done, and not just on Wobbly's old crossflow kart motors (back a page or two) but on a few loop-scavenged designs, one being the Anzani (back a bunch of pages). As long as block stiffness is maintained, and you can seal the covers, why not? And on the Anzani, the transfer covers weren't serving your purpose of installing timing-valves.

As to your valves (and your sketches are clear enough), I still wonder if the first one would flutter when passing through some harmonic range. The others are interesting, too. Yes, not simple; yes, a problem to actuate; and yes, maybe only doable on four of the six transfers.

I was trying to envision how a carrier with all the intake ports and passages could be done. Seems like a real can of worms, but there are better minds than mine. When Frits was speculating on this, he said he didn't want the exhaust timing tied to the intake timing; he just wanted to change the latter.

[peewee]

FOS transfer port lowering/restriction thoughts

E. The swing style valve could be incorporated by putting its spindle axis at the cover/cylinder interface. Actually I think this style of valve is not bad as it does provide an insulating layer for those passages under the exhaust passage.
F.

i had a idea like the swing valve with spindle, then maybe a electric motor to turn the spindle but im not sure what kind of cdi box would send the message to the electric motor, or how would you program it. what got me thinking about it was the ktm exh valve which just swings up and down on a spindle. even some of the hondas are simple swing valves as well. i didnt put much thought into it so i really didnt get as far as how to install the valves but a removable cover like you have would probly work

[Flettner]

FOS transfer port lowering/restriction thoughts
Possibly living in dreamworld, I thought it’d be neat to incorporate removable covers for the transfer passages. Given the liberty of doing some shitty pencil sketches that don’t have any semblance to accurate scaling, I have come up with the following:
A. General idea of covers. Probably possible for the front and rear passages, but could be a real task for those partially under the exhausts.
B. By using covers, this opens up the scope to be able to realistically manufacture and incorporate a range of valve types. This one is using the roller shutter principle where the valve is a thin spring steel strip (think tape measure) that follows the contour if the side slots
C. Thinking about covers, one would expect that the temps would not be too high that one could not use 3D ABS filament printed covers. However, if they were metal, eg cast aluminium, one might as well incorporate some finning
D. This got me thinking about regular rear exh facing cylinders, KZs being a case in point. I am sure that at any time, the external surfaces of the transfer passages (which are usually exposed in their contoured shape) are running much hotter than ambient. Given this, any form of additional air cooled finning must be beneficial. Note this is not like putting fins on the outside of a water jacket
E. The swing style valve could be incorporated by putting its spindle axis at the cover/cylinder interface. Actually I think this style of valve is not bad as it does provide an insulating layer for those passages under the exhaust passage.
F. Guillotine valve, with the top edge aimed at a level suitable for an appropriate step change in port height. Cover not really necessary for this.
G. Straddling valve
H. Sliding cylinder. In this case the head insert and crankcase are rigidly connected and the cylinder (inc exhaust & cooling) slide up and down. For a 54 stroke 120 rod engine at 190 exhaust and 130 transfers, if the cyl was dropped by 4 mm, these timings would become 173 and 108 respectively. Have no idea if this is enough. Certainly would be a monstrosity from the outside. The cylinder should be independent from any major axial forces.
Pretty much all these completely disobey the KISS principles with no consideration to any linkages & actuation mechanism etc. D is possibly the simplest.
However, there are many questions re what transfer duration reduction would be necessary and would partially blocking/restricting the passage somewhere upstream be also beneficial?
There you go. Back to work tomorrow.

Ken, yes D is what I had in mind but the floors of the transfers would be fixed so that as the cylinder came down the transfers nozzled untill eventully there is no transfer passages at all or a combination of both movements. But the exhaust! Well thats another can to open

[Yow Ling]

These are someone elses pictures of a RGV250 cylinder and port maps but I have a couple of these cylinders with three piece valves that I am building a motor with. Plenty of transfer STA straight off and the Exhaust can be modified to have the STA of a RS125's but better, as it will have a progressive power valve setup too.



I want the EFI for this air cooled RGV125 setup. I am hoping for high 30's with the RGV cylinder, larger rotary valve, 34mm Ball Valve inlet and 3L plenum chamber. The straight cut gear is out of a Chinese clone Honda monkey bike engine. It has the same C/C distance as my GP primary drive.

For a true "carburetion equivelent" of 24mm surley you would need to take into account that the carburettor does not supply a constant flow of air or air fuel mixture . if say the valve timing was 180 degrees then the restrictor feeding the plenum should be 180/360 x 24mm aprox 17mm, this would allow the throttlebody to flow a similar amount of air as a 24mm carburettor set up.

[lodgernz]

For a true "carburetion equivelent" of 24mm surley you would need to take into account that the carburettor does not supply a constant flow of air or air fuel mixture . if say the valve timing was 180 degrees then the restrictor feeding the plenum should be 180/360 x 24mm aprox 17mm, this would allow the throttlebody to flow a similar amount of air as a 24mm carburettor set up.

Then surely the same must apply to a normal carb arrangement.
Since the carb does not flow all the time perhaps 17mm should be the standard for 125 air cooled?

[Yow Ling]

Then surely the same must apply to a normal carb arrangement.
Since the carb does not flow all the time perhaps 17mm should be the standard for 125 air cooled?

no it would only apply to the plenum arrangment as the plenum refills continuously from the 24mm restrictor and then feeds intermittently into the engine via the 38mm throttle valve fited to the disk valve in this case, in a standard carburetted engine the feed is intermittent through a 24mm carb

[Grumph]

For a true "carburetion equivelent" of 24mm surley you would need to take into account that the carburettor does not supply a constant flow of air or air fuel mixture . if say the valve timing was 180 degrees then the restrictor feeding the plenum should be 180/360 x 24mm aprox 17mm, this would allow the throttlebody to flow a similar amount of air as a 24mm carburettor set up.

that ship's sailed mike - too many precedents in motorsport where a plenum intake has been taken as the inlet limiting factor...If it works and he comes within your reach, you could always try a protest. good luck.