Modern Vespa

That makes sense.
So it's an issue of availability rather than flow restriction. Interesting point.
That probably has some merit.

If we manage 22-23 with box - and 25-26 with pipe with it sucking a bit harder on the carb - this would be a killer build*.

*With a power curve fatter then a 50's round tail wide body of course.
🙂

Reeds are actually less efficient when compared to rotary. This is because the reeds are in the way and rotary is just an open hole. By making the reeds bigger, with a bigger entry hole they cancel out the fact that they are in the way and usually end up being way bigger. But this isn't why they are more powerful. For those still reading, reeds can open on the down stroke too, if there is a decent exhaust with enough pull. So opening much earlier/twice is the big advantage.

Oh - your on!
I'll have a look at these inputs and perhaps measure mine to compare (looks like you are close).
Little embarrassed to ask tho - what does it allow you to optimize for?

OH I CAN GUARAN-SCOOTIN-TEE we are still reading your wise words, Jack221.

Thanks for that clarification above. Nice simple way of explaining the basic advantage/disadvantage of both intake options.

What do you think of the Mikuni BN carbs?

Only seen one run on a scooter once and that bike did quite well. it seems a good fit as it's:
* downdraft
* built in pump
* less sensitive to vibration

https://www.summitracing.com/parts/mku-bn34-28-8010?rrec=true

Jack - seems both your responses we crossed over on typing last night.
Reeds explanation - thanks.

OCT - on thing I didn't follow with the engmod software. There are a hand full of fields to fill in - but no apparent "calculate" button, and I'm not sure what you would calculate? What is it optimizing for/figuring out when you give it those inputs?

Edit - OCT - what size do those carbs come in/ what is it about them that you think could make them interesting for Vespa?

In other news - some may have seen - I was awarded a Barista position - so I am off to brew my coffee extra good this morning.

If one of these carbs could be made to fit in the space with a short manifold and with the expertise to jet it in it should go fine. The butterfly throttle could be quite exciting on a high power scoot. Built in pump very helpful but would work more efficiently with reeds.
Not sure it would be any quicker than an SI carb or PHBH overall. You have one to try?

I've not tried one of those carbs, just always thought there was potential there.

EngMod screens:

The process below models the port and the STA uses an empirical model (like Jennings) to get the proportions close. I normally try and get the STA process for the intake, transfer, and exhaust all to about the same "potential power" before doing any simulation.


setting the intake port parameters: the open and closing timings are the crank angle, the port height and width are flipped from how we normally view it on the vespa crank. the height is measured along the axis of the crank.

when saving the intake parameters it computes the area, open and close ramps, and gives an equivalent diameter.

Specific Time Area: uses the port time area and target peak RPM to calculate an expected HP. Also gives a recommendation for the carb size.

also made this sketch to help when changing the values

OTC -thanks.
I watched a 30 min vid on Engmod.
Very nice tool.
Can see how having this can take some significant guess work out of tuning.

Update on build below.
Relates to above - and includes a question.
First, the update.

Long time readers of this thread (should question the direction of their life) may recall that Whodat convinced me on page 3 that I should use an EFL transmission.
Ginch said it seemed like it was possible - and the next thing I knew - I was in Germany buying what I thought was the magic needed part: a conversion shaft for the xmas tree.

When I did my build - it turned out that to fit the P200 based xmas tree in - I had to shim it about 1mm to keep the fatter primary gear from striking the case.
That set off a chain reaction of shimming.
Drive shaft - 1mm, kick start, 1mm, basically - every shaft had to shift 1mm.
That is something I wanted to avoid with this build.

So I rebuilt the Cush drive using two flat bottom plates.
Pics below.

OTC - last pic is for you.
Question: can Eng mod model any benefits or gains from flipping the carb as I show?
Makes intake path straight shot (you can see the con piston when you open the throttle...)


rebuilt the Cush drive - with some fancy yellow double springs - but forgot to shoot that picture...

Rebuilt the drive using the old back cover on the front - giving me two flat covers. Now there is sufficient clearance for the gears - so no shims needed on the rest of the build

Bit of 638. It's a "retained bearing" now, Jack. 🙂

Pulled the crank in and double checked alignment before the 638 set.

Would there be any benefit to flipping the carb backwards so that it was a straight shot into the cylinder?

You can see right up her (piston) skirt with the carb like this. I could do a bit more porting inside the cases to optimize a straight shot. Seems like rotary would really benefit based on Jack's prior comments vis-a-vis reed vs rotary

engmod can't really model the directness of the intake (it's a 1D simulation) but that layout gives a shorter intake than bringing it up and forward by 90°, and it does take those lengths into account. You have to model the whole thing (intake, case volume, transfer, exhaust, pipe) to do that simulation.

I was playing with that exact layout earlier today and realized it would need to hook down to maintain the same cross sectional as it narrows to fit into the rotary pad.

What size carb is that?

It's a 28mm.
I'm familiar with your post about the carb intake "hooking down" if you mean the need to match the cross section of the manifold with the top of case.

My cases are shaped a lot like the intake you designed.
Its just more compact in height.

I elongated the case base toward the back when I modified them.
So my rear carb stud has been moved back 10-15 mm and the deck is much longer.
This means I can have an angular tube cross section meet the cases without a bend.


The red dotted area is all added material. This would allow me to have a much longer inlet on the top - oval shaped - which is what happens when you cut a tube at an angle

You can see the tube cut at an angle that makes up the intake manifold here

28mm smart carb. Have pondered a 35mm version. I have a spare - and can sell/trade in towards a 35mm version - but they have a much larger body.

As the bearing is shimmed I guess the crank got widened? Any picture of that? Rebalancing procedure? Balance factor was x%?

Fitting the carb in reverse won't make any noticeable difference as the rpm target for Pmax is way too low to care. Now if it was maxing at 14,000 rpm, then probably.
The side panel won't fit must be an issue. Carb movement with suspension greatly increased. Not much positive to say on the subject.

Can only be weeks before this is running now.

Had I welded and widened - I would have taken the original balance factor and tried to match. Best guess - somewhere around 55%. SIP Doesn't Say™... I plan to come up a hair shy of that - but thought it was worth the discussion. in mere mortal world - that's true. The shock is also right in the path. But I had a cunning plan to deal with both those. This is the thing I don't like about it. Hit a bump and you send the carb from 30° to 45° of angle. rather than vice versa. If I don't re-paint.

If you do decide to repaint, at least it'll still be "The Unicorn".

Chuckled.
Repaint would be orange of course..
Little threat of this happening - tho some consideration of turning the silver to white still on the mind.

Jack - the image of the bearing not fully seated was as it was pressed in - not complete.
In order to bond the bearings in place - I first warmed the cases and pressed the bearing partially in - just enough to start.
Then I coated with the retaining compound - after it cooled a touch - and pressed the bearing in the rest of the way.
This ensured I didn't have a disaster with the bearing being at an angle as it went in.
Also avoided me overheating the retaining compound.
Fairly confident that bearing is not going to move now fully pressed in.

assuming you can make a backward facing carb clear all the other obstacles, keep in mind that:
1. the area of the duct needs to transition from round to rectangular with a width limited by the rotary pad (15mm?). So a 28mm carb needs to transition to a 15X42mm rectangle along the flow path to keep the same area.
2. the flow area through a port is reduced if it is at an angle to the port. depending on the timing and pad width you may not have the area to spare to bring it through at an angle.
3. as you add more design options you get into a nice combinatorics problem where all the choices impact each other. You will need a strategy for the order you will solve these as you pin down the design.


side view

front view
NOTE: I opened up the timing to 130° BTDC and 70° ATDC in this view.

OCT - interesting to see it modeled.
Below is a picture of how I had the carb mounted previously.
You can see - I fabricated a super short intake and angled it (this one is at 40°).
This allowed the carb to tuck in almost next to the motor.

For the rear facing concept - I planned to do something similar - only facing the other way.
Sketch in green line on top of yours to show it.




manifold (40°)

On scoot - tucks in nicely

OCT - having looked at engmod - and read your comments - would like to take you up on running a couple simulations (or calculations as the case may be).

Think the program will suggest a carb size - and that is the goal here.
You know my inlet timings - 119/70
My Tr and ex are approximately 123/181
60mm stroke.

Output I am curious about: What is carb size recommendation for these variables.
1. 13mm, 14mm, 15mm inlet widths
2. Timing moves to 125/185

What I am looking for is to understand
A. how inlet width and timing affects max carb size.
B. how timing increase affects max carb size.

I have both a resonance pipe with fairly low resonance (5500 kicks in) and Pollini box. Not sure how these will affect the calcs - so maybe just choose one or the other to keep it simple?

If to much of a PIA - just lemme know.
Curious tho!

Just to be clear on engine data:
bore: 63mm
stroke: 60mm
rod length: 110mm or is it 105mm?
Target max power RPM: 6500? 8500?

Thinking through the "test plan"... not a PITA at all, I gots to know!

Love this.
Ok.
62mm crank.
63 bore
110 rod
Target max power - can we do two?
7500
8500

Psyched to see.

I had given some thought previously* to a backwards (or any kind of directly facing at the cylinder) carb. While I'm not sure if I'm talking through my hat, directly or otherwise, isn't it the case that regardless of direction, the gasses are sucked into the crankcase first anyway?


*When that Indonesian guy was offering a backwards-facing VBC case if you remember?

I absolutely remember. What do you mean? Are you saying - rear facing doesn't help anyway because they flow with suction?

I would have thought that there would be reduced turbulence with a direct shot in.
Jack seems to have been down this road at least vicariously - and didn't see much in it.

I'm saying that I think it never actually goes directly into the cylinder anyway... the first cycle is crankcase compression - and then it goes through the transfers into the cylinder. In my head that sounds right but I'll stand to be corrected...

If that is correct then it shouldn't make so much difference as to which way it faces.

Ahh.
Interesting perspective.

This is well worth the watch. It's for piston ported chainsaws, but it really made it easier for me to understand just where the fuel and air are going in a 2 stroke.

If you take the time to look at the next 2 in the series, the reasons why different port timings will give higher revs with less power or vice-versa become really clear (to me at least.)

Thanks Craig- it's a good one.
Agreed - you can see the trade offs with higher timings shown.

Jack - as I am setting up cylinder - looks like I will have some space between BDC and bottom edge of transfers. Do I want to open those?
Same with Ex. port.

Recall one of these at least, may have some trade offs when opening.

before cutting the bottom of the ports, it's looking like it will be very hard to get enough intake area for all the exhaust and transfer you have. kind of wanting to plug it all in and simulate.

On smallframes the carb is turned around to get a straighter flow on the exhaust and shorter inlet/massive reed block. On a large frame is a bit pointless.
However, I may well in the future be building one with the carb on backwards for other reasons......It will all make sense, you'll see.

Important Exhaust port is flush at BDC, otherwise fumes get trapped and several other reasons.
Transfers flush at BDC is contentious in the tuning community. I think if you are able, do it. Transfers are 90% done by half open but for max power it all helps.

Let's see what your sim shows. I'll hold. Sounds like an interesting thread. Am sure other's would like to see just exactly what the backwards spin is for. Lemme know if you need something aluminum welded... Let's see what OCT comes up with on sim. BDC on ex. will get ground.

Here's the STA numbers for the rotary inlet with 119/70 timing:

One thing I didn't expect was for the "theoretical power" to go up as the rpm went down. but that increases the STA since the time is longer at a slower speed.

I'd say the max BMEP you could expect would be ~8, so that rules some out.

In looking at the way the big end of the crank blocks the inlet for the first 10° I think you could open earlier so that when the transfers close the intake really starting to open.

also included a width of 17 since you were doing the crazy welding and spacing of the crank.

Hey!
OTC - thanks!
I've gone all experimental on the turkey this year.
Sous vide then finishing on the grill.
Has left ample time for drinking while the turkey gets itself ready for the show.

Interesting - the widening looks like it would have been highly effective at raising HP?

Assume:
BMEP - is break mean effective pressure in the cases.
48.5 used is my current opening.

Wondering:
- can we run same with 10° more inlet length?
- why not extend closing?


Happy T-giving everyone!


Prepped and ready. De boned, dry brined, deboned, seasoned, rolled and tied. I would say these can produce about 20-22 HP on a good day.

Last minute realized I didnt have a pot big enough for the sous vide. So I improvised with my wife's silver punch bowl. shhhhh emoji. Just between us.


Last edited by charlieman22 on Thu Nov 25, 2021 5:13 pm; edited 1 time in total

Is 119/70 the rotary valve timing? 119 BTDC / 70 ATDC total of 189? I'm still thinking of pulling mine apart and opening it up.

Can you model a reed valve also?

Oh lord - here we go.
Timing is 119/60 = 179.
The 70 is an angle between a line through TDC intersecting with a line through the point on the pad that it has been cut to.
So the pad was cut at 70° to TDC.
But perhaps it should have been cut to 60 for some additional oomph!

Edited


Angle A is 70. Second number in timing, the /60 in this case, can be calculated E+F-A. E+F are the crank before and after TDC cut. So as A goes down - your timing duration /xx goes up. (if A had = 60, then 125+5-60 = 70 and 119/70/189 would be duratio


Last edited by charlieman22 on Thu Nov 25, 2021 5:28 pm; edited 1 time in total

That peak at 5000 rpm and a 30mm carburetor max, looks good to me.

see prior edit as well.

the "119/70" that I ran was the timing, not pad angles. so this was already with an extended closing from the 60° ATDC


Back to my gnocchi

Suspect that gnocchi looks like it might be locally famous.

Had a look at the chart.
If you believe Eng Mod - then for ea. mm of width of inlet opening, I'd get about 1hp.

OTC - the second run then would be interesting to hold all same but alter timing to 119/60 - to see how much that 10° of open time in length affects the HP and max carb size.

Interesting that Eng mod thinks I don't need a larger carb.

Jack - thoughts on extended duration?

did some spot runs with the 119/60 and reworked the chart so we can fit them in.

note that the STA method does not care about when in the cycle the port is open, all it's looking as is "do I have enough time and area to cram the air through to hit the HP target". or as we are doing here "what HP can I hit with a given time and area".

general method in engmod is to adjust the ports to get the STA of the intake, transfer, exhaust to a "balanced" output before starting with the simulations. Then look at the pressure waves and other data to adjust the design.

to get the port length I had to model the crank and I assumed you are using the SIP crank and that "F" in your diagram is 130°. when the duration drops by 10° to 179, the port length shortens to 37mm, so you take a hit in time and area.

It is incredible how the OCT method coincides pretty much with the actual experience. In as much that rotary inlet can take a 30mm carb and no more and even that might be slight overkill. Nice to have data to back it up.

Extending the duration makes more power but loses bottom end. The 10 degree overlap will make more power at high rpm but there will be bottom end losses. Extending the ATDC will make much more power but with a further increase in bottom end torque loss.
Need to ride it and see how it goes. If it wasn't pulling a sidecar the answer would be simple.

created a parametric sketch in fusion 360 to capture all your angles: https://a360.co/3rdIZ9Y

This should avoid confusion and make it faster to iterate on designs.


diagram is updated based on parameters. crank can be rotated to visualize port open and close

Input and calculated parameters.


Last edited by oopsclunkthud on Fri Nov 26, 2021 10:09 am; edited 1 time in total