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

[Fabian]

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.

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.

the reed valve stops the "one step back"

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.

 

[FurryOnTheInside]

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?

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.

 

[Deleted member 12676]

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

 

[Deleted member 12676]

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?

 

[Deleted member 12676]

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]

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.

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

 

[Fabian]

Did you ever even try an extended intake length?

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.

 

[Deleted member 12676]

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.

 

[gamer121ps]

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!
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!
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.

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.

 

[gamer121ps]

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

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).