I no this is probably the most basic question many of you have ever heard. I understand the basics of how your typical 2-stroke works and I hear all you guys gripe about how slow old saws are versus new saws. So what makes the new saws so much faster? Is the rpm limited by the rate of intake or just by the engineering of the saw. Thanks for helping me satisfy my curiousity.
what limits RPM
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kurtty
ArboristSite Operative
i think catastrophic failure is what holds the old saws back from high RPM's
gink595
Addicted to ArboristSite
Intake, Ignition (points vs.electronic), stroke, bore, rotating asssembly weight, exhaust, flywheel size/weight, port timing, compression ratio. Just about everything. To me that is a hard one to nail and point to one thing, everything effects another. Either positively or very negative. I'll be curious to see what others think.
Big Neb
ArboristSite Operative
I believe the newer saws have better lubrication, can handle the heat, and are designed for the higher rpms(Just better engineering).
You can probably get an older saw wound up pretty tight before you blow it to smitherings.
I think it boils down to the ability and integrity of the saw.
If you think about it, what limits our current saws at about the 14,000 mark? Mostly a lubrication, heat, and integrity of the saw problem. Same with the older saws.
You can probably get an older saw wound up pretty tight before you blow it to smitherings.
I think it boils down to the ability and integrity of the saw.
If you think about it, what limits our current saws at about the 14,000 mark? Mostly a lubrication, heat, and integrity of the saw problem. Same with the older saws.
1953greg
ArboristSite Guru
so then if a 2 stroke would stay glued together, would lets say 20k be possible??
has anyone intentionally taken one to the limit w/ a tach attached?
neat topic
Big Neb
ArboristSite Operative
Go ahead and try it...
Let us know what happens!
I would also let a friend do the trigger work for you while you hide behind a wall. I'm not sure sending a connecting rod out the side of the saw at a high rate of speed falls under my insurance coverage.
Let us know what happens!
I would also let a friend do the trigger work for you while you hide behind a wall. I'm not sure sending a connecting rod out the side of the saw at a high rate of speed falls under my insurance coverage.
1953greg
ArboristSite Guru
i did try to explode a briggs 4 stroke once that had the shaft sheared. filled it full of fuel, tied the carb wot and leter go. ran the tank empty. filled it up again, fired it up, and tweaked the carb to highest pitch. ran the tank empty again. filled it up again, ran empty again. didnt have a tach on it but it was definitely somewhat over 5500 as i have been around briggs kart engines turning 5500. i would guess around 6000-6500rpm.
as for 2 strokes, ultimately arent they limited by the speed of sound?
as for 2 strokes, ultimately arent they limited by the speed of sound?
gink595
Addicted to ArboristSite
There are 4 stroke turning those rpms, so I would imagine that is quite possible on a 2 stroker. I would think that ignition is a major factor in the newer saws. I would not think a points ignitoin could hold up to revs of newer saws, even with a dual point setup. Piston speed would proably blow the spark out. So what good would it be to have a saw engineered to run 15,000 if you couldn't fire the igniton at that speed, the older saws proably had to rely on torque due to the lack of tech back then. So now we proably shorter stroke engines that don't need torque but rely on engine speed coupled with newer ignitions and carbueration.
Holy s*** there are 4's that turn 20k. Read about some yamaha dirt bikes that turned 14K and thought that was absolutely ridiculous.
Ok another question for everybody, benefits of torque vs. hp? go to it, will be interested to hear what you have to say.
Ok another question for everybody, benefits of torque vs. hp? go to it, will be interested to hear what you have to say.
1953greg
ArboristSite Guru
has anyone else ever ran a 2 stroke w/o the muffler in the dark or dim light and looked into the exhaust??? the blue flame lights up the inside completely and the piston is moving so fast that it dissappears. really neat sight.
i still think the speed of sound, which limits the speed of air movement, is ultimately the limiter. of course that limit could be overcome w/ compressed air, maybe?
where is spacemule when we need him
i still think the speed of sound, which limits the speed of air movement, is ultimately the limiter. of course that limit could be overcome w/ compressed air, maybe?
where is spacemule when we need him
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Sawdustmaker
Addicted to ArboristSite
I have been wandering where all the energy that could be going into a log is going at WOT out of wood.
Art Martin racemodded a 090 and tached it at 18k on nitro
Walker saw shops did a 3120 that would run at 18k
Art Martin racemodded a 090 and tached it at 18k on nitro
Walker saw shops did a 3120 that would run at 18k
Sawdustmaker
Addicted to ArboristSite
i still think the speed of sound, which limits the speed of air movement, is ultimately the limiter. of course that limit could be overcome w/ compressed air, maybe?
What does that means?????? Sound travels in a medium. The speed of the medium has nothing to do with the speed of sound.
The speed of sound my be the limiter on other components but i don't think so.
gink595
Addicted to ArboristSite
I like torque, Thats why I'm so attracted to diesels and low turning rpm's, In dirt bike I always favored a 4 over a two. In street bikes the longer stroked 1000cc over shorter stroked 600cc. But that is my preference. I like the delivery of power earlier than ringing the crap out of it.
1953greg
ArboristSite Guru
isnt the speed of sound limited by the medium?
after all, in water the speed of sound is bout four times faster than in air
assuming the engine will stay together, isnt rpms limited by how fast air can be moved, and wouldnt that be limited by sound waves, ie the speed of sound? im thinking bout the roll expansion chambers play.
somewhat outside the box
i really know nothing bout what im saying, just thinking/asking.
gink595
Addicted to ArboristSite
yeah but that is where porting and bigger carbs and exhausts come in to play. To be able to handle and exchange more airflow, besides you can manipulate airflow with super chargers and turbos.
1953greg
ArboristSite Guru
i guess im thinking more bout theoretical than practical.
the original question was bout actual saws.
i drifted off line
so i guess the practical limit would be the expansion speed of the gas vapor, right?
so maybe thats why the newer saws, ie shorter stroke, turn more rpms?
looks like the practical 2 stroke limit would be bout 40k, if the stroke is short enough!!!!!
http://www.rc-trucks.org/rc-nitro-engines.htm
the original question was bout actual saws.
i drifted off line
so i guess the practical limit would be the expansion speed of the gas vapor, right?
so maybe thats why the newer saws, ie shorter stroke, turn more rpms?
looks like the practical 2 stroke limit would be bout 40k, if the stroke is short enough!!!!!
http://www.rc-trucks.org/rc-nitro-engines.htm
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safeT1st
ArboristSite Operative
Volumetric Efficiency and time
Speed of engine is limited by volumetric efficiency . Design determines how the air / fuel mixture is allowed to pass into , combust and exhuast from the engine . 2 and 4 stroke engines have many breathing obstacles to overcome combined with the inertia developed from the rotating mass . If it turns too fast things will start to fly apart all by itself ( runaway is the term used if a diesel becomes ungoverned) .Look at your typical turbine engine ...Virtually no restriction or impedence to air flow , quality brgs , fine balance resulting in load distributed evenly on thrust and axial load surfaces , not to mention more than adequate lubrication provided . I would suggest that ignition also limits speed as you must start and stop each cycle every time whereas the turbines are a continuous burn . There simply isn't enough time between strokes to charge and exhaust the cylinder . That's my 2 cents .
Speed of engine is limited by volumetric efficiency . Design determines how the air / fuel mixture is allowed to pass into , combust and exhuast from the engine . 2 and 4 stroke engines have many breathing obstacles to overcome combined with the inertia developed from the rotating mass . If it turns too fast things will start to fly apart all by itself ( runaway is the term used if a diesel becomes ungoverned) .Look at your typical turbine engine ...Virtually no restriction or impedence to air flow , quality brgs , fine balance resulting in load distributed evenly on thrust and axial load surfaces , not to mention more than adequate lubrication provided . I would suggest that ignition also limits speed as you must start and stop each cycle every time whereas the turbines are a continuous burn . There simply isn't enough time between strokes to charge and exhaust the cylinder . That's my 2 cents .
Cerran
Addicted to ArboristSite
They are also limited by flame front speed during combustion. You can only burn a fuel/air mixture so fast. This is one of the primary limitations of RPM in a Diesel, the flame front speed is quite slow compared to gasoline.
With a larger cylinder the larger it takes for a flame front to completely fill the combustion volume which is why larger engines have practical combustion related limitations.
This is also why RC engines can turn so fast, they have a very small combustion volume and hence a short flame time.
Gas flame front speed is around 70 to 170 ft/s (Dependant on octane and fuel/air mix) with diesel being around 40ft/s if I remember right.
With a larger cylinder the larger it takes for a flame front to completely fill the combustion volume which is why larger engines have practical combustion related limitations.
This is also why RC engines can turn so fast, they have a very small combustion volume and hence a short flame time.
Gas flame front speed is around 70 to 170 ft/s (Dependant on octane and fuel/air mix) with diesel being around 40ft/s if I remember right.
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