There's a fair difference between the two. A saw made to run at high RPM's will cut fast in small wood. A saw made for torque will shine in larger wood. Also with a torque or mid tune mod run can run an 8T sprocket to keep good rpm's. With a rev tune you can put a 7T sprocket on it but you still won't have the torque that a torque modded saw has. Speed is kind of a way to measure the saws power, but only in the wood rpm's. Plain and simple torque mods will hold their rpm's in the wood better than a rev tuned saw.
A question about torque, speed, RPMs, and the relation among all three.
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There's a fair difference between the two. A saw made to run at high RPM's will cut fast in small wood. A saw made for torque will shine in larger wood. Also with a torque or mid tune mod run can run an 8T sprocket to keep good rpm's. With a rev tune you can put a 7T sprocket on it but you still won't have the torque that a torque modded saw has. Speed is kind of a way to measure the saws power, but only in the wood rpm's. Plain and simple torque mods will hold their rpm's in the wood better than a rev tuned saw.
Yes I had this happen with the first modded saw I owned. It did alright with light pressure, but with much force a stock saw would outcut it.
Tzed250
Addicted to ArboristSite
As I remember TW, ignition timing is usually optimized to time the the peak cylinder pressure with the moment that the con-rod and crank are at a right angle with one another(not 90° ATDC). This is the point at which the piston can apply maximum force to create the greatest torque.
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fredmc
Addicted to ArboristSite
Wouldn't it be easier to put a 7900 topend on a 681
Are the cases essentially the same?
They looks to be from the exact same casting to me. However, I think they have different crank bearing. I don't know if the crank would swap or not.
No the bearings are larger in the 681, so the openings in the case halves are larger.
fredmc
Addicted to ArboristSite
If that is the case, I wonder if the case volume differs?
True, but perhaps the ID of the inner race is the same and the cranks may still swap?
timberwolf
Addicted to ArboristSite
Yep, lots of things to balance, peek too soon and there is not much leverage on the crank, peek too late and the piston is moving away from the charge making for poor energy transfer and effectivly holding back the RPM.
From the looks of it peek cylinder pressures come somewhere about 10-20 deg ATDC. By the 2 stroke books, any burning after 20 degs past TDC never really contributes anything power wise to the crank.
Wild Knight
ArboristSite Guru
I’ve struggled with these questions for a while. I think I understand the concepts, but figuring out which combinations of all the permutations will work best can make you go crazy. It really requires a lost of data, understanding of gas laws and thermodynamics and a good bit of computer modeling to put it all together. To the best of my knowledge, Timberwolf is the only one trying this right now….other than the engineers at Stihl/Dolmar/Husky/etc.
I’ve learned a lot about saws and theory from Erick. I’ll try to synthesize some of this with the knowledge of Bell and Jennings. I’m sure he’ll set things straight if I get off track.
Speed is nothing more than how fast the chain goes around the bar. The faster it goes, then faster you will cut. Now, how do we get the chain to go fast? How do we get the chain to go fast in the cut? With a big bar? Small bar?
A chainsaw is an air pump that uses fuel to work the pump. Essentially, the piston goes up and down and draws air in and pushes it out. When the spark plug fires, fuel is ignited and pushes the piston down. As the piston goes down, pressure builds in the crankcase until finally the transfers open and the fuel flows into the cylinder. As the piston goes back up, it creates an area of low pressure in the crankcase, pulling in fresh fuel and the begins to compress the fuel and air in the cylinder. Once the exhaust ports close, the volume of gases trapped compress to the limits of the squish, are ignited and the process starts over. So, you can clearly see all the places that can be modified to make this air pump flow better and potentially produce more power.
When does the intake port open and close? What is the duration? Longer durations can allow more gases to flow, but pressure and gas velocity may be lost. Shorter durations may starve the engine.
What is the capacity of the crankcase? How much fuel can come in? Does it favor high volume or high pressure?
How many transfers are there? How big are the tunnels? When do they open and when do they close? Do they operate under high pressure or high volume? Do you move slower, larger volumes of gases over a longer period or move smaller, high pressure gases over a shorter duration?
How much volume is in the cylinder once the exhaust port closes? How much fuel can you fit in there under ambient conditions? How does air temperature (cold air is more dense) and altitude (high altitude = lower density and less oxygen) affect the maximum charge you can fit into the combustion chamber? What is the shape of the combustion chamber? Is it deep or shallow? Is it offset or centered?
When does the exhaust port open? Too soon and less power is transfered to the piston to drive it down. If it is open too long, some of the new charge coming in is lost out the exhaust before ever being burned.
Ultimately, power only comes from the charge being burned to drive the piston down. The questions is, how do you get the most potential energy into the chamber to then do work once ignited? When you go through the zillions of permutations of timing, duration, volumes, pressures, etc. etc. You find the combination that produces maximum torque at the specific RPM you want the saw to operate at in the cut. We have our opinions and engineers have theirs. They design a saw to be durable and reliable. A lot less stress is put on a saw that turns 8-9000 RPM in the cut than one that turns 11-12000 RPM. Remember, the piston comes to a full stop twice for every revolution; this is a lot of stress on the rod, crank, bearings, etc. We take what the engineers designed and move things around to raise the RPM where the saw generates maximum torque - where the work of the expansion of combusting gases is producing power. Yeah, you can get a saw to turn faster without resistance b/c once inertial forces begin helping the piston go up and down, torque is not as important. Once something resists the piston going up and down (e.g., wood resisting the cutters on the chain), then inertial help is lost - it cannot sustain itself and now torque is important again.
Think of it like this. A truck accelerating must overcome the weight on the car to get it going. It has no inertia, no momentum yet. Torque is required to do this. Once the car is going fast enough, the inertia of the car is helping take some of the load off the engine, allowing the motor to run with less resistance to the work it is trying to do. Now, if you start driving into gale force winds, resistance is increased and HP alone may not help you. You slow down to the speed at which the maximum torque of the engine can sustain. Clear as mud?
Thus is the case where saws drop down to their RPM in the cut. I believe this is the point at which the saw is making maximum torque. Now, how do you build a saw for just torque? I don’t know. How big is the combustion chamber? What is the bore to stroke ratio? How is it ported? How many transfers? I guess you error on the side of doing anything that would hurt torque (like raising the exhaust port). You can change port sizes, timings, and durations to flow the maximum amount of fuel charge you can get into the combustion chamber, at particular volumes, under particular pressures at precise timings and durations to get the desired RPM. Ultimately, the higher your torque curve, the higher the RPM in the cut, the fast the chain will go around the bar, the faster you will cut.
You can gain a false sense of maximum power by tuning out of the cut. You can lean out a saw to go real fast, especially since inertia is helping. Once the saw hits wood, it falls on its face. The porting numbers, gas flow, etc. are not sufficient to pack the combustion chamber with sufficient fuel to maintain its torque at high RPM. It’s running too lean at lower RPM, and the saw dogs down. A saw designed to produce maximum torque at in-the-cut speeds vs a saw designed to produce maximum HP at no-load speeds will always cut faster. Saw engineers aren’t stupid. They have the sophisticated models and specific knowledge to help them design all of the parameters. It doesn’t take much to screw up what they spent a lot of time and knowledge designing.
I think an interesting question is how close to maximum RPM can we raise the torque curve? How close can we get the cutting RPM to the maximum RPM?
I’ve learned a lot about saws and theory from Erick. I’ll try to synthesize some of this with the knowledge of Bell and Jennings. I’m sure he’ll set things straight if I get off track.
Speed is nothing more than how fast the chain goes around the bar. The faster it goes, then faster you will cut. Now, how do we get the chain to go fast? How do we get the chain to go fast in the cut? With a big bar? Small bar?
A chainsaw is an air pump that uses fuel to work the pump. Essentially, the piston goes up and down and draws air in and pushes it out. When the spark plug fires, fuel is ignited and pushes the piston down. As the piston goes down, pressure builds in the crankcase until finally the transfers open and the fuel flows into the cylinder. As the piston goes back up, it creates an area of low pressure in the crankcase, pulling in fresh fuel and the begins to compress the fuel and air in the cylinder. Once the exhaust ports close, the volume of gases trapped compress to the limits of the squish, are ignited and the process starts over. So, you can clearly see all the places that can be modified to make this air pump flow better and potentially produce more power.
When does the intake port open and close? What is the duration? Longer durations can allow more gases to flow, but pressure and gas velocity may be lost. Shorter durations may starve the engine.
What is the capacity of the crankcase? How much fuel can come in? Does it favor high volume or high pressure?
How many transfers are there? How big are the tunnels? When do they open and when do they close? Do they operate under high pressure or high volume? Do you move slower, larger volumes of gases over a longer period or move smaller, high pressure gases over a shorter duration?
How much volume is in the cylinder once the exhaust port closes? How much fuel can you fit in there under ambient conditions? How does air temperature (cold air is more dense) and altitude (high altitude = lower density and less oxygen) affect the maximum charge you can fit into the combustion chamber? What is the shape of the combustion chamber? Is it deep or shallow? Is it offset or centered?
When does the exhaust port open? Too soon and less power is transfered to the piston to drive it down. If it is open too long, some of the new charge coming in is lost out the exhaust before ever being burned.
Ultimately, power only comes from the charge being burned to drive the piston down. The questions is, how do you get the most potential energy into the chamber to then do work once ignited? When you go through the zillions of permutations of timing, duration, volumes, pressures, etc. etc. You find the combination that produces maximum torque at the specific RPM you want the saw to operate at in the cut. We have our opinions and engineers have theirs. They design a saw to be durable and reliable. A lot less stress is put on a saw that turns 8-9000 RPM in the cut than one that turns 11-12000 RPM. Remember, the piston comes to a full stop twice for every revolution; this is a lot of stress on the rod, crank, bearings, etc. We take what the engineers designed and move things around to raise the RPM where the saw generates maximum torque - where the work of the expansion of combusting gases is producing power. Yeah, you can get a saw to turn faster without resistance b/c once inertial forces begin helping the piston go up and down, torque is not as important. Once something resists the piston going up and down (e.g., wood resisting the cutters on the chain), then inertial help is lost - it cannot sustain itself and now torque is important again.
Think of it like this. A truck accelerating must overcome the weight on the car to get it going. It has no inertia, no momentum yet. Torque is required to do this. Once the car is going fast enough, the inertia of the car is helping take some of the load off the engine, allowing the motor to run with less resistance to the work it is trying to do. Now, if you start driving into gale force winds, resistance is increased and HP alone may not help you. You slow down to the speed at which the maximum torque of the engine can sustain. Clear as mud?
Thus is the case where saws drop down to their RPM in the cut. I believe this is the point at which the saw is making maximum torque. Now, how do you build a saw for just torque? I don’t know. How big is the combustion chamber? What is the bore to stroke ratio? How is it ported? How many transfers? I guess you error on the side of doing anything that would hurt torque (like raising the exhaust port). You can change port sizes, timings, and durations to flow the maximum amount of fuel charge you can get into the combustion chamber, at particular volumes, under particular pressures at precise timings and durations to get the desired RPM. Ultimately, the higher your torque curve, the higher the RPM in the cut, the fast the chain will go around the bar, the faster you will cut.
You can gain a false sense of maximum power by tuning out of the cut. You can lean out a saw to go real fast, especially since inertia is helping. Once the saw hits wood, it falls on its face. The porting numbers, gas flow, etc. are not sufficient to pack the combustion chamber with sufficient fuel to maintain its torque at high RPM. It’s running too lean at lower RPM, and the saw dogs down. A saw designed to produce maximum torque at in-the-cut speeds vs a saw designed to produce maximum HP at no-load speeds will always cut faster. Saw engineers aren’t stupid. They have the sophisticated models and specific knowledge to help them design all of the parameters. It doesn’t take much to screw up what they spent a lot of time and knowledge designing.
I think an interesting question is how close to maximum RPM can we raise the torque curve? How close can we get the cutting RPM to the maximum RPM?
TreeClimber57
Addicted to ArboristSite
Agreed. Which is why the big saws can be easily beaten in smaller wood by a much smaller and lower horsepower saw.. however put in its own element and it will smoke the smaller saw every time.
Tzed250
Addicted to ArboristSite
It is easy enough to get the torque peak close to the max RPM, this is how two-stroke roadrace engines are tuned.
The real question is will you like the way the saw runs if it falls off the torque peak in the cut?
.
Yeap just like how a ms460 will have more "zip" and less torque, but the ms660 will have more torque and less zip. But if you ran a 460 with stock bearings and a 660 with silicone nitride bearings the "zip" factor would be better in the bigger saw.
TreeClimber57
Addicted to ArboristSite
Well if I remember correctly, the engine should be utilized between the two peaks, that being peak of torque and peak of power. (which defines the power band)
So if these two are close together, then you need to maintain that specific rpm (where peak torque is) to really make the saw/engine usable. Having the peak torque lower widens the power band, and actually makes the saw/engine more usable across a larger variance of rpm if I remember correctly. Engines that have the two close together are typically racing engines, and utilize transmissions to offset the narrow power band. Although there are some low cost foreign cars that have a fairly narrow power band, which is why they are gutless when accelerating.
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Tzed250
Addicted to ArboristSite
Spot on. When an engine slows from the power peak toward the torque peak it experiences torque rise, as it is riding upth back side of the torque curve. Great engines are usually characterized by a large spread between the two peaks, but also having a high specific output.
timberwolf
Addicted to ArboristSite
I disagree a bit here, if gearing and chain is exactly the same then yes, possible to out run the big saw, but adjust the gearing or bite of the chain to match the saw and bye-bye small saw. It HP that does the work but it the chain and dive line is left as the bottle neck then it does not matter how much HP. What is impressive though is HP/cc the smaller the bore and overall cylinder for that matter the more HP can be pulled from each cc. On the down side though the smaller the cylinders the less efficient fuel to output wise.
Makes me think of the old for pickups classic 300-I6 vs. the 302-V8. same displacement. Inline six produced about 170hp and 260ft/lb or so and could get into the upper 20s MPG. V8 was something like 220hp with basic carb and 300ft/lb, but needed a tail wind to get 20 MPG. Fewer bigger cylinders is good for efficiency but does not produce the power.
LOL nope, there are a number of others, SRT is one good example, he has done some real good stuff on fuel injection. lots of guys just do a better job keeping the lips sealed.
Wild Knight
ArboristSite Guru
The peak of torque is the point where the motor has the most potential to accelerate when resistance is met. Here is an interesting primer...
If you were able to produce maximum torque at a higher RPM, then a saw could hold its power at a higher RPM (or higher gearing) and consequently would turn the bar faster. HP doesn't seem to be your friend while sawing logs b/c the wood is constantly resisting the chain and trying to slow it down. The resistance will always pull the saw down to max torque RPM.
This may seem peaky, but saws don't have the luxuries of cars with 4 strokes, tranny, etc. How is a broad power band even sustainable in a 2-stroke?
If you were able to produce maximum torque at a higher RPM, then a saw could hold its power at a higher RPM (or higher gearing) and consequently would turn the bar faster. HP doesn't seem to be your friend while sawing logs b/c the wood is constantly resisting the chain and trying to slow it down. The resistance will always pull the saw down to max torque RPM.
This may seem peaky, but saws don't have the luxuries of cars with 4 strokes, tranny, etc. How is a broad power band even sustainable in a 2-stroke?
Wow.
I think I'm more confused than when I started this thread!
Tzed, timberwolf, and Wild Knight - all three of you guys have some fantabulous posts in this thread.
I would ask some new questions at this point but I don't understand the information in this thread well enough yet to put all the thoughts together collectively in order to come up with a question.
I've read parts over a few times. But I just need to continue to do so - I want to get the most out of this thread that I can.
Thanks.
I'll give credit - you guys are good. 
I think I'm more confused than when I started this thread!
Tzed, timberwolf, and Wild Knight - all three of you guys have some fantabulous posts in this thread.
I would ask some new questions at this point but I don't understand the information in this thread well enough yet to put all the thoughts together collectively in order to come up with a question.
I've read parts over a few times. But I just need to continue to do so - I want to get the most out of this thread that I can.
Thanks.
I'll give credit - you guys are good.
TreeClimber57
Addicted to ArboristSite
It is not. However, typically the peak torque and rpm do not match even on a typical chainsaw. Not just the 2-cycle vs 4-cycle, but also typically the displacement.
TreeClimber57
Addicted to ArboristSite
good stuff.
