TORQUE!

[Robin Wood]

Something rarely discussed here when we mod or speak about specs, crank counter weight
We always talk about compression, port timing and etc. But me and my dealer always believed that raw torque comes from crank counter weight, when the heavy weight gets going it becomes a tractor like power.

Here's a few comparisons of crank weight among some big saws
stihl 070: 945g 40mm stroke
Stihl 088: 742g 42mm stroke
Cs1201: 1,028g 44mm stroke

Cs1201 and stihl 070 has longer shaft ends so we can assume they weigh less about say 50g against stihl 088 to be fair. Even then they weigh alot more, which explains their low down torque and broad power delivery. This also explains why cs1201/1200 has crazy amount of low end torque, with that heavy crank multiplied with longer stroke it has exponentially higher torque output.
So technically 088 is high rev saw with lower torque against 070, but the torque ratings on both these model are almost the same. Could it be cc difference and bore size which increases some overall hp and torque? Majority of torque comes from stroke and crank weight?

 

[1hander]

i think mostly counterweights is there to help balancing, because of the balancing thing
the weight of the counterweights during rotation is more or less a wash when counteracting the weight of piston etc etc
they can only be heavy enough to counteract piston/rod assembly rotating mass to within certain rpm range
im not sure how much torque would actually come from the counterweights mass. size but there would definitely be a good bit coming from them
the flywheel/fan rotating weight is also a big player in torque delivery department
port timing,compression numbers, ignition timing, and bore all come into play but i dont know enough about that
the extra displacement/compression, piston surface area, i think make up the difference your mentioning
the 070 and the 088 while having very similar torque numbers will deliver that torque at different rpms
theres alot of i thinks in my post huh haaaa

 

[Huskybill]

I bought everything to balance two stroke rotating elements. I hope to get into it soon. How close are they balanced now? I’m not sure. Porting, port timing, big bore kits but no talk about balancing, torque and pulling more rpm out of the engine.

Why port and run the saw at its orginal max rpm? What’s the gain? Maybe adding weight on the flywheel side? I think there is more torque and rpm we can squeeze out of it.

On the large dynamometers I built we put heavy mass weights to test torque.

I need a torque transducer to build a chain saw dynamometer. I think it’s around $4 k. A wet disc brake setup can be used as a load.

 

[1hander]

I bought everything to balance two stroke rotating elements. I hope to get into it soon. How close are they balanced now? I’m not sure. Porting, port timing, big bore kits but no talk about balancing, torque and pulling more rpm out of the engine.

Why port and run the saw at its orginal max rpm? What’s the gain? Maybe adding weight on the flywheel side? I think there is more torque and rpm we can squeeze out of it.

On the large dynamometers I built we put heavy mass weights to test torque.

I need a torque transducer to build a chain saw dynamometer. I think it’s around $4 k. A wet disc brake setup can be used as a load.

what setup did you buy if you dont mind me askin
these saws could definitely benefit from some good balancing, i would think they could probly rev another 1500 2000 with a balance oriented more towards the top end of the rev range
i was looking at some software that is supposed to be really good for 2 smoke dirtbikes, i wouldthink it would work equally well. on saws
i have a balancing radle that i designed and cncd for dynamic balancing but have yet to find a suitable balancing system that is affordable
static balancing is the best i could do at this point.
i really want to do some type of precision balancing on the 08 im restoring right now but dont know where to begin.

 

[toadman]

We used to machine the flywheels on VW bugs (not a saw, I know) the Dyno showed 23-25 hp gained at the wheels on an average one of our more heavily modified motors, and a slight torque boost as well. but normally only >5-ish ft lbs more torque.
I tried this on a Dodge Dakota, and lost torque. I only gained 7 hp, which was negated by lost torque and it felt weaker.
There's a lot more playing into torque and HP numbers than just adding or removing extra rotating mass... especially in a two stroke. Port timing is huge.
I am not an expert on two strokes by a long shot, but I suspect the difference is more perceived by the RPM bands it plays in than by raw numbers.
My old mans mac 250 cuts right on the rev limit under load and feels super powerful because of this, but thats still far lower RPM and thus slower cutting than my ms460 pulling chain in the same wood.
Both bog out at about the same amount of load, but behave very differently to the ear, giving the [impression] of having more power to the mac.

 

[Huskybill]

I purchased bits and pieces like alloy I beam, live adjustable centers, on eBay. I have twin indicators to change out crank pins on my husky bikes. A 12 ton press from eBay. A arbor press from eBay, smaller work.

On a dirtbike we can feel the difference in porting. I’m thinking with the saws they just stay at higher rpm’s while under load. There’s no rpm drop.

 

[rd35]

Very interesting discussion! I'll throw in my 2 cents. So, technically, rotating mass has nothing to do with torque on a running engine when measured at a sustained rpm. There are two things rotating mass accomplishes. 1) it prevents the engine from stopping at low speed under full throttle conditions or low idle speed. 2) it stores power for a quick application of high torque when needed, say at the beginning of a cut when the chain wants to grab the wood all at once. In that moment the rotating mass of the crank, flywheel, clutch, etc will allow the engine, for an instant, to produce huge torque and horsepower as that stored rotating energy gets used up as the engine slows down. But once a steady state speed is reached at full throttle/full load, then the torque and horsepower are only dependent on the bore, stroke, air flow (which includes carb, intake, exhaust, porting, etc), and system losses (friction). Torque is the average rotational force required to produce a measured horsepower at a particular rotational speed (RPM).

 

[Robin Wood]

My terms may have been layman but you guys are right by correcting me, that mass increase will not change torque but widen the power band with acceleration sacrificed?

Am i right?

 

[frank_]

Very interesting discussion! I'll throw in my 2 cents. So, technically, rotating mass has nothing to do with torque on a running engine when measured at a sustained rpm. There are two things rotating mass accomplishes. 1) it prevents the engine from stopping at low speed under full throttle conditions or low idle speed. 2) it stores power for a quick application of high torque when needed, say at the beginning of a cut when the chain wants to grab the wood all at once. In that moment the rotating mass of the crank, flywheel, clutch, etc will allow the engine, for an instant, to produce huge torque and horsepower as that stored rotating energy gets used up as the engine slows down. But once a steady state speed is reached at full throttle/full load, then the torque and horsepower are only dependent on the bore, stroke, air flow (which includes carb, intake, exhaust, porting, etc), and system losses (friction). Torque is the average rotational force required to produce a measured horsepower at a particular rotational speed (RPM).

+1, the rotating mass wont alter torque or hp, you need to widen the powerband to increase the torque of a 2 stroke

 

[rd35]

It really doesn't widen the power band either, However....that rotating mass does make it "feel" like a wider band because when load is applied the engine doesn't lose speed as quick. The rotating flywheel mass causes the engine to be able to resist a speed change better as its stored up rotational energy is dissipated. We used to compete in garden tractor pulling. We had a massively large flywheel on that little tractor. The engine (10hp kohler block - 30 cu-in modified class) was capable of producing around 50hp at 8500 rpm (yes it sounded like it was going to explode). However with that flywheel on there we could start the sled much better than most folks because that tractor (coming out of the hole) would make well over 100 hp for a second or two until we got the wheels spinning. The extra hp was solely from the flywheel spinning down under load along with the engine doing all it could do! In circle track racing (like sprint cars) flywheels are as light weight as possible to allow quicker acceleration and deceleration.

 

[1hander]

what you guys are saying makes sense to me ..

 

[Robin Wood]

Well it doesnt widen the power band technically but holds rpm better when resistance is applied since it has more inertia stored via the heavy reciprocating mass, which in return gives the more torque effect.
In essence it just slows down at slower rate when load increases compared to lighter crank counterpart

 

[Huskybill]

Porting changes your power band envelope. You can port for mid range or more top end. I’ve seen the smaller cc engines come on like gang busters top end. But very little from bottom to mid. Where the bigger bores end up with a wider power band in the bikes. I’m sure the saws are no different.

Lighter flywheel more power quicker acceleration. Less mass turning.
Heavier flywheel more calm, more torque, controllable acceleration. more mass turning.

 

[ironman_gq]

i think mostly counterweights is there to help balancing, because of the balancing thing
the weight of the counterweights during rotation is more or less a wash when counteracting the weight of piston etc etc
they can only be heavy enough to counteract piston/rod assembly rotating mass to within certain rpm range
im not sure how much torque would actually come from the counterweights mass. size but there would definitely be a good bit coming from them
the flywheel/fan rotating weight is also a big player in torque delivery department
port timing,compression numbers, ignition timing, and bore all come into play but i dont know enough about that
the extra displacement/compression, piston surface area, i think make up the difference your mentioning
the 070 and the 088 while having very similar torque numbers will deliver that torque at different rpms
theres alot of i thinks in my post huh haaaa

Single cylinder engines are nearly impossible to balance, you have a single reciprocating mass producing vibration in a linear way (up and down) and then you try to balance it with a rotating mass producing vibration rotationally (all directions 90 degrees to the crank axis). At best you can cancel out a portion of the vibration from the piston and rod and changing the mass of either the crank (shaved weights, heavier flywheel) or the piston/rod (big bore kit) has little effect on balance. That's why you see so many model lines that have 3-4 saws of different displacments that all use the same rotating assembly.

 

[1hander]

Single cylinder engines are nearly impossible to balance, you have a single reciprocating mass producing vibration in a linear way (up and down) and then you try to balance it with a rotating mass producing vibration rotationally (all directions 90 degrees to the crank axis). At best you can cancel out a portion of the vibration from the piston and rod and changing the mass of either the crank (shaved weights, heavier flywheel) or the piston/rod (big bore kit) has little effect on balance. That's why you see so many model lines that have 3-4 saws of different displacments that all use the same rotating assembly.

I agree.. The most I have ever been able to achieve is to reduce vibration at the upper end of the Rev range.. The saws I balanced would kinda hit a point where the rpms were greatly reduced and everywhere else was not so good.. But one can definitely do better than a factory balance.

 

[Piston Skirt]

I agree.. The most I have ever been able to achieve is to reduce vibration at the upper end of the Rev range.. The saws I balanced would kinda hit a point where the rpms were greatly reduced and everywhere else was not so good.. But one can definitely do better than a factory balance.

Factories actually balance them quite precise but for the targeted RPMs.
Some manufacturers look for best numbers to get lowest values in catalogue (as per ISO measuring procedure) while some look for least vibrations in the actual cutting range.
As it was stated it's physically impossible to balance single-cylinder unit. The only way is to decide at which spot and for what reason the vibrations should be reduced and what needs to be sacrificed.

 

[sawfun]

Robin is correct about crank weight, especially the bobweight of the rod journal vs its distance from the crank centerline. More rotating mass will prefer to stay in motion, though, all things being equal, be slower to accelerate, the trade off. I remember doing a tow up a hill experiment with a couple of similarly geared Chevys. One had a big block 400 (402), the other a small block 400. The big block out towed the small block by a good margin. Then I pulled a plug wire off #5 cylinder on both. The small block shook and really struggled, the big block with its heavier rotating mass had little trouble. With #5 & #7 wires off the small block barely ran, the big block shook a little but pulled the load, though not well. Both vehicles had the same size torque convertor and turbo 400. My 500 caddy engine can easily idle down to 450 rpm in gear and is hard to kill with its giant crank. While heavier cranks do give the flywheel effect, there seems to be more going on than just that.

 

[Huskybill]

Balancing a two stroke.


Here’s another idea I mentioned about centering the crank and using a cold chisel to remove the stress between the porkchops. We put the rod at bdc. And tap the chisel lightly. If the crank feels tight in the case assembly this will help it.

Here’s another interesting idea,,,looking for every hp/torque gain.

 

[Robin Wood]

Robin is correct about crank weight, especially the bobweight of the rod journal vs its distance from the crank centerline. More rotating mass will prefer to stay in motion, though, all things being equal, be slower to accelerate, the trade off. I remember doing a tow up a hill experiment with a couple of similarly geared Chevys. One had a big block 400 (402), the other a small block 400. The big block out towed the small block by a good margin. Then I pulled a plug wire off #5 cylinder on both. The small block shook and really struggled, the big block with its heavier rotating mass had little trouble. With #5 & #7 wires off the small block barely ran, the big block shook a little but pulled the load, though not well. Both vehicles had the same size torque convertor and turbo 400. My 500 caddy engine can easily idle down to 450 rpm in gear and is hard to kill with its giant crank. While heavier cranks do give the flywheel effect, there seems to be more going on than just that.

Finally someone that understands what i meant, keep up the discussion guys

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[sawfun]

Finally someone that understands what i meant, keep up the discussion guys

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Oh I learned that plenty well in my early 20's.