rse reed valve drill holes in piston or raise the piston skirt and extend intake port

gamer121ps

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Feb 23, 2014
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i have 3 questions.

1. im getting a rse reed valve for a grubee 48cc. i know that i should drill holes in the piston skirt on the intake side to extend intake duration for any kind of power increase with a reed valve. but instead, can i raise the piston skirt and lower the intake port to achieve the same affect? source: http://www.dragonfly75.com/motorbike/reedvalve.html

2. according to the link i provided, i can take 5mm off of the skirt and lower the intake port 5mm for a total of 10mm. is 10mm total too much or too little or just right?

3. after all is said and done, i know i will have more power, but what will all this do in terms of fuel economy?
 


Fabian

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After it is all said and done, you may only have a small improvement in peak power, but a noticeable improvement in low and midrange torque.

Just port match the piston skirt to the top of the intake port and things will be all good.
The low and midrange torque improvement with a reed valve intake will still be noticeable without modifying the piston
 

jaguar

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If you don't modify the piston then the reed valve will act as an intake flow restriction with the only advantage being that it prevents fuel mixture flow reversal when the piston is descending. Dyno charts show that even doing all the right mods the reed valve gives less low end power than a piston port (only) engine.
The best low end power is had with a piston port intake with extended intake tract.
 

Fabian

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If you don't modify the piston then the reed valve will act as an intake flow restriction
Not sure about that; logic says it should be the other way around. I can't see the reed valve being an intake restriction, because that being the case, Japanese manufacturers of motorcycle 2-stroke engines wouldn't use a reed valve system.


the only advantage being that it prevents fuel mixture flow reversal when the piston is descending.
That's exactly the reason why a reed valve is used and the reason why it's so beneficial to low and midrange torque.


Dyno charts show that even doing all the right mods the reed valve gives less low end power than a piston port (only) engine.
My experience has proven to show the opposite, hence the reason why my bike is fitted with a reed valve intake, and will continue to be fitted with a reed valve intake.
 

gamer121ps

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If you don't modify the piston then the reed valve will act as an intake flow restriction with the only advantage being that it prevents fuel mixture flow reversal when the piston is descending. Dyno charts show that even doing all the right mods the reed valve gives less low end power than a piston port (only) engine.
The best low end power is had with a piston port intake with extended intake tract.
ok, i am willing to modify. the thing is, should i drill holes in the piston skirt? or instead will shortening the skirt 5mm and lowering the the intake port 5mm give the same power increase as just drilling holes in the piston skirt?
 

jaguar

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The advantage of reed valves is that they work fairly well over a huge range of RPM whereas a piston port intake is confined to a narrow band of maybe 4000 RPM wherein it is advantageous.
I once rode a Hodaka 100 that had piston port intake and it was ported for high RPM power. It was a dog till it got up to 8000 RPM and then it took off like a scalded cat! Piston port intakes are just fine if the engines main use is within a powerband of 4000 RPM.
I tested the RSE reed valve against a piston port intake with extended intake and the latter gave the best low RPM power and would be my pick if I was over 180 lbs and used it mainly in town.
 

jaguar

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Fabian, a restriction to flow is that which causes a pressure drop. Such is the case with reed valves. If it didn't cause a pressure drop then there would be no vacuum to pull the reeds open.
 

gamer121ps

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The advantage of reed valves is that they work fairly well over a huge range of RPM whereas a piston port intake is confined to a narrow band of maybe 4000 RPM wherein it is advantageous.
I once rode a Hodaka 100 that had piston port intake and it was ported for high RPM power. It was a dog till it got up to 8000 RPM and then it took off like a scalded cat! Piston port intakes are just fine if the engines main use is within a powerband of 4000 RPM.
I tested the RSE reed valve against a piston port intake with extended intake and the latter gave the best low RPM power and would be my pick if I was over 180 lbs and used it mainly in town.
first off jaguar and Fabian, i want to thank you for your time. i just turned 21 and im really starting to get into 2 strokes. so i have ALOT to learn. but jaguar, your saying with all the right mods a reed valve will just decrease power no matter what? i wanted to install one on my next build which is going to have a grubee 48cc engine, sbp expansion chamber, jaguar cdi, light weight titanium wrist pin, and a extended tip ngk spark plug to increase compression. according to this website: http://www.dragonfly75.com/motorbike/reedvalve.html

you will see a performance decrease with just a reed valve alone (the testing was done on a Zundapp which i have no clue what that is). but with a reed valve and supporting mods to the piston and intake port, there was a nice increase across the whole rpm range vs no reed valve at all (piston port engine).
so, according to all that, i should see a increase with a reed valve across the entire rpm range if i do the right mods to the piston and the intake port. but here is my problem and it is the whole reason i created this thread. i dont know if i should drill holes in the piston skirt or just shorten the piston skirt and make the intake port longer by grinding it down at the bottom of the port. i was going to make the piston skirt shorter by 5mm on the intake side, and take 5mm off of the bottom of the intake port for total of 10mm.
 

keatonx

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Fabian, a restriction to flow is that which causes a pressure drop. Such is the case with reed valves. If it didn't cause a pressure drop then there would be no vacuum to pull the reeds open.
This makes me think of how people think there is no such thing as 'suck', and that instead of negative pressure drawing fuel into the engine, that the atmospheric pressure pushes it in. My thinking is if you take two chambers, both in a vacuum but one with more vacuum than the other, fluid will still flow from the less vacuumed chamber to the more vacuumed one, meaning it can't be solely atmospheric pressure that pushes things from high to low pressure. Correct me if I'm wrong
 

FurryOnTheInside

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Well they're all above zero, there's no actual negative values in pressure, a deficit seen from one perspective is just looking at things from the perspective of the lower pressure, but they're all numbers above zero so would be technically incorrect I suppose.
Anyway, it's the pushing agains the reed valve that slows the flow, but the reed valve stops the "one step back", thre's a trade off.

The really important question IMHO is why are two experienced MAB builder/riders saying their experience contradicts?
I'm here to learn so I can't stand this! :rolleyes:
I've spent an hour thinking about it and all I can come up with is it could be differences between their engines**, or it's the vague terms "low and mid range"; but even taking the language out of it their choices contradict so that screws up that theory; so back to differences in the detail of the engine mods (pistons drilling, boost port...).
**but jaguar says "even with all the right mods", though that's not a direct experience, and the references to 3000 and 3500rpm on his dragonfly75 are also from published research on other, better engines?

hmmm.. I'd really love to know the answer! :geek:
BUT
If the OP already ordered the reed valve then this may be moot in the context of the thread. The question of what mods to do if you are fitting an RSE reed valve is the focus of this thread. :)
 
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