Limits on reed sizing?

I’ve been thinking of the bigger is better reed block theory.

Let’s say one were making cases from scratch. You could put a massive reed block or multiple reed blocks on the cases along with either one massive carb or multiple carbs.

Where is the limit?

If the cylinder was ported with mega scavenging and very nice flowing there a way to figure out the hard limit as to reed area?

Because at some point the cyl regardless of porting just couldn’t flow any more. I’d like to find out that limit. Did anyone ever do a gila on a case reed to fool with this in a dual carb kinda way? Like with double 24mm carbs?

I know Snordly ran a dual carb e50 case/cyl reed that was a monster on pinball. Used a 16mm and a 24mm.

Just have to have a big ass gas tank (edited)

Re: Limits on reed sizing?

I don’t have any reed moped experience. However, I know that intake area controls velocity and that controls pressure changes which make carburetors work.

Too big of an intake area would reduce the pressure drop in the carb(s). That would make an ever increasing size of jet necessary. Maybe making tuning harder if not impossible at a usable RPM range.

Multiple carbs with smaller, more controllable venturi would seem like the easier way to go.

I always default to the: “If it worked, they would have done it years ago” line of reason.

The most power, that I have heard of, comes from rotary valves. Unless I missed some stuff. So, that means its more or a timing thing than area. I could be wrong.

And, I am definitely drunk now. So... hope any of this makes sense tomorrow. Sorry if im crazy talking.

Re: Limits on reed sizing?

It doesn’t because with reeds you can have 360deg intake. It’s a one way valve.

Get with the times old man.


Re: Limits on reed sizing?

I had a professor in college that was a hot shot engine dude at mercury marine back in the day, he designed reeds that looked like this.


Crazy huh?

Re: Limits on reed sizing?

That’s what I’m talking about!

Big 2t or not so big?

Re: Limits on reed sizing?

I think that is like a 20hp twin, probably 200 cc per cylinder?

It's weird, they are 2 strokes but totally optimized for torque, like, 6000 rpm tops but mostly run all day at like 1000 rpm.

Re: Limits on reed sizing?

whaddya think about these studies?

on top of Papa's comments concerning charge lag issues in the carb:

I would imagine that to compensate for charge lag in the block, you would want to maintain cross section area and mass flow in balance with pressure gradient. so i think the coolest problem that you could run into is intake tract resistance.

let's say instead of conventional style reed block, you could use duckbill valves. you have an intake manifold that leads from 1 - infinite valves. (e.g. one big gulp straw, or 4 regular straws, or 20 coffee stirrers. and on). so each one of these has a duck bill valve on the end of it. a series of tubes (-_-) is real nice to think about from a fluid dynamics perspective, compared to the transition to a big square thing.

you can start to imagine the pros and cons of more tracts vs less, how much to increase the cross sectional area, how far the valves have to open, tract surface area resistance. and on and on.

I think you'll always have the lag issue of when your charge is expanding sideways, it is not moving forward.

unless you have regulated forced air induction, and could tune your intake charge to overcome these tendencies, but I don't really understand how forced intake works on a reed valve.

that being said, rotary vs reed depends a little on the application. from a singular peak HP perspective, i believe rotary is superior, but reeds are a great "problem solver" of low rpm blow back and power harvesting. making them generally more usable. i bet they are also easier and cheaper to manufacture.

as far the "if it worked they would've already done it..." i disagree. Most of what we have is fueled by market, application, cost, politics, cultural training, and necessity. remember how young the combustion engine really is! i think we see a lot more of "if it isn't broken, don't fix it"

diminishing returns...what gains are there? is it worth it?

interesting exercise though!

Re: Limits on reed sizing?

I’ve never hit a reed size limit yet. Every time I put more flowey reeds there’s more power.

So I’m considering running a pyramid reed derbi with a treats cyl reed or something, or add a second case reed block to a hobbit

Even on lower timed bikes I’ve run 360deg intake and the power went thru the roof without losing any width in the band.

Re: Limits on reed sizing?

I looked at those links, and decided I can’t understand all that.

Giving more sober thought to the OP, I have this to say.

Reeds are ideal for letting flow happen whenever the engine is asking for it. Not in peaky pulses like a piston port, but smoother.

One weakness to reeds is the speed they have to open and close and how far they deflect. Challenges related to that can be reduced by increasing the number of reeds and their area.

I would assume a large, low profile set of reeds that only had to open a little to get the same volume through, would open and shut faster with less flexing. It would just be complicated and expensive.

So, I still can’t see the gains in having too big of carbs. But maybe a case designed to allow maximum coverage in teeny little reeds that are super responsive and stable at even extreme high rpm. Well, that would have to be an improvement.

Am I getting closer to the idea now JBOT?

Re: Limits on reed sizing?

I’m just looking to find out max reed area possible for a high flow cyl with a hard hitting pipe.. that’s it.

At some point one would hit max, where adding more would not add any more power or rpms.

I’ve never hit that maximum though. But I will

Re: Limits on reed sizing?

So there are two factors that I'm aware of with designing a reed valve,

First you have the mechanical dynamics of the valve itself. It's a bending spring basically, it can only open and close so fast because the internal force of the spring is only so strong and F=m x a. If you are at 10,k rpm and the valve is trying to close in a tiny fraction of a second, it can essentially mess up the port timing and hang open because it doesn't have enough force to get it's mass accelerated and moved to the shut position, therefore a smaller thicker reed has more closing force than a bigger thinner reed, and more force allows you to run higher rpm just like a valve spring in a 4t.

Second is the gas dynamics. Opening a reed valve takes work, atmospheric pressure has to exert a force x time to do the work of deflecting the spring. The piston is creating a vacuum in the crankcase when it goes up, and this vacuum is a finite amount of energy (or lack of energy, so to speak) that is filled by atmospheric pressure pushing the reeds open. The more work the reeds are using up just to open and close them, the less work is available to move air into the bottom end. Larger, thicker reeds use more work to deflect, because they are a bigger spring, essentially reducing the volumetric efficiency of the charge in the crankcase.

So, as your reeds get bigger they can let in more air, that's good, but at a certain point they get too big and you gotta make them thicker to hit high rpm, then you're going to start to loose efficiency.

The 2 wild card factors are the pipe being able to draw direct from the reeds, and the trapping efficiency that you can gain from an airbox, essentially recovering some of that energy to raise the atmospheric pressure at the reeds.

If you ever want to see how a 2t should be designed look at stuff like the NSRs and RGDs... The tail end of MotoGP 2t was an insane drag out fight to the last iota of energy. There are always people coming up with new and novel designs in their garage- Boyesen is a great example of some kinda crackpotty stuff that actually works- but it's hard to improve on the work of major companies with the best engineers in the world dumping millions of dollars into getting an extra 1/10 of a hp.

Re: Limits on reed sizing?

Why not just run reedless tesla valves?

Re: Limits on reed sizing?

Another thing to think about is bigger the reeds bigger the case/plentum volume theres deff a limit on that too for max efficiency

Re: Limits on reed sizing?

I bet the next step in reed performance would be some exotic carbon nano-reed membrane surrounding the entire crank area. Allowing 0 impedance of flow in but instantly stopping any backflow at any rpm.

Which one of you college types has the lab to make that happen? Just for a few mopeds.

Re: Limits on reed sizing?

> Rebel Moby Wrote:

> -------------------------------------------------------

> Why not just run reedless tesla valves?

Found this :

Re: Limits on reed sizing?

How does carb diameter relate to cubic foot per minute? I think it'd be finding that, and then the pipe supercharged displacement. Like figure displacement * 1.5 for a hard hitting pipe packing it in * rpm = gives you the volume the cylinder moves at that rpm. Basically cc/min. Just needa figure out the intake side of that.

Like that'd get you the approximate cc/min your engine flows. Then finding a carb that has has a similar cc/min. From there taking the area of that carb and relating it to area of the reeds? There's a whole lot of pressure changing going on in there too which makes the math a cluster

^ whole buncha speculation here

Re: Limits on reed sizing?

That tesla valve intake looks cool. I bet it would backflow like crazy at low rpm.

I just saw a youtube video with some guy playing with a one way straw. I wouldn’t say it worked for slow moving fluid.

Re: Limits on reed sizing?

reed blocks it always was a weird concept, a block for an intake that you have to close off for it to work. So large for such a small volume, etc.

Re: Limits on reed sizing?

Calculator for "CFM" here, just dont get all mish mash calculating in metric and those ancient colonial measures of feet inch pound miles or what the heck...

(those guys have other calculators online as well)

> jordan * Wrote:

> -------------------------------------------------------

> How does carb diameter relate to cubic foot per minute? I think it'd be

> finding that, and then the pipe supercharged displacement. Like figure

> displacement * 1.5 for a hard hitting pipe packing it in * rpm = gives

> you the volume the cylinder moves at that rpm. Basically cc/min. Just

> needa figure out the intake side of that.


> Like that'd get you the approximate cc/min your engine flows. Then

> finding a carb that has has a similar cc/min. From there taking the area

> of that carb and relating it to area of the reeds? There's a whole lot

> of pressure changing going on in there too which makes the math a

> cluster


> ^ whole buncha speculation here

Re: Limits on reed sizing?

Yeah CFM flow has to be rated at a certain pressure drop so it's not a great way to rate a carb.

A 19 carb on a good designed 62cc reed kit can out flow a 21 on a poorly designed piston port. Since there are really only about 4 good choices for moped carbs, it's pretty easy to decide what's best for each application just by knowing them.

Re: Limits on reed sizing?

I will not depend on math for this.

Math isn’t my guy for this job.

I will figure it out with real facts and myth and legend

Re: Limits on reed sizing?


Even if you had all the computers, it would take 10x longer to do the math than just try some shit.

I just find a good designed engine with about the size and hp I want to make and copy the smart people. I use cobras, kx65, stuff like that and copy what they did. I bet the kx65 reeds are exactly what we want for a 15-20 hp mopeds. I have a set for a case reed e50 that has been on the back burner about 6 years...

Re: Limits on reed sizing?

I use kx65 reeds on my hobbit, just need to clearance the block adapter and cases a lil. Best reeds for the price point in my opinion

Re: Limits on reed sizing?

♣Slew Foot♣ /

Get a junk m48 and find out put a reedkit and an adapter plate and have at it. Case induction and an extra carb bit how do you sync the throttles?

Re: Limits on reed sizing?

You know I was thinking about this double carb thing and I've always been kinda dismissive since it seems kinda hokey, but you know what would work really good would be a rotary induction with a cylinder reed to draw through from the pipe. Those biturbo vespa kits was what I was thinking about.

Re: Limits on reed sizing?

My experience has been more is better, I have been running the same kx65 v-force reeds and have never replaced the reed material in over 10 years. It seems splitting the intake area over a larger number of reeds just helps with the longevity if nothing else.

Re: Limits on reed sizing?

Jason ( ͡° ͜ʖ ͡°) /

Funny you mention the Biturbo kit because I just managed to get my hands on one to try exactly that after the whole small case half (through crank) + rotary intake combo. And now that I think about it I completely agree with you. I certainly thing I am going to have a weird time pairing a rotary intake with a reed valve intake . . . we shall see. It will be interesting to see how intake timing between the biturbo and stock rotary inlet match up.

That vespa biturbo kit is wild. Looks like a clone of the illusive merkloos 70cc. Super rare and probably gone from the face of the planet by now so all we have are the poorly cast biturbo clones.

Ideally I'd like to try out every possible dual carb combination of:

- stock rotary intake

- small case half through crank

- small case half in front of crank

- biturbo piston port

Naturally this would require a custom intake for the biturbo that plays nicely with the small case intake since as is they would interfere.

Re: Limits on reed sizing?

Jason ( ͡° ͜ʖ ͡°) /



Re: Limits on reed sizing?

Omg wtf.

Mind blown

Re: Limits on reed sizing?

I am not super familiar with reeds other than my limited tomos use. I never got too far with trying much beyond stock reeds as I only ported and ran stock cyl and there isnt much room for different reeds unless spacers got involved and I just never did all of that yet...not to divert from the overall question but I also think it plays into the possible success of all of this.

After reading thru this and what Graham said about the limiting factors of reed size and thickness and the speeds at which it can function properly. Has anyone played with using two different reed sizes in the same block? Or would this be what the multi stage reed blocks are all about?

Thinking that at lower rpms you can have a set of reeds that function optimally until you hit over 10k rpm at that time the thicker set of reeds are powered and can function optimally to the bikes limit. Yest the first set of reeds lose most of their ability to close but they only allow half of the mix the engine needs anyway so it should not affect anything since the thicker reeds are flowing the rest.

So you could have lower rpm reeds on the cylinder intake, and higher rpm reeds on the case intake or half and half on each block.

I'm probably way off.

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