RES
ArboristSite Operative
I don't recall seeing a reed valve on any of the Stihl saws I have taken apart. On the 2 cycle illustrations they always show a reed valve. Can a 2-cycle operate without a reed valve?j
Reed Valves
- Thread starter RES
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timberwolf
Addicted to ArboristSite
Most if not all modern saws are timed by transfer ports cast into the jug. Some older stuff used read valves, but if I am not mistaken this limited the RPM, as the reads could only move so fast. Modern bikes use carbon or other exotic materials that lighten them up and allow for higher RPM, but I doubt there are reads that could handle 15-16K RPM without floating or breaking.
RES
ArboristSite Operative
But isn't the reed valve needed to hold pressure?
timberwolf
Addicted to ArboristSite
No, when the piston goes down towards BDC, the intake port from the carb is closed, then when it starts to go back up the transfer port into the cylinder opens before the intake, pumping the charge from the carb up into the cylinder.
ozflea
Good ol' BP Zoom 25-1
Reeds are a very handy item to have, in the older Mac's it allowed more wear and tear plus allowed the saw to produce more torque. Something the newer saw lack is torque relying on revs instead.
Mc Bob.
Mc Bob.
The reed valve are a intake timing device. A chaisaw uses the piston skirt as a timing device. Nothing to do with the transfers at all.
Reed valves are usefull in making more midrange HP, since a saw is only run it full throttle it isnt needed. Plus it adds cost and complexity.
Reed valves are usefull in making more midrange HP, since a saw is only run it full throttle it isnt needed. Plus it adds cost and complexity.
timberwolf
Addicted to ArboristSite
Ben, the transfers must be coordinated with the intake if the transsfers did not open before the intake on the up stroke the charge could just as well go back out the carb. But because the transfers open before the intake, a flow is established in the forward direction, some reversals do hapen, but once a flow is started in one direction it generally wants to continues in that direction.
Tell Dean that,LOL
Did you know that some bike engines with reed vlaves have 360 degree intake port timing?
BTW My comments where in regards to your use of the terms "Most if not all modern saws are timed by transfer ports" Its kind of ambigous and some people may misunderstand it.
timberwolf
Addicted to ArboristSite
I did know that read valves would allow a much longer intake window, but 360 deg? There would only be intake flow at times when the intake pressure was higher than the cylinder pressure, correct? So even though it is technically open is still can only flow at some number less than 360, maybe 200, 210?
Yes thats correct. No matter how long the port opening is it will only flow when pressure allows it.
How does this apply to Dean's Boost ports?
xander9727
The Silverback
Wasn't the stihl 191t a reed saw?
I thought the 200T was as well?
Lakeside53
Stihl Wrenching
yesxander9727 said:
Lakeside53
Stihl Wrenching
bwalker said:
nope...
timberwolf
Addicted to ArboristSite
Ben, Back to Deans boost ports, I did not post last night because the server was slow.
I think he has giving a 20 year old bone a chew to see if there is any meat on it, The idea was tried on bikes before, From what I can find, sounds like the bone was reburied by anyone who tried due to an unusably narrow power band and low end problems. But maybe it could work on a saw that could be limited to a few hundred RPM in the hands of a good saw racer.
They did not have much luck with the idea on bikes, but maybe it could work on a race saw with pipe and intake tuned run by an operator that had the ear to keep it in the powerband.
The idea as I understand it is to use the high boost ports to shoot a bit of the outgoing charge (before full bleed down)down into the base and bump the base pressure up before the transfers open, then when the transfers open the extra pressure helps in getting the charge up into the cylinder adding to the super charging effect.
One of the problems is, that when the saw is turning slow or when loaded heavy. There is ether a longer duration or more pressure put down into the base, bases are not designed to take exhaust pressure and if enough pressure is built in the base it is possible to light the base up, esp if a bit of carbon is blown down one of the boost ports. I think the boost ports are small enough with enough length to snuff flame propagation, but a hot particle could ignite the base.
I think he has giving a 20 year old bone a chew to see if there is any meat on it, The idea was tried on bikes before, From what I can find, sounds like the bone was reburied by anyone who tried due to an unusably narrow power band and low end problems. But maybe it could work on a saw that could be limited to a few hundred RPM in the hands of a good saw racer.
They did not have much luck with the idea on bikes, but maybe it could work on a race saw with pipe and intake tuned run by an operator that had the ear to keep it in the powerband.
The idea as I understand it is to use the high boost ports to shoot a bit of the outgoing charge (before full bleed down)down into the base and bump the base pressure up before the transfers open, then when the transfers open the extra pressure helps in getting the charge up into the cylinder adding to the super charging effect.
One of the problems is, that when the saw is turning slow or when loaded heavy. There is ether a longer duration or more pressure put down into the base, bases are not designed to take exhaust pressure and if enough pressure is built in the base it is possible to light the base up, esp if a bit of carbon is blown down one of the boost ports. I think the boost ports are small enough with enough length to snuff flame propagation, but a hot particle could ignite the base.
Reed valves are actually a lot more than a timeing device. On a high performance two stroke engine the intake charge actually comes into the cylinder and part way out the exhaust port, a returning pressure wave then forces most of the fresh charge back into the cylinder. A reed valve is needed to prevent this pressure wave from pushing the charge back into the carb. Thus, with a good expansion chamber and reed valve you can get a positively charged cylinder yeilding great power.
On a very mildy tuned non-expansion chamber equiped two stroke engine a reed valve is not really needed I suppose, but on a high performance two stroke engine you'll have difficulty getting top power out of it without a reed valve. For example, look at the old Kawasaki JS550's compared to the newer 550's - only real engine diff was a reed valve in the newer ones, but they sure run a lot better and start easier too.
I would really like to see a modern saw design that utilized a compact expansion chamber and reed valve. I suspect you could see a lot more power and a broader power spread with such a design.
Just FYI all modern motorcycle, kart, light aircraft and jetski two stroke engines that I know of use reed valves. 125cc two stroke GP bikes turn 13-14k rpm making 40hp and the reed valves handle that rpm just fine (they also have variable exhaust port timeing/compression raito/volume). They are now constructed of carbon fiber, but formerly were fiberglass and before that aluminum. All the materiels worked and could handle the rpm, its just that with the more modern materiels the reeds can be accomodating of different rpm's making it possible to get a broader power spread from the engine rather than having to use a stiff top end reed or a soft low end tq reed.
On a very mildy tuned non-expansion chamber equiped two stroke engine a reed valve is not really needed I suppose, but on a high performance two stroke engine you'll have difficulty getting top power out of it without a reed valve. For example, look at the old Kawasaki JS550's compared to the newer 550's - only real engine diff was a reed valve in the newer ones, but they sure run a lot better and start easier too.
I would really like to see a modern saw design that utilized a compact expansion chamber and reed valve. I suspect you could see a lot more power and a broader power spread with such a design.
Just FYI all modern motorcycle, kart, light aircraft and jetski two stroke engines that I know of use reed valves. 125cc two stroke GP bikes turn 13-14k rpm making 40hp and the reed valves handle that rpm just fine (they also have variable exhaust port timeing/compression raito/volume). They are now constructed of carbon fiber, but formerly were fiberglass and before that aluminum. All the materiels worked and could handle the rpm, its just that with the more modern materiels the reeds can be accomodating of different rpm's making it possible to get a broader power spread from the engine rather than having to use a stiff top end reed or a soft low end tq reed.
timberwolf
Addicted to ArboristSite
Hitting 13 or 14k once and a while on a dirt track is a bit different than running 13-14k, your talking redline RPM vs running at 12-14k day in day out.
You could go to a rotary valve also, but it is all space and weight, most power in a usable package that is also reliable and in recnet years must also fall within EPA guidelines.
A dirt bike turning 13-14k is a race toy, for many a saw turning 14k is workhorse used daily, a hundred or more days a year.
You could go to a rotary valve also, but it is all space and weight, most power in a usable package that is also reliable and in recnet years must also fall within EPA guidelines.
A dirt bike turning 13-14k is a race toy, for many a saw turning 14k is workhorse used daily, a hundred or more days a year.
amissionaryman
ArboristSite Lurker
Timberwolf,
Hitting high rpm is not the limiting factor for reed valves. The motorcycle industry has tested reed valve material that can withstand in excess of 20k rpm on a 350 cc gp race engine for over 3 months continuous use. And the emissions of a reed valve engine on a motorcycle are less than the emissions on a piston proted motorcycle engine. Of course there hasen't been any real technology advances on piston ported motorcycle engines for about 20 years.
God Bless,
Brent J Cox
Hitting high rpm is not the limiting factor for reed valves. The motorcycle industry has tested reed valve material that can withstand in excess of 20k rpm on a 350 cc gp race engine for over 3 months continuous use. And the emissions of a reed valve engine on a motorcycle are less than the emissions on a piston proted motorcycle engine. Of course there hasen't been any real technology advances on piston ported motorcycle engines for about 20 years.
God Bless,
Brent J Cox
timberwolf
Addicted to ArboristSite
Come on, was that 3 months at 20k?
Look at % of time a saw spends at max RPM vs a bike, sure modern reeds can take some high rpm but not for extended periods of time for hundreds if not thousands of hours of service. And if reads come apart what does that do to an engine?
Sure it could be done, you might well even make a great race saw with reeds, but would it hold up as a work saw, what about the carbon reeds you would need to run that kind of RPM, some of those reeds alone cost more than a cosumer saw.
Look at % of time a saw spends at max RPM vs a bike, sure modern reeds can take some high rpm but not for extended periods of time for hundreds if not thousands of hours of service. And if reads come apart what does that do to an engine?
Sure it could be done, you might well even make a great race saw with reeds, but would it hold up as a work saw, what about the carbon reeds you would need to run that kind of RPM, some of those reeds alone cost more than a cosumer saw.
