Reed Valve V Rotary Disc Intake and why small 2 stroke engines favor the later.
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- Опубликовано: 28 фев 2018
- The reasons why small 2 stroke engines favor Rotary disc over Reed valve intake systems.
The main reason for this being the large capacity engine has more gas inertia to overcome back flow in transfer port during low speed operation.
At higher speeds the Rotary discs has better gas flows due to the fact a Reed valve has a pressure differential each side of it.
that is also somewhat overcome by the large capacity 2 stroke engine.
For these reason you will only find rotary disc valve intake system on smaller capacity 2 stroke engine and reed valve on all large capacity 2 strokes form about 125 cc per cylinder upwards. 
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Thanks for the mention! Great video and explanation as always!
Thank's Alex
I might do a second one as there are a few points I didn't cover.
It s a bit of a juggle avoiding too much detail.
hi you are here bro....!
Don't know if you still watch this but back in the 70's I had a Bighorn flattracker. My tuner was Jeff Bratten who was VERY good with porting two stroke engines for racing. He did the tuning for one of if not the fastest Yamaha 360 flat trackers it had dual 38 mm Mikuni carbs to feed the engine.
When he did the work for my Bighorn he came to the conclusion that I could put dual 54mm carbs on it and it would out perform the Yamaha. I never did that as it simply cost too much. But I really wish I could have.
But even with the single carb I could match that 360 in a drag race when going against the wind and he would pull me by no more than 2 bike lengths with the wind.
I still have the port diagrams the Jeff made and was thinking of building a replica for fun today. All I need to do is find a Champion frame and a F5 or 9 engine. That's all.
I rebuilt my seadoo a couple of years ago it had a direct injected oil anf fuel, also had reed valves and power valves. The engineering that went into it blew my mind.
It was a rotax 951.
Hi Stevo
Water craft have some interesting reed arrangements and used reed very early on.
Sorry for the delay with the video I had a pile of work to get done by end of Feb for a customer and had an angry week with YTube
Next video will get back to come expansion chamber handy equations to use as requested by viewers.
Thank you for this. I am studying the differences between 2 strokes and 4 strokes, and I am on a section about reed valves and transfer ports. The book glazed over rotary disc valves, but this video was very informative.
I would really like to learn more about the reed valve system. Things like increasing performance/flow and reed designs. How they effect performance. Designs for the entrance into the crankcase and how that would effect things. Your videos are awesome. Thank you for sharing some of your wisdom.
I was planning on doing a video just on reed design and covering some equations used for it, Ill get that done at some stage.
Very interesting. Content. Longer vids are fine. We who watch your channel can watch and try to understand / learn some serious schematics. Thanks for the lessons. Keep up the excellent work.
Thank you for the comment
I will be doing more on each topic, this video just covers the basic difference between the two types of intake.
Congrats brother, I was wondering if ya made it, keep up the good work.
Cheers Kyle
Stuffing sent me here. I'm staying, great informative channel :)
Very informative many thanks
This one was an interesting video too.
Thanks, Dave
You're welcome, and thanks for the comment.
A wealth of knowledge, more on reeds, material types, tensions, lengths and the effects on power characteristics would be great. I’m in the process of trying to generate more power from a KTM 50SX 2021 with a centrifugal clutch that’s very tunable, my grandson is 70 lbs (no gear) his competition is 40-55 lbs so getting a good start has been a challenge. He’s a good rider for 6 yr but he’s having to push to hard getting up to his real competition and doesn’t like losing. So it’s gramps job to get him to the first turn with the lighter kids, and getting any (real) info on a good engine package is next to impossible. Some shops will modify the cylinder and head but it’s not a package (carb-stinger) and I have the tools and ability just not the data as what will produce the best power mid-top Rpm’s I’ve heard you mention software for engine & pipe design do you sell some that I could purchase or could I pay you to generate the specs for our cyl, head, intake & pipe designs. From watching lots of your videos and being very impressed with your depth of knowledge that must have come from countless hours research and development over YEARS, I just don’t have that much time and testing for a 6 yr old is difficult. We’ve done it with his suspension and he was a trooper but I NEED some help with this unique 50 cc moto X mini maybe he can pull a hole-shot! If you sell your software please let me know how to purchase it or how to pay your for a build sheet for the pipe and engine combo, he thinks Gramps can fix anything and I’m going to try FOR SURE! Thanks for the inspiration to make more power!
GOD BLESS! Ray Harold
Very nice video, thanks! Could you please go into more detail about the reed valve intake design in a later video?
Thanks for the comment.
Yes I plan on doing more on both Reed and Disc Valve design.
What rotary valve intake need is a split disk that can be tuned for rpm for optimal intake and closing timing. Like a four stroke cam phasing on intake and exhaust timing for optimal rpm V.E.
RG500 had a variable rotary disc intake.
Google it and you can see pics.
Great video...thanks. I would like to see a little more detailed comparison between just the piston port and the rotary valve for high rpm high horsepower. My particular application is a 100cc 2 stroke engine that is going to be used for land speed racing.
The video does shows a basic a comparison to Piston Port to Rotary Disc at the beginning of the video.
How do you mean more detailed ?
thanks very helpful.
Thanks a lot for the video. Very educational. Why do you think that most 50cc-500cc 2T bikes come from the factory with reed vales and cage rather than the rotary disc? Doesn't seem sensible if they are cheaper to build that way and then doing just the reverse.
Note in the video that the benefit of rotary disc is for engines that have low air mass and thus inertia so the smaller the engine is the more benefit a rotary disc intake is, conversely in large engines that rotary disc offers little to no advantage once you get to 500 CC single, in 500 cc GP bikes they are mostly 4 cylinder so each cylinder is 125 cc in which case rotary discs does offer some benefit.
The issues for rotary disc is for one thing it is more expensive to manufacture and also requires the carb to be fitted to the side of the engine that can provide some obstacles for motor cycles as it makes the engine wider and the carb needs some protection.
But in general its a cost factor in smaller 2 strokes that reeds are cheaper and require less service.
am i right in thinking that if you tune the disc for hi rpm you loose out at low rpm due to the port being open to long and the charge reversing back out the open disc and this is what give the advantage to reed valves on lower rpm
Not quite, keep in mind the disc timing is set relative to the timing of the other ports so if the exhaust and transfers suit higher RPM then of course the timing of all ports will be less suited to lower RPM even if you have reed or disc.
Note in the video (at about 6 minutes onward) the disc opening is relative to the transfer closing point.
If the transfers and exhaust suit lower RPM then the disc timing would be closer to transfer closing.
So in short the reed is not beneficial in low speed more than the disc, its just that the benefits of the disc reduce the larger the engine capacity.
ok thanks ill watch again try and get that into my head
Well worth the wait! I hadn't considered the rotary ability to basically negate reversion of cylinder charge via early intake timing, which is basically impossible for reeds sans some complex mechanical wizardry that would be prone to breakage or at least compromise flow. Far easier to engineer too, I would imagine, lacking the pitfalls of inertia, flexibility vs sealing, and "valve float", never mind the box volume vs flow characteristics conundrum. Are rotaries less suited to larger cylinder volumes, or is your cutoff just one of obvious advantage?
That is the main benefit at low and mid, when you compare to piston port that relies in the expansion chamber pulling a good suction wave to counter that the RV will manage if off the pipe giving it more low down.
If you tried to increase piston port timing you would end up with late closing causing nasty reversion at the carby spitting fuel out at low revs.
And yes to the later, reed has no advantage with large capacities, its a case of the rotary has diminishing benefit the larger you go which leaves the reed as the better option on the basis of simplicity and compactness over a RV, being behind the barrel not side of the motor.
AuMechanic Piston ports seem fine for low performance, but are basically a high wire balancing act of compromise. Reeds obviously need at least some crankcase low pressure compared to the carb to just open, and require compromise as well in that rigidity, flexibility, strength set of requirements. Rotaries can open at dead, positive, or negative pressure compared to the carb, huge advantage, the only real compromise is port cross sectional area changes as the valve transitions state between open and closed. But, that "just cut in a crankcase port and drop in a Reed case" requires a LOT less head scratching, lol. I can see the diminishing returns on big rotary set ups just on crankcase volume/pumping volume/port clearance realities for actual gas flow to occur, though I imagine you could force it to work through offset or "cake pan" valves. You certainly stimulate my old brain cells, lol.
Suzuki had the Reed Piston port hybird, working as a piston port primarily and having the reed to feed the crankcase by gas inertia.
But was only really beneficial around the mid range.
So reed can be more versatile in that way.
AuMechanic I guess if you toss enough tech and sliderule you can make just about anything work, but the actual return for the effort and complexity, and the cost in reliability is where the rubber meets the road, lol. That hybrid idea actually sounds intriguing, but I can see how the advantage would diminish as the "port signal time" duration falls off at high revs. It actually sounds like those bicycle engine Reed kits in a way, only adding the Reed to the case and y piping the carb outlet, allowing a very mild Piston port timing but good packing until the Reed state latency eats too much time in the cycle. And I thought packing 4 strokes took brains, lol!
It was used on the old RM Suzuki off road bikes which made use of the good mid range.
!! Super
I'm seeking information on the importance of intake length of small two-stroke for the purposes of achieving higher efficiency in my bicycle engine.
The benefits of having a very long "tuned" intake are exceeded by the benefits of having the carb as close to the cylinder as possible.
So you only need an intake just long enough to convert the carb size to the cylinder port.
nice video! how to improve the rv system efficiency? to make it open earlier and close quicker? are using harder reeds and altering the reed stoper angle would help?
Hi Cer
Tempting as it is to bend the reed stop back increasing the lift of the reed could be terminal, it could cause the reed to break off at the root. There are other methods to improve the reed and I can go into that in another video as there is a few issues to resolve doing it.
Improving the RV disc is helped by proper port matching at the disc, some other mods are possible to avoid collision of mixture with the side of the crank, but overall tuning the disc timing itself will have the most profound effect.
AuMechanic Great! Learned so much from your video. Can't wait for next one!
Glad you got some info from that, Be another video this week.
HI, i completetely removed the stopper from my bike, and my power barelly increased( i didnt feel any increasing) but im sure it helped, but i dont have a good setup and only 70ccm with not more than 12k rpm, im sure they would break if my rpms were higher or had more cc
Hi ACreativeName
It will depend on the engines ability to open the reed to its stops but that's a bit hard to test for so the safe bet is don't bend the stops out.
A break at the root of the reed from doing that means the whole reed petal comes off and gets sucked into the crank case.
It can get mashed by the crank and bits of it get on the big end and up the transfers.
There are other safer mods you can make to them and we might cover that in a video.
Different reed thickness, 2 stage reeds, working the cage to get better flow, slight increases to the opening at each reed to name a few.
what will happen when I cut or cut my rotary valve by several millimeters on both sides what will happen? and if I do that some millimeters are contaminated?
Hi Lance
I've covered this in comments further down with Jason.
would it be a idea, that each transfer, had its own small reed valve ? so it wont try to backflow, into the crankcase.
No, looses created by it would like exceed any gains doing it.
Would the little nitro RC 2 strokes be considered to be disc intakes as it's actually the crankshaft that is cutting on and off the carby from the crankcase?
Missed seeing any videos from you for the last few years. Hope that you are keeping well and if you're looking for video ideas then an in depth analysis of how nitro rc engines work would be great.
Even though it has no disc as such the intake is "timed" by the crank slot at the carb intake so you could treat it much the same way.
And I'm doing well, thanks for asking.
I used to fly RC Heli's but I never really did any mods to engines or worked on them enough to cover more than the basics.
Aside from the different ignition they are mechanically a 2 stroke so a lot of the videos cover them too.
do u know anything bout this KH125 (GTO125 the disc has a small hole on the other side of the inlet i read it gives the boost port more kick and I don't see how that is ,to me it looks like the small hole did not line up with the inlet and if it did it would bleed crank air fuel back to the card unless im missing something and I would like to see what you think of boost bottles and how they help
Hi jt
I had a look at a parts list for it, hard to see the intake cover for the carb but I see the small hole.
That hole is so small there wont be much flow through it so I doubt the boost claim.
It also seems to align with the clearance slot on the engine plate as it rotates
It only has one gas relief slot around the face.
Note the rotary disc hub looks like a floating disc not rigidly fixed to its hub.
I would hazard a guess is it allows pressure to equalise each side of the plate so it doesn't get sucked against the surface being the disc floats on the hub not fixed, which would cause it to wear on the surface.
Without seeing more of it that's about the best explanation I can think of.
Does the oil injection pipe bolts to the DV cover plate anywhere, if so that may have some bearing in its purpose too.
its crazy I read a lot of people go on and on bout this small hole makes more power monkey see monkey do , but I know what your saying bout surface wear , I think the hole is bad and will bleed when you don't want it to but maybe its needed, glad to see some1 in to this stuff like I am your show is the best I seen with making tune pipe and really getting in to it so I don't have to buy the soft wear to make a pipe I know I should buy it ,I make couple pipes and getting better always looking for the new trick with anything do
With such a small flow any significant pressure differential will be a "choked flow", that means the speed of gas flow through the hole remains the same from that point on regardless of an increase in the pressure differential.
And the period of time involved is milliseconds.
Meaning there wont be enough gas flow through a very small hole or time to do anything like boost.
Id leave the hole there as it is likely there to resolve mechanical issues as I mentioned.
looking at that small hole ... its most likely a timing orientation feature for installing the rotary valve, nothing more .. a look at the assembly manual would be telling.
Why is it necessary to window the piston with reed valves? Is this something I'd need to do if i converted my piston port to reed valve?
The main reason aside from any benefits of reed is that the reed becomes the inlet timing control instead of the piston skirt. Since the reeds are controlling inlet timing having a hole in the skirt effectively bypasses the piston skirt controlling inlet timing
As for the benefits of reeds I have a video on Reeds vs Rotary Disc intake that covers much of this.
@@AuMechanic I watched it, was pretty good. Looking forward to more of your videos. Would i be right in saying the inlet port duration could be set to 360 degrees if reeds are being used?
Can someone recommend a video or a site with detailed information regarding setting up the timing of the rotary disc valve?
In short you need to get some starting figures by using a port time area calculator software.
Not aware of any free ones around though.
@@AuMechanic Thanks! any program that does not cost a fortune?
@@erikjohansson9039 Try torqsoft
www.torqsoft.xyz/disc-valve-inlet-time-area-two.html
Not sure of prices but they have a disc intake time area calculator
Rotary valves and oil injection really brought the Big Boy pants to 2 strokes
Yes rotary disc intake were a great innovation for small capacity engines and great for square fours even with the fact side mounting carbs caused ground clearance issue on cornering.
But oil injection was not without problems, when it was first used on street bikes in the late 60's it was to make it simpler for commuters to fuel up without having to premix however before they started fitting oil level sensors and warning lamps it was not uncommon for some to forget to fill up the oil tanks and then discover that after the engine had seized due to lack of oil.
And performance wise I can tell you having road raced street 2 strokes the first thing we did was to remove the oil injection and run premix instead.
@@AuMechanic you cant tune autolube up for power as easily as you can the rest of the engine. I run 32:1 premix in my RD350 racer
@@hardwaylearner Yes that's true.
There is also the potential of the oil pump failing so there are a few good reasons to get rid of it for racing.
@@AuMechanic Can mixing oil affect the performance of the carburetor in misting fuel? If that's true, does that mean you need to adjust the carburetor?
@@agameffendi6971 It doesn't effect the "performance" of the carb per se, the carb will still work fine and as we use premix in racing it also applies in high performance carbs both gravity fed carbs with a bowl and those with integral pumps such as used in Karting and chainsaws.
However as the oil makes the liquid slightly more viscous (thicker) as you increase oil ratio it means less will flow through the same size jet orifice per air, also the amount of fluid has less fuel and more oil so oil has less "combustion" value than fuel, both above effects will cause the air fuel ratio to be a bit leaner so you need to tune the carb a bit richer to compensate if you increase the oil mix...
Hi used to have a yamaha fs1-e fizzy 50cc we used to cut the disc down a little bit each side it seemed to make the bikes go better would doing this make a bike go better all just do nothing at all if it do make a different how much could cut away to make most power
Yes increasing the timing should produce more peak power, just be mindful of the relationship between DV opening and transfer closing point.
Video covers that to some degree at 6:00 onwards.
If you are going to mod the DV best to start by increasing the opening point first.
There must be a limit to how much you can cut away all can you cut a lot away as long as you can still time every think in
There is a limit for sure, you will reach a point where there is no gain by increasing the DV opening.
That will depend on the exhaust and transfer timing.
All three port timings need to be set to suit the peak RPM and are all relevant to one another.
That is where specific port time area analysis comes into play.
Open it too early and close too late and you are going to get some serious flow back or reversion that will upset mixture control at the carb and effect power adversely.
Very interesting nice to talk to someone who knows what they on about
Cheers
Just need to keep in mind that as RPM increases the less time there is for mixtures to enter and exit the engine.
So for high RPM we need more port timing period / port area.
The second part is the power spread or peak is more down to the relationship of port timings to one another.
does it make any difference if the reed is on the cylinder or if the reed is on the crankshaftcase?
Case fitted reeds are common in small weed eater engines for a more compact engine, not many performance 2 strokes with reeds direct to case, closer they are to the transfer intakes the better.
@@AuMechanic Thank u, so mainly talking about experiences with cylinder-reeds. Cause many tuning-cylinders in the market don´t have any cylinder intakes, buyers have to cut out a piece of their crankcases to make a port/adapter for the reed valve... Talking about small cylinders, under 150cc, for Kreidler, Zündapp, Simson, Puch etc., etc. ...
the rotary valve make more power
?
In short yes.
In Sprint Karting they were separated into different classes to race because the rotary disc intake engines had the advantage.
Can you do a video on piston ported engines?
Ill probably cover that if and when I do more videos on porting in general.
@@AuMechanic awesome thanks! I’ll be watching for them! I’m rebuilding a bultaco alpina 350, there’s not a lot of information on them out there. Thanks for the videos, cheers!
Hi. Very interesting videos. Could you do more un rotary valve or if you have the time for me to ask you some questions i would really appreciate it. Thanks in advance
Hi Ben
Any questions feel free to ask.
If you prefer email
ausmecham at gmail com
AuMechanic Hi. I have transformed a derbi 50cc with longer stroke and a 80cc cylinder 90cc in total, in to a rotary valve intake. I want the bike for a top speed drag bike. Exhaust duration is 201° and 135 port duration, what would you recommend has a starting point in disc timing. The disc is 112mm wide. Is your email address right? I can send you some pics if you want. I appreciate you're help and time
Hi Ben, just need to ask
Is that 135 total transfer timing ?
Also what is the peak RPM ?
Email is ausmecham at gmail com, I just removed the @, YT sees everything as spam even my own comments.
23rd hope some monetization comes back soon.
The volume is too low.
Would it be possible to have a rotary disc valve with variable timing?
Yes it does exist, basically uses a moving aperture on the intake plate to effect a timing change on the rotary disc. It can be operated by centrifugal or electronic servo by lever or cable.
@@AuMechanic That makes sense, thank you! Centripetal force sounds a lot more like my kinda jam, I'm still traumatized from trying to fix exhaust valve servos on a Honda NSR250 😅
@Christopher des Baux If you search google images you can find some pics posted in thumper talk forum..
You need to turn up volume on your recording. I've got it turned up all the way and can barely hear you
Yes some of the older videos hod low volume, my Sure mic was not compatible with my PC mic input, its improved in later videos.
Can you convert a scooter minarelli vertical from reed valve to rotary disc? 70cc polini
Not seen one but at a guess but you would have quite a bit of fabrication on your hands to do it for limited benefits.
@@AuMechanic is it worth it for me to install a 4 reed valve stage 6 intake vs regular (2 reeds) wirh a little bigger intake manifold into a 70cc polini with Yasuni R exhaust? It will improve the breathing intake of the system but I have a question, would I benefit from 4 reeds? Its for Yamaha Spy (CW50RSP) Minarelli vertical 2t AC
@@OnlyLifts. Cant help with specifics on engines Ive not seen but in general if the exhaust port is increased in size then you would benefit from a larger reed intake and carb.
@@AuMechanic could I contact you for some details if I needed any advice?
@@OnlyLifts. Just post here in comments as answers may help others.
Glad you made it through the pootube bull.
What kind of gain would you estimate going from piston port to rotary valve. On a small engine. Say low to mid 30s
To arrive at some figures of gain you would need to use a 2t modeling software the crunch the numbers for each engine to compare and provide power figures to compare.
Otherwise it would be difficult to quantify with any values.
But in short the smaller the engine capacity is the more benefit to Rotary Disc has over reed and piston port.
I know that it would be quite a few variables. But figure both were tuned well. Would you estimate like 1% or 2%. Or you thinking like 10% or more. Just kinda trying to ball park. Or would you have no idea with out modelling.
Its a question best posed to Sprint Kart owners who race piston port, reed and rotary disc 100cc as to the observed differences in power output.
I can only give you the technical differences of each type to compare to.
Only comparison I've had myself is driving RV, Reed and Piston port Karts on the same track.
However the RV (Parilla 100 cc) and Reed (DAP 100 cc) were both open class not homologated stock engines and both modified, but the Piston Port was homologated stock (Yamaha KT100S)
So the PP was slow by a great margin, the Parilla RV creamed the DAP in the top end, it pulled hard to very high revs, but it was also the one that grenaded pistons twice.
Not sure if that is any help.
Sorry. Didn't think it would have been such difficult question. Thx for the info tho. I will try to do a bit of looking around and see. Just learned about the rotary disk a few weeks ago. Be a lot if work to incorporate into use on weed string trimmer engine on a rc car. But kinda neat how they work. Thx again.
Meby the! Rotary Disk aaa litle Bit Name: präziser fore Open + Close.