NiSi is not prone to chipping or flaking. Chrome is. I've never seen the NiSi chip on the port edge after a port job.
Porting 101
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That's not to say it can't. Always put a bevel back on the port.
AUSSIE1
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You can enlarge the windows, just don't sacrifice the strength too much.
The window can look like a jellybean tucking under the pin boss a tad if you get my meaning.
Woodman 460
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Very helpful infor. Thx for the thread!:greenchainsaw:
Ok..a question for the pro's...based on an observation and a little experience from the Dirt bike years with old Piston ported two strokes of the late 1970's ..
Subject is strictly intake port & air box relative to crank case volume..assuming things like muffler mods and standard flow related mods to the casting with out changing port timing... YET. (Yea yea I know all these things are inter related)
So I think I have encountered a situation where I think the exhaust flow & scavenging is way more efficient than the intake flow as the saw turned into a torque monster but didn't gain on top, feels like it just ran out of air. And it dawned on me there are a few ways to deal with that.. (Any one remember old KTM 420's? WAY way back when..a story 4 another time )
1) Open up the air box...as the opening is roughly 3/4 the cross sectional area of the exhaust. Also the first link in the (performance) chain that extends all the way to the outlet of the exhaust.
2) Widen the intake port..but it already is....so
3) In crease the intake port duration....and here is the observation:
Small crank case volume..lower the intake at your peril! The less volume means that the pressure rises faster and there is a chance you get the charge to spit back out the intake tract with no reed valve on a piston port machine, Larger crank case volume..more latitude as there is less pressure rise and the column of intake air can pack into that "softer" crankcase.
So on my older big bore J-reds (both an 820 and a 2094) lowering the intake made a tangible change. I want to try to lower the intake on a on a Husqvarna 272 a few degrees, but wonder if the crank case volume is too small to gamble that way.
Anyone ever graft a reed cage from a 60cc dirt bike motor onto a chain saw?
What do you think? And set me straight if I'm way off base.
Subject is strictly intake port & air box relative to crank case volume..assuming things like muffler mods and standard flow related mods to the casting with out changing port timing... YET. (Yea yea I know all these things are inter related)
So I think I have encountered a situation where I think the exhaust flow & scavenging is way more efficient than the intake flow as the saw turned into a torque monster but didn't gain on top, feels like it just ran out of air. And it dawned on me there are a few ways to deal with that.. (Any one remember old KTM 420's? WAY way back when..a story 4 another time )
1) Open up the air box...as the opening is roughly 3/4 the cross sectional area of the exhaust. Also the first link in the (performance) chain that extends all the way to the outlet of the exhaust.
2) Widen the intake port..but it already is....so
3) In crease the intake port duration....and here is the observation:
Small crank case volume..lower the intake at your peril! The less volume means that the pressure rises faster and there is a chance you get the charge to spit back out the intake tract with no reed valve on a piston port machine, Larger crank case volume..more latitude as there is less pressure rise and the column of intake air can pack into that "softer" crankcase.
So on my older big bore J-reds (both an 820 and a 2094) lowering the intake made a tangible change. I want to try to lower the intake on a on a Husqvarna 272 a few degrees, but wonder if the crank case volume is too small to gamble that way.
Anyone ever graft a reed cage from a 60cc dirt bike motor onto a chain saw?
What do you think? And set me straight if I'm way off base.
Mastermind
Work Saw Specialist
opcorn:Terry Syd
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I'll start, these type of discussions can lead to all sorts of observations.
First, I'd like to hear about the mod you made to the exhaust to turn the saw into a 'torque monster'.
As far as exhaust scavenging on a saw (I assume this comment is related to pressure waves in the exhaust system), I don't see this as possible without an expansion chamber. It would be desireable to get a returning negative wave to the cylinder during the blowdown period, unfortunately the escaping exhaust gases are traveling near the speed of sound - that makes it very difficult for a sonic wave to travel back up to the cylinder when the medium it is traveling in is going as fast as the wave is.
The exhaust could be tuned to return the negative wave after the cylinder pressure had dropped, which is what happens in an expansion chamber, however with the negative wave pulling the contents of the cylinder out the exhaust port you really need that returning positive wave from the reverse cone to pack the charge back in prior to the port closing.
Essentially, the only viable mod I see to the exhaust on a work saw is dropping the BACKPRESSURE as quickly as possible.
As far as the intake timing and spit back issues. The volume of mixture through the transfer ports is determined by time/area of the ports and crankcase pressure.
At peak torque, the flow of the mixture into the cylinder will have just come to a halt as the piston closes off the transfer ports. Above the speed of peak torque the time is too short to allow all the mixture to flow into the cylinder. If the transfer ports are to remain stock, we can move the point of peak torque higher up the powerband by increasing the crankcase pressure. Along with peak torque being moved higher up the powerband, so does the point of peak horsepower, as now there is more pressure trying to push the mixture through the rapidly closing transfer ports.
So what happens when we increase the intake timing to increase peak power? - We are trying to get a better fill of the crankcase at the higher speeds.
Which means, since the time/area of the transfer ports remains the same and the calculated crankcase compression ratio remains the same - we are raising the crankcase compression pressure by increasing the volume of mixture that entered the crankcase. More pressure in the crankcase, the faster the mixture flows into the cylinder.
IMO, the first way to improve the intake flow is to reduce pressure drops in the intake system - timing changes should be the last resort. You have widened the intake port, which is good to increase the 'gulp' factor (rate of demand) of the intake system, however, that does not address the issue of what causes the greatest pressure drop (constriction) in the intake system.
It may be that a slight increase in the size of the carburetor venturi and/or a better filtration system would allow the engine to breathe sufficiently at the higher engine speeds - and allow you to retain the stock intake timing.
First, I'd like to hear about the mod you made to the exhaust to turn the saw into a 'torque monster'.
As far as exhaust scavenging on a saw (I assume this comment is related to pressure waves in the exhaust system), I don't see this as possible without an expansion chamber. It would be desireable to get a returning negative wave to the cylinder during the blowdown period, unfortunately the escaping exhaust gases are traveling near the speed of sound - that makes it very difficult for a sonic wave to travel back up to the cylinder when the medium it is traveling in is going as fast as the wave is.
The exhaust could be tuned to return the negative wave after the cylinder pressure had dropped, which is what happens in an expansion chamber, however with the negative wave pulling the contents of the cylinder out the exhaust port you really need that returning positive wave from the reverse cone to pack the charge back in prior to the port closing.
Essentially, the only viable mod I see to the exhaust on a work saw is dropping the BACKPRESSURE as quickly as possible.
As far as the intake timing and spit back issues. The volume of mixture through the transfer ports is determined by time/area of the ports and crankcase pressure.
At peak torque, the flow of the mixture into the cylinder will have just come to a halt as the piston closes off the transfer ports. Above the speed of peak torque the time is too short to allow all the mixture to flow into the cylinder. If the transfer ports are to remain stock, we can move the point of peak torque higher up the powerband by increasing the crankcase pressure. Along with peak torque being moved higher up the powerband, so does the point of peak horsepower, as now there is more pressure trying to push the mixture through the rapidly closing transfer ports.
So what happens when we increase the intake timing to increase peak power? - We are trying to get a better fill of the crankcase at the higher speeds.
Which means, since the time/area of the transfer ports remains the same and the calculated crankcase compression ratio remains the same - we are raising the crankcase compression pressure by increasing the volume of mixture that entered the crankcase. More pressure in the crankcase, the faster the mixture flows into the cylinder.
IMO, the first way to improve the intake flow is to reduce pressure drops in the intake system - timing changes should be the last resort. You have widened the intake port, which is good to increase the 'gulp' factor (rate of demand) of the intake system, however, that does not address the issue of what causes the greatest pressure drop (constriction) in the intake system.
It may be that a slight increase in the size of the carburetor venturi and/or a better filtration system would allow the engine to breathe sufficiently at the higher engine speeds - and allow you to retain the stock intake timing.
One factor that seems to get little discussion is intake resonance. Symmetrical opening and closing of the intake create a period where crankcase pressure can overcome the inertia of the charge and push part of the charge back into the intake. Reed valves are one way to deal with this, but intake resonance is another.
Fluid moving through a pipe will return a negative pressure wave when it reaches the end of the pipe. This is the first thing that happens in a tuned exhaust pipe, assisting in scavenging (the second cone on the exhaust generates a positive pressure wave to stuff back some of the charge that was drawn into the exhaust by the negative wave).
The intake functions similarly to the first half of the exhaust, but because it runs into the crankcase, it functions as a resonating flask. The research appears to indicate that the volume of the flask (crankcase) has little effect on resonance relative to the neck of the flask.
If you change the shape of the intake, you change the resonance, and alter the timing at which the negative wave occurs. In other words, the "pull" on the charge provided by the wave no longer occurs at the point where crankcase pressure starts to push mix out of the intake.
The only way to get the right timing for the negative pressure wave is to alter the length of the intake. If you try to find this experimentally, you will need to use a stub exhaust to remove the effects of the exhaust system on the dynamics of the system.
Fluid moving through a pipe will return a negative pressure wave when it reaches the end of the pipe. This is the first thing that happens in a tuned exhaust pipe, assisting in scavenging (the second cone on the exhaust generates a positive pressure wave to stuff back some of the charge that was drawn into the exhaust by the negative wave).
The intake functions similarly to the first half of the exhaust, but because it runs into the crankcase, it functions as a resonating flask. The research appears to indicate that the volume of the flask (crankcase) has little effect on resonance relative to the neck of the flask.
If you change the shape of the intake, you change the resonance, and alter the timing at which the negative wave occurs. In other words, the "pull" on the charge provided by the wave no longer occurs at the point where crankcase pressure starts to push mix out of the intake.
The only way to get the right timing for the negative pressure wave is to alter the length of the intake. If you try to find this experimentally, you will need to use a stub exhaust to remove the effects of the exhaust system on the dynamics of the system.
BloodOnTheIce
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With porting a lot of times less, is more.
To both who responded thank you! I'm pretty familiar with expansion chamber theory and realize that's not happening on a work saw! Also familiar with the analogy in theory applied to intake tracts.To Terry, scavenging was the wrong concept i was trying to express..u r right takes a cone, a chamber or some other active device to do that...reduced restriction to increase flow is the right description for this situation. To an extreme I believe.
Intake dynamics are where I'm interested. In particular case volume and intake tract mods. The space restrictions on most saws pretty much put an end to the games played in the motorcycle world with later designed two strokes..but the older piston port designs seem to have similar characteristics to these saw motors...so that's why I'm asking if some of the things that worked with them are a valid place to explore on these saws. I understand things are not "apples to apple" because of differences in fundamental design... like bore & stroke and application requirements. Motorcycles were "square" typically. So the entire crank volume was effected by that relative the radically over square saw designs. But I do remember "stuffing" cranks and building reed blocks to allow for more radical (4 the time) intake port timing during that era..and also remember the power output was also somewhat limited by available cooling until water cooling came along!
This makes sense and I guess I wouldn't have expected the crank volume to literally effect intake resonance frequency, maybe the intensity of the pressure waves both positive and negative. Back to the motorcycle world, there are a variety of intake scheme's trying to find the blend between raw horsepower & usable torque.. I know intake geometry & volume has a whole lot to do with power characteristics. Given that intake geometry is tough to change on saw, my bet also is intake port duration is a place power can be found on piston port saws just as those old motorcycles. Again a place of interest as there isn't much you can do to the intake geometry short of designing one from scratch..not practical..but also where too much duration can ruin your porting day.
So back to the question:
Input on intake port timing relative to crank case volume. There has to be a definable inter relationship between the two. I think I have seen this but I'm not certain, has anyone else? More looking for empirical data vs. theory to either support or refute the rationalizations I have conjured up based I am looking at on a few saws crossed with the experiences I had 30 years ago! Things like the intake port timing experiences on those big slower RPM (stock) saws.
As an aside, I'm real interested in experiences on the 200 series Husqvarna's (600 series Jreds) 272.s 670.s etc. They seem to have small transfers & limited space to open up intake & exhaust so looking else where for power.
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Terry Syd
Addicted to ArboristSite
I'm sure your aware of the concept of the 'Boost Bottles' where the intake resonance is tuned. There is an easy way to tune the intake using a straight hose.
The formula is different for a hose vs a resonanting flask such as the Boost Bottle, but I gave up trying to work it out via a formula. I simply put a long, reinforced, plastic hose on the intake system and then used a needle nosed vice-grips to clamp it off at various locations.
As you move the vice-grips along the hose, different volumes are created. Those different volumes would in turn create different resonance characteristics in the intake.
On an enduro bike, a long hose could give me two resonant peaks. I could move one peak to just above idle for torquing along in the rocks, the second peak would occur higher in the powerband. I would adjust the two peaks to give the broadest powerband.
It may be possible to use the hose concept on a saw. The hose should be of a fairly large diameter, but small enough that you could slip it inside the handle of the saw.
At the engine speed that saws run, it may not need a very long hose. You probably won't be able to get the primary (and therefore the strongest) wave where you want it, but you may be able to get one of the other waves to a useful position in the powerband.
After you find the length of hose that gives you the powerband you want, you could then cut it, plug it, and position it inside the handle.
The formula is different for a hose vs a resonanting flask such as the Boost Bottle, but I gave up trying to work it out via a formula. I simply put a long, reinforced, plastic hose on the intake system and then used a needle nosed vice-grips to clamp it off at various locations.
As you move the vice-grips along the hose, different volumes are created. Those different volumes would in turn create different resonance characteristics in the intake.
On an enduro bike, a long hose could give me two resonant peaks. I could move one peak to just above idle for torquing along in the rocks, the second peak would occur higher in the powerband. I would adjust the two peaks to give the broadest powerband.
It may be possible to use the hose concept on a saw. The hose should be of a fairly large diameter, but small enough that you could slip it inside the handle of the saw.
At the engine speed that saws run, it may not need a very long hose. You probably won't be able to get the primary (and therefore the strongest) wave where you want it, but you may be able to get one of the other waves to a useful position in the powerband.
After you find the length of hose that gives you the powerband you want, you could then cut it, plug it, and position it inside the handle.
Thanks to all for the time and text book. Good info! And terry's "tuning" concept is a clever concept that should make a difference. Lots of design now goes into the size and shape of everything from the reed block back to the air box & filter these days on the dirt bikes..always a space vs, optimum shape compromise. And while "tuning" is a very important component..its actually the next step after the hard mechanical things like port size & timing along with intake tract and carb configurations are done in my mind to try and optimize the final system.
Yea I remember the Boost bottles, along with other aftermarket wiz bang stuff to hang off the intake tracts over the years, and then the power valves in the exhaust ports..pretty much grew up with and racing motorcycles from my first real race bike, a 1970 Bultaco Sherpa S 200 to the suspension revolution mid seventies to water cooling, active exhaust timing (devices (power valves) thru to the four strokes now. As a former Engineer all that theory was real interesting...but.
I was more curious if any one had real vs. theoretical experience in that area. Some thing quantitative because I know from a life time in the technology world, theory usually comes before empirical data and then gets modified to explain the empirical!!...SO I guess what I have learned is no one has a clue if there is a real relationship..so I'll go out on a limb and say there is..both from what I have seen in the distant past and what I saw again with a Jred 2094 and again with an 820. AND I guess no one has tried to lower the intake on a 272 who's willing to discuss so I'll have to be the one who ruins a few barrels to find out...and report! (Something tangible like how many degree's of rotation gains power if at all and at what point is goes the wrong way! (And that's assuming the other stuff I always do now so what may work for me won't for someone else s configuration..and knowing as a saw building novice, I'm just at the very beginning of a very long curve so please NO one assume I know a damn thing! All I can do is report what I see)
Yea I remember the Boost bottles, along with other aftermarket wiz bang stuff to hang off the intake tracts over the years, and then the power valves in the exhaust ports..pretty much grew up with and racing motorcycles from my first real race bike, a 1970 Bultaco Sherpa S 200 to the suspension revolution mid seventies to water cooling, active exhaust timing (devices (power valves) thru to the four strokes now. As a former Engineer all that theory was real interesting...but.
I was more curious if any one had real vs. theoretical experience in that area. Some thing quantitative because I know from a life time in the technology world, theory usually comes before empirical data and then gets modified to explain the empirical!!...SO I guess what I have learned is no one has a clue if there is a real relationship..so I'll go out on a limb and say there is..both from what I have seen in the distant past and what I saw again with a Jred 2094 and again with an 820. AND I guess no one has tried to lower the intake on a 272 who's willing to discuss so I'll have to be the one who ruins a few barrels to find out...and report! (Something tangible like how many degree's of rotation gains power if at all and at what point is goes the wrong way! (And that's assuming the other stuff I always do now so what may work for me won't for someone else s configuration..and knowing as a saw building novice, I'm just at the very beginning of a very long curve so please NO one assume I know a damn thing! All I can do is report what I see)
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AUSSIE1
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Srcarr52 looks to have played with a few 272's extensively and crankcase compression along with some other 1 and 2 series saws. You don't see him on here very often. Maybe a PM.
Timberwolf has played with the resonance and tuning of the inlet to some extent. He is often lurking around in the background.
Having the means to measure and test seams to be out of reach of most of us.
Increasing the length in increments wouldn't be difficult but shortening would be another matter with some saws.
Timberwolf has played with the resonance and tuning of the inlet to some extent. He is often lurking around in the background.
Having the means to measure and test seams to be out of reach of most of us.
Increasing the length in increments wouldn't be difficult but shortening would be another matter with some saws.
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Since this is porting 101, I think I'll ask a couple of questions.
1) How close can you make the ports to the ring ends?
1a) Is it better to move the rings ends, or take the easy way out and put the piston in backwards?
2) On cylinders that have a large chamfer around the ports, would you leave the ports alone or enlarge the ports to get a crisper port opening?
1) How close can you make the ports to the ring ends?
1a) Is it better to move the rings ends, or take the easy way out and put the piston in backwards?
2) On cylinders that have a large chamfer around the ports, would you leave the ports alone or enlarge the ports to get a crisper port opening?
AUSSIE1
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Coupla mil roughly for a work saw.
You can run a piston backwards if the ring ends/ports allow. The 064 piston is run backwards in the 371/2 BB kits for example.
I'd concentrate more on your port timing firstly.
You can run a piston backwards if the ring ends/ports allow. The 064 piston is run backwards in the 371/2 BB kits for example.
I'd concentrate more on your port timing firstly.
The roof of the exahust port I'm looking at is flat, it lines up perfectly with the ring groves. And the chamfer is largest off to one side of the port and seems to be the smallest near the center. Which is not what you would want the chamfer to look like. The exhaust duration is 170, as measured with a port map which uses the edges of the chamfers.
Would you fix the chamfer and/or raise the port to give it a slight arch?
Would you fix the chamfer and/or raise the port to give it a slight arch?
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