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

Discussion in 'Performance Mods' started by gamer121ps, Feb 24, 2014.

  1. gamer121ps

    gamer121ps New Member

    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?
     

  2. Fabian

    Fabian Well-Known Member

    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
     
  3. jaguar

    jaguar Well-Known Member

    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.
     
  4. Fabian

    Fabian Well-Known Member

    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.


    That's exactly the reason why a reed valve is used and the reason why it's so beneficial to low and midrange torque.


    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.
     
  5. gamer121ps

    gamer121ps New Member

    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?
     
  6. jaguar

    jaguar Well-Known Member

    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.
     
  7. jaguar

    jaguar Well-Known Member

    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.
     
  8. gamer121ps

    gamer121ps New Member

    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.
     
  9. keatonx

    keatonx Member

    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
     
  10. FurryOnTheInside

    FurryOnTheInside Active Member

    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! :dunce:
    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! :detective:
    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. :)
     
    Last edited: Feb 25, 2014
  11. Fabian

    Fabian Well-Known Member

    There is really no such thing as "suck", but a pressure differential that's always trying to find equilibrium. If a low pressure situation exists, atmospheric pressure will push it's way into that place to regain equilibrium.


    That's the reason why the reed valve is such an effective device at low and mid rpms, because at low rpms, there's more 'time' for the intake gas to be pushed back out of the crankcase, for the period that the piston needs to close the intake port. Secondly, at low rpms, there is much less intake velocity to counter the "push back effect" of gasses being forced out of the intake port on a piston ported engine.

    For this reason, high performance 2-stroke engines eliminate "intake" piston porting and have the reed valve directly installed in the crankcase. Incidently, Arrow Motorized Cycles makes a modified Chinese 2-stroke bicycle engine that uses a crankcase reed valve system.
    I want to purchase that engine, but Arrow Motorized Cycles won't sell it to me; that's just their version of customer service, which still leaves me scratching my head about their business model.
     
    Last edited: Feb 25, 2014
  12. FurryOnTheInside

    FurryOnTheInside Active Member

    1. From reading the dragonfly75.com page you linked: "The inertia of the air/gas pushed up into the cylinder through the transfers at BDC requires that there is at least one row of holes completely open when the piston is at bottom dead center" and then the reason why: "so that the vacuum caused by the upward air/gas movement [though the transfers] will draw more charge in through the reeds which allows more charge to enter the upper cylinder." Only raising the skirt and lowering the intake wouldn't achieve this same effect.

    2. Well you said "according to the link" so that's "just right", answer can't be anything else unless you drop the according to the link!
    From the dragonfly75.com page you linked: "Like they show here it is also advantageous to lower the intake port. For the HT you can lower it 5mm (to 60.5mm from cyl. top) at the intake port and cut 5mm from the bottom of the pistons intake skirt to the same width as the intake port. With this 10mm total the piston skirt starts to fully open the intake port at 85° BTDC. But this is only good in conjunction with a reed valve."

    3. No idea, it's not like I built one of these things and then went on a forum to learn how to. :whistling:
     
    Last edited by a moderator: Dec 18, 2015
  13. jaguar

    jaguar Well-Known Member

    Drilled holes are more essential than altering when the skirt opens the port.
     
  14. jaguar

    jaguar Well-Known Member

    Fabian, if the piston port intake with extended intake length works good for 4000 RPM, say from 1500 to 5500, then that rightly matches these engines which peak at 5500. I ported mine to rev to 8500 and so needed a greater range which is why I made myself a reed valve and then later bought one from RSE. The same reason is why reed valves superceeded piston port intakes, greater range.
    Did you ever even try an extended intake length?
     
  15. jaguar

    jaguar Well-Known Member

    Piston port intakes work well within their "resonant range" because the flow inertia within that range is enough to partially or fully counteract the propensity to back flow when the piston is lowering. An extended intake length lowers the resonant RPM range. If an engine was made that adjusted the intake length with the RPM then that engine would be superior to a reed valve engine.
     
    FurryOnTheInside likes this.
  16. FurryOnTheInside

    FurryOnTheInside Active Member

    I like the sound of that. An extended intake is less expensive than a reed valve too! :)
    Not putting a reed valve inside the intake port omits the "increase in crankcase compression ratio from 1.1 to 1.2 (for about twice as much transfer flow pressure) which is important for power"*, does that matter? How much of the benefit of the piston port intake is lost in this trade off?
    *from the page linked by the OP on dragonfly75.com
     
  17. Fabian

    Fabian Well-Known Member

    Yes i did.
    In my case, it didn't provide any extra torque over a standard intake pipe, so i reverted back to the reed valve intake and regained the improved torque that i was familiar with and the reduced intake noise that i was familiar with.

    Of all the benefits of the reed valve intake, it's the reduced intake noise that gives a much more enjoyable riding experience.
     
    FurryOnTheInside likes this.
  18. jaguar

    jaguar Well-Known Member

    Furry, for a non-modified engine reving only to 5500 rpm I am not sure if the CCR makes much of a difference. It did for mine reving to 8500.
     
    FurryOnTheInside likes this.
  19. gamer121ps

    gamer121ps New Member

    yes! :) i want to know the "reed valve supporting mods". specifically, intake duration to compensate for the reed valve. should i drill holes in the piston skirt in te intake side OR shorten the piston skirt 5mm on the intake side AND make the intake port longer by 5mm. would shortening the piston skirt 5mm on the intake side AND make the intake port longer by 5mm have the same or maybe even more of a performance increase then just holes in the piston skirt alone? and if anyone has tested if a reed valve AND reed valve supporting mods beats beats no reed valve at all in terms of performance on the grubee 48cc specifically, let me know.
     
  20. gamer121ps

    gamer121ps New Member

    that is what i feared...now i have no clue on where to drill the holes on the piston, what size holes, how many holes, or if i should have multiple rows of holes, or how many holes in each row... :( but, have you tested it? are you sure on the grubee 48cc engine specifically, even if i put the holes in the piston, i wont see any performance increase with the reed valve? if anything, i hope the reed valve will at least help with fuel economy since there will be no "spit back" coming out of the front of the carburetor soaking the air filter with oil (my last gas bike did that).
     
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