Two Stroke Motocross
Two Stroke Motocross Forum => Technical => Topic started by: MattiasRyman on October 19, 2010, 02:08:57 AM
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Hi!
A question for all engineers and others with great knowledge.
Why will a dual stage reed, like boysen, work better than a single one?
I can understand the benefits of the V-force since it has twice the area the reeds only needs to open halv the distance and they will therefore be quicker to close. You can use a softer reed for max bottom end and still it will be quick enough at high rpm. It also has a straighter intake path.
But how on earth can a dual stage reed be of any benefit at all? Please can someone tell me?
Thanks a lot
Mattias
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maybe becouse the distance between the reeds its bigger
and it alows the fuel mixture to get in easyer
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maybe becouse the distance between the reeds its bigger
and it alows the fuel mixture to get in easyer
As far as I know it would have the opposit effect?
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The dual stage is so they can react to both bottom and top end. A single stage reed can't do this as efficiently obviously.
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This is what I am curious about. How can it react faster than the weakest link at high rpm?? I mean the soft reed petals also needs to close: Or are they thogether when they both are open at high rpm? Anybody got a clue?
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This is what I am curious about. How can it react faster than the weakest link at high rpm?? I mean the soft reed petals also needs to close: Or are they thogether when they both are open at high rpm? Anybody got a clue?
The small ones open at low rpm.. both sets open together at high rpm. Each set is designed to open at a certain velocity to help optimize power delivery.
The reeds close when the pressure reverses and tries to escape out the intake manifold. This process happens literally thousands of times per minute.
Hopefully this helps to explain.
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Hi,
The tricky thing is that at medium rpm when the bottom stiffer reed starts to open, will they immideately open together or will the soft one open more than the stiff bottom? If the dual stage tech actually work they have to open together, because if the soft one opens more than the bottom one, the dual stage tech is not effective at high rpm. Does this make any sense?
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You are correct that the "soft" one could open more than the "stiff" one at certain rpms... to hopefully clarify, if the "stiff" reed opens all the way it will push the "soft" reed into the casting of the cylinder, meaning that all the flow will be going through the largest opening at high rpm.
This process was developed by Eyvin Boyesen in the 1970's... the technology works very well. I've heard many stories about how he developed these reeds over the course of many years.
I know in the 1970's and 1980's that was one of the few mods that I did to every single bike I owned. It made a difference.
Keep in mind that the reeds "flutter" at amazingly high speeds, they do not "stay" at one point for very long. The more velocity going through the reeds the more they open, as soon as the velocity slows, they close preventing fuel-air mixture from being squeezed out of the carburetor.
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boyesen dual stage reeds was the best bang for buck item i bought for my bike. although it had all the surrounding mods, ported, pipes etc. soon as i put them on i noticed a huge increase of power! now im in the market for a flywheel weight as it is a wheelspin machine!
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Thanks for clearing things up :)
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Mattias you are 100% correct, there is NO WAY the bottom petal can lift when you have a very soft petal on the top. Ron Chinoy (RD Dreams) was the first person to strobe and film reeds in action. He found s as rpm rose the bottom petal barely lifted and the top petal flapped about uncontrolably.
The dual stage petal has long been ecilpsed by a VForce.
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Mattias you are 100% correct, there is NO WAY the bottom petal can lift when you have a very soft petal on the top. Ron Chinoy (RD Dreams) was the first person to strobe and film reeds in action. He found s as rpm rose the bottom petal barely lifted and the top petal flapped about uncontrolably.
The dual stage petal has long been ecilpsed by a VForce.
I'd disagree with that last part.I've run both and prefer the Boyesen pro series carbon's hands down.
You have to remember that as the top(softer) reed opens far enough the tension increases to the point that it is in contact with the bottom reed and the two are working as one at higher rpm.
They don't necessarily flow any better but simply regulate better throughout the rpm range.
Stock reeds are set from the factory as either high,mid or low tension depending on what power characteristics the engineers are after.
I've always found that as a rule of thumb,I've ended up jetting down two sizes on the main and pilot both after intalling the boyesens.
Better spread of power and better mileage.What's not to like? :)
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The simple fact that as rpm rises the top soft reed flaps about with no control. It is impossible for the top reed after it is open to allow the bottom petal to open far enough. Later model OEM reed cages have been designed to to direct flow at the reed tip as that is where it bends easiest. Not half way up the petal.
The fuel is actualy metered before the reed, a reeed is simply a one way check valve.
High end factory reed engines may have a duel reed set up but no ports are in the bottom reed.
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Of course it's possible for the bottom to open far enough-especially given the fact that the vast majority of the surface area is taken up by the top reed-the lower in a Boyesen set have a cut out close to the same area as the upper reed.The rev plate stops the top from opening too far and as you reach high rpm they are ,for all intents and purposes,functioning as a single reed.If,as you say "as the rpm rises the soft top reed flaps about with no control" then as you were trying to accelerate they couldn't possibly seal and some of the mixture would be pushed back out through the carb venturi-resulting in a bike that would blubber and stubble horrendously on acceleration.Sorry,I call BS on that.Boyesens don't work like that or they never would have sold a single set-and certainly not multiple sets to me.After jetting I gain more lower,mid and top and mileage with the Boyesens-every time,without exception. :)
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Now we are talking ;D
I like to hear all different teories. But a few more questions start to come up in my head.
1. Why do you need to jet differently with a dual stage read? It is after all just a "backvent" that prevent already mixed gases to go back through the carb!?
2. The larger the hole in the bottom reed. the less will the bottom reed open. Have not Boysen now a smaller hole in the bottom reed than before? To make the bottom reed open more(or at all).
3. I heard a roumor that most pro uses really soft reed that will need frequently replacement, anybody else heard this?
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A reed at 6000 rpm opens and closes 100 times per second at 12,000 it's 200 times a second. The reed remains seated for approximately 1/200th of a second at 6000, that is a single petal. If for a moment you still don't belive the top petal flaps about then maybe all those camera's that Queens University Belfast pointed at reed petals , lied also? That test under running conditions has been repeated a few times with strobe lights and high speed camera's. All with the same result. Top petal flaps about uncontrollably as rpm rises. I have the QUB paper if you care to read it?
As a footnote it is with much sadness that Prof Gordon Blair of QUB passed away during the week after a long battle with cancer. RIP and Godspeed Prof GP Blair.
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A reed at 6000 rpm opens and closes 100 times per second at 12,000 it's 200 times a second. The reed remains seated for approximately 1/200th of a second at 6000, that is a single petal. If for a moment you still don't belive the top petal flaps about then maybe all those camera's that Queens University Belfast pointed at reed petals , lied also? That test under running conditions has been repeated a few times with strobe lights and high speed camera's. All with the same result. Top petal flaps about uncontrollably as rpm rises. I have the QUB paper if you care to read it?
As a footnote it is with much sadness that Prof Gordon Blair of QUB passed away during the week after a long battle with cancer. RIP and Godspeed Prof GP Blair.
I'd love to read it.I don't see though(and am quite willing to be proven wrong)how you could set up a strobe and hs camera to capture relevant data.The strobe would have to be timed to something otherwise it would just capture the reed at various positions with no way of telling if it was on the way open,staring to close or vice versa.Regardless,the simple fact of the matter is that if the reed is staying open-or not sealing(anyone ever had a chipped reed?)the bike will run like a bag of crap.That said,I have no doubt they will flap at higher rpm in the same manner that valves will float but at that point you are outside of your effective power range so it's irrelevant what they do.
Mattias,that is their intended purpose but a dual stage reed meters better due to a smaller opening at lower rpm's and a larger opening at higher rpm's.Smaller openings produce more velocity at lower speeds.It's the same idea as how a smaller carb works better at lower rpm's and a larger carb works better for high RPM's.The power wings that are sold everywhere now are an attempt to produce the best of both worlds.
Before I changed to the Boyesens on my SX,I took the stiffener off the back of the reed(as recommended in my KTM manual)to see what effect it had.It was indeed much better for a short time,then fell on it's face badly-but it depends on where you ride and your riding style as to what would suit you the best.
I don't know if Carbon Tech is still in business or not but they used to sell low,mid and high tension reeds-depending on what you were after.Pretty narrow focused but it's the ideal if you ride mainly at one particular rpm.
2t institute,condolences with regards to prof Blair.Obviously I never knew him but when you are touched by it yourself you appreciate what it feels like to think you are at the end of your days.
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I'll try to find the SAE paper number, but simply there was a vaccum tank attached to a rotary valve (simulating engine pulses) a rotary encoder attached to the valve a reed valve housed in a perspex box with a high speed flash and a high speed video camera recording the events.
The rotary valve could be turned at various RPM and the data recorded with annometers and pressure transducers before and after reed valve.
I did get one part incorrect the test is on dual stage reeds but not with ports in the bottom reed , but I do know Ron Chinoy(RD Dreams) made basicaly the same test.
I'll see if I can scam these somehow
http://papers.sae.org/790842 (http://papers.sae.org/790842)
http://papers.sae.org/942528 (http://papers.sae.org/942528)
I didn't know Prof Blair but anyone with a passing interest in engines has benefited from his life times work.
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Excellent,thanks,it looks like it would be a good read 8)
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Two cycle engines are nothing more then "Air pumps" and from a theoretical vantage point a reed valve could be considered an obstruction in the intake passage. Some of the early RM's and the Rotax 370/400's were a combination of case reed and piston port induction.I wonder just how much a reed assembly inhibits gas flow?.
Is this the same Belfast institute mentioned every time a Greeves 370 Griffon comes up for auction on Ebay?.
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A reed at 6000 rpm opens and closes 100 times per second at 12,000 it's 200 times a second. The reed remains seated for approximately 1/200th of a second at 6000, that is a single petal. If for a moment you still don't belive the top petal flaps about then maybe all those camera's that Queens University Belfast pointed at reed petals , lied also? That test under running conditions has been repeated a few times with strobe lights and high speed camera's. All with the same result. Top petal flaps about uncontrollably as rpm rises. I have the QUB paper if you care to read it?
As a footnote it is with much sadness that Prof Gordon Blair of QUB passed away during the week after a long battle with cancer. RIP and Godspeed Prof GP Blair.
I would really like to read that paper, if you are in a position to share it that is.
The footnote is sad news indeed..