Why so few reed valves these days?

[drmiller100]

drmille, you mixed 2 differant things, so I will stick to one item at a time from now. When talking about boundry layers do not forget to mention surface tension. The higher the RPM the more it robs power.

surface tension is when them little water skipper bugs float around on top of the water. add a bit of soap (sulfactant) and buggy sinks.

I'd be curious to learn how this applies to friction of air.

 

[drmiller100]

Yes...the crankcase volume is important...but some engines will benefit from increased crankcase volume. Yamaha made long rod kits for the reverse cylinder TZs and early nineties YZ250s. The longer rod allowed a spacer to be placed between the cylinder and the cases, increasing the crankcase volume. Some builders will space the reed blocks back to accomplish the same thing.

I don't think any 2 stroke gas engine is improved by increasing crankcase volume in and of itself.
My GUESS would be the longer rod added horsepower, and required a spacer to make it work.
I think the reed spacers improvements can be traced to interference of the reed cages themselves, and the air not being able to get out of the reeds and up the transfers/boost ports. Sure willing to listen though!

 

[bwalker]

as for frictional drags along the intake tract, can you tell when you have opened the transfers on an engine? Get the right surface, and you have effectively increased the cross section for freely flowing air up the transfers.

likewise, if you took a good running engine, and welded a washer into the transfers restricting the air flow 20 percent in one little spot, could you feel a difference? My bet would be your engine would fall off the pipe at WOT, and she'd be a an ugly pooch. For this reason, I doubt putting 6 reed cages into your 2 stroke is a very good idea.

Let alone considering the effects of increasing the crankcase volume with all your transfer reed cages.

I agree with you, relax.:biggrinbounce2:

so you didn't answer the question about the oil in suspension hurting horsepower. or, if you did, i didn't understand it.

If the oil doesnt drop out of suspension prior to reaching the combustion chamber it means the fuel is in liquid form as well. Liquid fuel prior to combusting must be converted to a vapor so if you have oil/fuel mix making it to the copmbustion chamber you are leaching energy out of the combustion event. This causes a loss in power and incidently is the reason for a two cycle smokingex cessivly and making less power when cold.

 

[Tzed250]

?

isn't that a surfactant?

 

[klickitatsacket]

reeds build presure in the crank case forcing more fuel into the cylinder, unless you have the harmonics of the intake down to a nats ars. For most practical applications you will never be able to do this. Aside from this there are the limitations of drag from the reed cage. Which is better. That is the question and personally do not think it is cut and dry. There are guys out there right now who are getting 22k out of honda 125 engines in karts.
Oh and oil very much has to do with HP. Why can you get more compression and HP on certain engines running 16:1 vrs. 32:1?

 

[Tzed250]

Well

Its tough for a connecting rod to add horsepower, allthough the engine having the correct rod length ratio is important. The cylinder spacers "do " add power. If the case volume is insufficient the the cylinder filling will be likewise. I have worked on yamaha long rod engines and I do know for a fact this is why the technology existed!

 

[bwalker]

reeds build presure in the crank case forcing more fuel into the cylinder, unless you have the harmonics of the intake down to a nats ars.

Intuitivly that would seem reasonable but in the field it s adifferant can o worms. High crankcase compression motors for the most part do not work very well as the increased crank case pressure blows to much fresh mix out the exhaust, is only effective over a very small rpm window, etc. The jap mfg's went this rout in the 60's with not so hot results.

 

[bwalker]

Its tough for a connecting rod to add horsepower, allthough the engine having the correct rod length ratio is important. The cylinder spacers "do " add power. If the case volume is insufficient the the cylinder filling will be likewise. I have worked on yamaha long rod engines and I do know for a fact this is why the technology existed!

That was the 94 YZ 250, right?

 

[klickitatsacket]

I do not have a huge back ground in the reed cae engines. Only a few and most of what little I know about them is theory from those who know way more about them then I do. One of the things I have been told is that engines with more ports and huge timing areas is where the reed cages shine. Ben, your comment on the narrow RPM range makes a lot of sense. It would seem to me that a very narrow and wide exhaust port and well advanced with a short stroked motor would be a good bet for a cage design.

 

[Tzed250]

!

Yes Ben, the 94 YZ250 was the first YZ to use the long rod kits, but the 1990 TZ250 twin was Yamahas first application of the technology.

 

[bwalker]

I actually rode a long rod kitted yz 250 and one that was just ported. I liked the ported one better, but I was never a A class rider.
Neither had the bark of a 96 cr 250. Stock that motor was my favorite.

 

[drmiller100]

Intuitivly that would seem reasonable but in the field it s adifferant can o worms. High crankcase compression motors for the most part do not work very well as the increased crank case pressure blows to much fresh mix out the exhaust, is only effective over a very small rpm window, etc. The jap mfg's went this rout in the 60's with not so hot results.

ummm, modern sleds do it today. as do dirt bikes. as do pretty much any current 2 strokes.
Except saws of course.

it is pretty easy to get the back pressure right.

 

[bwalker]

The sledfs I have played with all had generous cc volumes. Polaris domestic , large block twins have huge cases.

 

[Sparky_NY]

Let me walk you through this real quick. At BDC the intake port is closed and the transfers are fully opened. Now as the piston moves up, it is expanding the crankcase volume and creating vacume. Until the transfers are closed that vacume is filled by the open transfers pulling fuel back down. Now when the transfers close then the leak is shut off and the crank case begins to build vacume again until the intake is opened. Reeds in the transfers could make the pp engine more efficient. Until it was built to see if the advantages out way the flow restrictions; it is just conjecture.

BTW, do not ASSume that every one knows less than you. You will not make many friends that way.

Dean, that concept of reeds in the transfers is extremely interesting. Even if not for chainsaws, what about motorcyles and other 2-stroke apps.
I used to have a Yamaha TD350, one mean 2-stroke engine... lately I have been fooling around with a 50cc scooter that I take on the motorhome camping. The scooter has reeds... I am ordering a set of Boyerson (spelling?) carbon fiber reeds to bump up the low end.
As for transfer port reeds and flow restrictions you mentioned... that was my first thought when you mentioned it... fitting reeds into the ports/crankcase, that would be the real trick! As for flow, they use reeds in the intakes without too serious a restriction issue and the transfer ports would each only be flowing half (or less) of the intake flow.
The point of the crankcase going vacuum after BDC and drawing part of the charge back down through the transfers from the top never occured to me. Guess thats why your the hotsaw man and I am happy to just keep em running.

You are going to prototype a reed equipped transfer port engine and report back, right? LOL Please? LOL

 

[PES+]

Oil shear and surface tension

Sure more oil can make more HP due to better sealing in the case after the piston closes the intake and between the ring or rings and the piston groove.
BUT you are then losing also due to oil shear which is worse the faster two surfaces pass each other. So too much on the bearings causes more drag
A crazy balancing act.
There are two schools of thought on that as near as I can figure. One school is use more oil and gain compression or use less oil and seal in a different way

Again the reeds and some other designs on pistons

 

[klickitatsacket]

You are going to prototype a reed equipped transfer port engine and report back, right? LOL Please? LOL

This is actually an idea that I have had only for a few months now. Cutting the transfers out and inserting reed blocks is a project I hope to do in the future. Right now though I have another possible solution, that if it works I will share at least a little bit of it

 

[PES+]

I do not make assumptions

I make chainsaws

I found an old friend and fellow builder and he is coming out of retirement

Stay Tuned For Updates


"BTW, do not ASSume that every one knows less than you. You will not make many friends that way."

Could not agree more

 

[Big feller]

What a great thread!

Full marks to Pest for getting the ball rolling on this one. Sorry to see swords crossed early on, but anything that gets the great minds on this site to spill some beans and teach us all plenty can't be all bad.

Looking forward to hearing the results of the reeds in the transfer ports...

 

[olyman]

. I am ordering a set of Boyerson (spelling?) carbon fiber reeds to bump up the low end.
Boysen reeds ---i have them on my boat--yamaha 50 hp--they work--after reading this whole thread--wonder if these were applied to the old saws--if theyd really howl--near no resistance with Boysen--carbon fiber--if they break--they just disintegrate in the cyl--and--theyre two stage--for low end--and top end both--neat--

 

[PES+]

Carbon is not very well suited for hyper saws......I used metal and most used the Phenolics I am serching now for a set of phenolics from an old CR 125 none of my new suppliers know why in the world anyone would want them.....

and I STILL haven't found my box of Yamaha bits