A question about torque, speed, RPMs, and the relation among all three.

[billmartin]

I can apply 50 lbs/ft to a bolt for 10 minutes, but if the bolt never moves then no work is done.


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Very well put

Not being sarcastic. I think that is a very good way to put it.

 

[parrisw]

Parris, if you put a degree wheel on the saw you can then be sure what modifications you are making. If you have an old piston, then pop in the old piston and nip a bit off the front of the crown next to the exhaust port. The timing will change just like it would as if you had raised the exhaust port.

You can then keep going up in 2 degree increments to see how it changes the powercurve. When you go too far for your preference in power characteristics - then you can port the cylinder to the duration that you did like and put in the good piston.

If the amount off the front of the piston is only a few degrees, then you might even stay with trimming the piston crown and thus retaining the stock exhaust port with the factory chrome at the top of the port. It will also always allow you to go back to stock timing anytime you want.

If you stay with the trimmed crown, you will loose a very small amount of mixture under the squishband from the removed material, but this isn't a race saw that needs every percentage of power possible. The gains you will get from the increased timing will more than offset the tiny mixture loss.

Hmm, ???? Don't really like that idea, plus wont work on my case on the 395 since its got a modded 2101 piston, and I don't just have a few of those to carve up.

 

[Outlaw5.0]

Most saws will be in the 9000 rpm to 13,000 rpm range in the cut. The highest average torque in that range will be the quickest. Average torque/hp is what wins, not peak numbers.

 

[Andyshine77]

To me power is power, but it really comes down to how it's delivered, or used. Torque is the amount of force applied to a rotating axis, change the size of the axis, you change the torque, 8 pin vs 7. Change the chain you change the resistance. To say one saw has more torque you must first explore the entire power curve before you come to this conclusion. Sprocket size, chain angles, depth gauge settings, tuning, operator technique and so on.

The 681 likely has more HP and torque than the 7900, however it may be at a more specific rpm. You really can't judge a saw, or group of saws by making a few cuts with different bars, temperatures, without retuning, and in a log that has branches allover the place, and really think you know what's what. In fact it may take you months or years before you can come to a more educated conclusion.

I'm not saying Jason and Dan's findings are incorrect, however I do believe, more R&D would allow them to understand an inch can actually be a mile and a mile may actually be just an inch.

Nevertheless this is a great discussion, and myself and others enjoy constructive threads like this. Last, but certainly not least, Dan and Jason are having fun with their hobby, and that's what it's all about.

 

[fredmc]

Has anyone thought to put the 681 crank in the dolmar with a 7900 top end?

 

[WoodChuck'r]

To me power is power, but it really comes down to how it's delivered, or used. Torque is the amount of force applied to a rotating axis, change the size of the axis, you change the torque, 8 pin vs 7. Change the chain you change the resistance. To say one saw has more torque you must first explore the entire power curve before you come to this conclusion. Sprocket size, chain angles, depth gauge settings, tuning, operator technique and so on.

The 681 likely has more HP and torque than the 7900, however it may be at a more specific rpm. You really can't judge a saw, or group of saws by making a few cuts with different bars, temperatures, without retuning, and in a log that has branches allover the place, and really think you know what's what. In fact it may take you months or years before you can come to a more educated conclusion.

I'm not saying Jason and Dan's findings are incorrect, however I do believe, more R&D would allow them to understand an inch can actually be a mile and a mile may actually be just an inch.

Nevertheless this is a great discussion, and myself and others enjoy constructive threads like this. Last, but certainly not least, Dan and Jason are having fun with their hobby, and that's what it's all about.

+1000 :kilt: :wave:

--

I do find this thread extremely informative and very interesting to say the least. Like I've said in the past (and I'm speaking for Dan as well) this isn't about who's saw is faster, or "better". It's not a bragging ego booster by any means. I've enjoyed running chainsaws for the past 10 years or more now - but the recent saws (681, 7900, 441) have really put a few questions in my mind since they have been "turbocharged" so to say.

I'm not just trying to improve my understanding of the chainsaw mechanically. I'm taking a step a bit further to try and understand the why behind it all.

I've run lot's of saws in the past, and had a blast doing it ever time. But other than understanding the technique of how to use one as a tool, I never really put much though into things like this. It was only when I got my hands on the (awesome) built saws that I really started wondering about power bands and the physics of it all.

Awesome information. I find it sort of interesting that cut times (when applied to this subject) are rather put on the back burner and don't necessarily mean as much when you look at "the whole picture".

Thanks to all that have contributed so far.

 

[WoodChuck'r]

Has anyone thought to put the 681 crank in the dolmar with a 7900 top end?

And have a hybrid?? A Solmar 7981?? Now THAT would be a cool project!! :chainsawguy:

 

[blsnelling]

Has anyone thought to put the 681 crank in the dolmar with a 7900 top end?

Wouldn't it be easier to put a 7900 topend on a 681

 

[blsnelling]

100 degrees?? Exhaust duration!?

The exhaust port opens at 100° ATDC. That gives it a duration of 160°.

HP = Torque x RPM / 5252

Ignore the 5252, that's just a constant to make the units consistent. Ignoring the unit issue for the moment, we could say that:

HP = Torque x RPM

Your 7900 may have higher torque, but it produces less power because it revs slower.

Your 681 may have less torque, but still produces more power because it revs faster.

The Walkerized 372 sounds low on torque but I bet the revs are way up there.

In all cases best results will be obtained when the chain, particularly the raker depth, is matched to the powerhead. The chain that cuts best on a 120cc saw spinning 10,000 rpm may perform poorly on a Walkerized 372 at 14,000 rpm, and visa versa.

I'm not sure how I missed this post last night. This one really helps me.

 

[bitzer]

One thing to throw in here. A more effcient burn should produce more torque. Transfer volume, flow and shape, squish band shape, blowdown, I would think would also be factors in the amount of torque created. At 13k you are burning more fuel than at 14.5k. As long as the burn is efficient or more effcient than at 14.5K more torque should be produced. In other words, there is more fuel to be used at 13K when leaning on a saw than at 14.5k. In a saw the force is applied by the expansion of gases during combustion. Bigger bang=bigger force. Add a longer stroke and there is greater torque.

Also the longer the stroke for the same displacement the greater the torque. Torque is force applied X the length. Just like extending a socket wrench with a piece of pipe. The extra length will give you more torque. There is also less pressure on the cylinder walls which should result in slightly less heat.

Like the others said, torque x rotational speed = power. Torque= force x distance (angular or rotational)

 

[troutfisher]

How about ignition timing? Chainsaws have a fixed timing. Most bigger motors have an more complex ignition system that will retard or advance the timing based on load and RPM. When we pull a saw down off the powerband we're also taking it out of the timing range it's meant to run in. That could be felt as a loss of "torque". Maybe some saws have a more forgiving timing curve.

 

[bitzer]

How about ignition timing? Chainsaws have a fixed timing. Most bigger motors have an more complex ignition system that will retard or advance the timing based on load and RPM. When we pull a saw down off the powerband we're also taking it out of the timing range it's meant to run in. That could be felt as a loss of "torque". Maybe some saws have a more forgiving timing curve.

If I remember correctly I believe Brad advanced the timing on the 681. That would effect the burn rate/effciency.

 

[troutfisher]

If I remember correctly I believe Brad advanced the timing on the 681. That would effect the burn rate/effciency.

It would also affect how that motor acts at lower rpms.

 

[Andyshine77]

I also believe the coil on the 681 has no advance or retardation, the stock coil would change timing based on engine load and speed. Like you said some low end was likely lost in favor of high rpm in the cut power.

 

[mdavlee]

I know what woodchucker is saying about one being stronger than the other. My 7900 and 390 are like that. They cut almost the same speed with whatever combination I have tried so far. I don't know how big of wood I have to get in to make the 390 really outcut the 7900. I know in oak that goes right to the tip of a 28" bar both with 8 pins they are neck and neck. I'm gonna guess it's going to be right around 32" or so all the way buried for the torque of the 390 to really shine. My 372 will cut right with the 7900 in smaller wood and will turn more rpms also with the unlimited coil.

 

[timberwolf]

Agree, I have yet to see a saw coil that has an actual advance curve, some might have a retard at starting RPM, but from idle RPM to WOT timing on most coils does not apear to move more than a degree one way or the other.

Also the longer the stroke for the same displacement the greater the torque

There is a catch though, yes there is a longer leaver, but if the cylinder pressure is the same than the surface area of the piston must be counted too. Which makes the gains much less than they are at first look.

Lets say 800 psi peek cylinder pressure and 2.9 in2 for a 50 mm piston = 2325 lb of force. vs. 2.36 in2 for a 44 mm piston = 1885 psi. 50mm with a 38mm stroke (0.75 in radius) comes out to 75cc, a 44mm piston would need a 49.06mm stroke (0.97 in radiu)to get the same 75cc.

End result vs 50mm piston and 38mm stroke, the 44mm piston with 49mm stroke has 81% of the force down on the piston and rod but 129% of the force effect to the crank. = a net gain in torque of only 4.5% going from 1.32:1 (over square) bore to stroke to 0.90:1 (under square).

A huge factor is rod and crank angle relationships with respect to the cylinder pressure, if the rod has no mechanical advantage on the crank when the pressure happens it just pushes on the main bearings and does not translate the force into rotation. If the piston speed is higher than the charge expands then no force gets transfered to the crank. Math gets out of hand real fast on this stuff.

 

[bitzer]

Agree, I have yet to see a saw coil that has an actual advance curve, some might have a retard at starting RPM, but from idle RPM to WOT timing on most coils does not apear to move more than a degree one way or the other.



There is a catch though, yes there is a longer leaver, but if the cylinder pressure is the same than the surface area of the piston must be counted too. Which makes the gains much less than they are at first look.

Lets say 800 psi peek cylinder pressure and 2.9 in2 for a 50 mm piston = 2325 lb of force. vs. 2.36 in2 for a 44 mm piston = 1885 psi. 50mm with a 38mm stroke (0.75 in radius) comes out to 75cc, a 44mm piston would need a 49.06mm stroke (0.97 in radiu)to get the same 75cc.

End result vs 50mm piston and 38mm stroke, the 44mm piston with 49mm stroke has 81% of the force down on the piston and rod but 129% of the force effect to the crank. = a net gain in torque of only 4.5% going from 1.32:1 (over square) bore to stroke to 0.90:1 (under square).

A huge factor is rod and crank angle relationships with respect to the cylinder pressure, if the rod has no mechanical advantage on the crank when the pressure happens it just pushes on the main bearings and does not pl
translate the force ;into rotation. If the piston speed is higher than the charge expands then no force gets transfered to the crank. Math gets out of hand real fast on this stuff.

For some reason I thought Brad was talking about moving the key slot.

I thought about the optimum stroke length and relationship to the bore size after that post. There is an optimum correlation I am sure and that is where most saw manufacturers would be trying to spec there saws around. I knew you'd be around with numbers shortly. I always learn a little more from your extensive testing and number crunching TW! I've had some Physics courses in the past and unless you are hard down on it everday it does get out of hand quickly, especially when there are so many variables when messing around with saws. Human error and judgement, testing wood variation, fuel and air compostion variations, etc... I am sure there is a percent error for dynos too.

 

[TreeClimber57]

From what little I understand about physics the longer stroke of the 681 should allow it to make more torque.

Normally the case, but other factors do come into play.

 

[TreeClimber57]

I have seen the equation horsepower = (torquexrpm)/5250.

True, but each engine will have an optimal rpm to torque to maximize the horsepower. As rpm increases the torque will at some point drop off, so one needs to find the balance between torque and rpm to find the peak horsepower. A fine example of this is my diesel truck, which has peak torque at 1600 rpm. Interestingly enough though the peak horse power is actually at a slightly higher rpm, where the torque is lower.

 

[FATGUY]

another awesome thread

excellent gentlemen, excellent. Particularly Jason for starting it and Tzed & Timberwolf for their insite.