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Reed Valves-the modern ticket to 2-stroke power

trappermike

trappermike

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The reed valve has many advantages over it's earlier piston-ported intake power.
Yamaha really pioneered reed valve intakes: piston-port reed intakes and had much success.
The reed valve has a much better intake-crankcase charge and timing than a piston port,and also has much better crankcase compression than a piston port. Of course we're talking modern hi-flow reed valves,not some Homelite XL-12 thing. Piston intake ports open at a way too late period,and close at a way too late period,but they can make great power. If only you could make an intake port open at the right time and close at the right time,well reed valves come close to that. Not only that the reed valve stays closed during the piston's downstroke to create more crankcase compression,there is much higher pressure in the crankcase to push gases up the transfer ports.They also create a much better intake period,almost ideal,a really strong well-timed intake charge of up to 180 degrees or more.. In piston port motors there may only be 30-40 degrees of crankcase compression when the intake port closes to push fuel up the transfers into the cyl.,whereas the reed motor has about 120 degrees compression on the crankcase-far more pumping pressure.
Of course to adapt reed valves to a saw you must use modern design,a big reed cage and well designed intake. With reed valves you can also use a larger carb than can piston-port and still run like stock.
 
trappermike

trappermike

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The rotary-valve intake can provide exact intake timing and make exceptional power but are too big and complicated for saws,used in some kart,motorcycle and snowmobile engines,but you can't convert one to a saw...
 
trappermike

trappermike

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On a 94cc saw if you had a modern reed-valve intake you could use the big HT Tillotson carb with no problems,very stock running, and gain a lot of power.
Reed valve conversions generally create much more low-speed and midrange power,but done right will greatly enhance high speed power. That's why there are almost no hi-performance 2-stroke piston port motors anymore,dinosaurs of the late 60's,70's. Just really cheap and light for saw makers to build still.
 
buzz sawyer

buzz sawyer

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The rotary-valve intake can provide exact intake timing and make exceptional power but are too big and complicated for saws,used in some kart,motorcycle and snowmobile engines,but you can't convert one to a saw...
Wasn't rotary valve intake used on the McCulloch 3-25, 4-30, 5-49?
 
Ted Jenkins

Ted Jenkins

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Trapp rotary valves are not quite as complicated as you make them out to be. Setting them up in a chainsaw might be a challenge though. I grew up tuning them. As far a reed induction goes their big advantage would be to extend the power band, but with out an expansion chamber variable exhaust it likely would not be to practical. Thanks
 
trappermike

trappermike

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Piston port motors were abandoned in the motorcycle world in the late 70's,poor low speed power,piston wear and less transfer ports. Yes a reed valve intake and a bigger carb would add a little weight to a modern falling saw, it could double power,but of course all modern manufacturers want to use less metal and more plastic so that day will never come. The piston port is the cheapest and easiest and lightest,and that is all that counts,unlike other engines more HP will not come to factory chainsaws,only cheaper construction. I think some of the older saws will increase in value and collectivity.
 
sean donato

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Being into 1/5 scale rc with heavily modified 2 stroke engines, I can say reed ports are great, but not the end all be all for power. The difference between a piston port and reed case engine isnt much for hp numbers. More so where the power comes along in the som range. Pipe selection is much more important. The Induction method a well Made pipe will make up for a crappy intake reed set up, or even a mediocre port job with a case reed. The engine in the rig bellow is a 34cc zenoha based engine, mild port job, with a vrc tuned pipe. Piston port. Makes right around 6.5 hp. Imagine what we could do to our saws if we had room for a decent exhaust system, and a good carb set up.
 

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Ted Jenkins

Ted Jenkins

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Piston port, reed induction, rotary valve, and variable exhaust all have pros and cons. In general they would be more developed and used except for EPA restrictions. I remember being introduced to reed induction fifty years ago and not too much has changes since then. The development of fast moving reeds with exotic materials enabled Moto X motors to take advantage of the variation in the reed response times. The fast response times of the reeds with all the other developments enabled motors to have an incredible broad torque band. Well designed four stroke motors of today lack the raw HP that the earlier two stokes produced easily so larger displacement has been incorporated to make up for it. I still have quite an array of carbon fiber, fiber glass, Kevlar reeds that I would enjoy using, Thanks
 
hillwilliam

hillwilliam

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The reed valve has many advantages over it's earlier piston-ported intake power.
Yamaha really pioneered reed valve intakes: piston-port reed intakes and had much success.
The reed valve has a much better intake-crankcase charge and timing than a piston port,and also has much better crankcase compression than a piston port. Of course we're talking modern hi-flow reed valves,not some Homelite XL-12 thing. Piston intake ports open at a way too late period,and close at a way too late period,but they can make great power. If only you could make an intake port open at the right time and close at the right time,well reed valves come close to that. Not only that the reed valve stays closed during the piston's downstroke to create more crankcase compression,there is much higher pressure in the crankcase to push gases up the transfer ports.They also create a much better intake period,almost ideal,a really strong well-timed intake charge of up to 180 degrees or more.. In piston port motors there may only be 30-40 degrees of crankcase compression when the intake port closes to push fuel up the transfers into the cyl.,whereas the reed motor has about 120 degrees compression on the crankcase-far more pumping pressure.
Of course to adapt reed valves to a saw you must use modern design,a big reed cage and well designed intake. With reed valves you can also use a larger carb than can piston-port and still run like stock.
That's an excellent explanation of what I've come to understand myself, though I doubt I could have stated it as well.

I do love my ported Pioneers - torque and power too. Some think that reed valve saws have to run at relatively low RPM, but it's not true.

You started an interesting conversation and got some astute replies.
 
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