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bar length vrs rpm.

Started by Timbercruiser, December 04, 2014, 06:15:17 PM

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Timbercruiser

I was told that to run a longer bar you need to richen up the saw mixture a bit to give more power under load ?  Just wonder what you guys think ? :snowball:

celliott

Hmm... I've always just tuned by ear, or in the wood.

IMO (what I think) it would be the opposite. A longer bar and chain would take more power to spin, more load. So RPM's would be lower, say, if you tuned it for a 20" bar, then threw a 36" bar on. Right? Wrong...?
Short bar would be less of a load, so I'd think the saw would want to rev higher, then you would want it tuned richer.
So do you tune the saw for a 7 tooth VS an 8 tooth sprocket?
I'd think that's getting a little nit-picky at that point.

Chris Elliott

Clark 666C cable skidder
Husqvarna and Jonsered pro saws
265rx clearing saw
Professional maple tubing installer and maple sugaring worker, part time logger

HolmenTree

Throttle response is quicker with short bars.
Making a living with a saw since age 16.

ehp

yes you need to be richer on longer bars , the longer bar buts more load on the motor and more load equals more heat in the motor , the more heat the leaner the motor becomes , if left at same setting say for a 16 inch bar then but a 36 inch bar on the saw means the saw once warmed up after a couple cuts in bigger wood will be very lean compared to when you had the 16 inch bar on it , If lean enough you can cause engine damage

Gary_C

Power is directly related to RPM's. So if you need more power for longer bars, turn up the RPM's and run the saw at wide open throttle.
Never take life seriously. Nobody gets out alive anyway.

ehp

you need more torque to run longer bars as the drag is alot higher so if saw is lean you produce less torque compared to a proper set carb for the length of bar , do yourself a test , set your saw up with a 20 inch bar on it and make a couple hard cuts and see what rpms its turning at wide open throttle now put a 36 inch bar on it and do some back to back cuts in 36 inch in diameter wood and then check your saw roms, if the saw doesnot have a rev limited coil in it the rpms will be alot higher cause the saw is going to be running alot leaner caused by the motor having to work alot harder so the motor produces alot more heat , remember the saw is only cooled by air and the fuel charge , On normal setting for bars a good place to start is for every 4 inches of bar length over 20 inches you set the saw 200 rpms less , if your milling or cutting hardwood then set saw richer than that

gimpy

Or a smaller sprocket? For more power.
Gimpy old man
Lucky to have a great wife
John Deere 210LE tractor w/Gannon Box

Spartan

In my experience, every time I put on my longer bar, (32) I have to lean the saw out a bit.  I am assuming due to the additional drag.

ladylake

With a short bar and not leaning on it very hard you might by with a lean tuned saw without burning it up, long bars take torque to turn and need more fuel.   Steve
Timberking B20  18000  hours +  Case75xt grapple + forks+8" snow bucket + dirt bucket   770 Oliver   Lots(too many) of chainsaws, Like the Echo saws and the Stihl and Husky     W5  Case loader   1  trailers  Wright sharpener     Suffolk  setter Volvo MCT125c skid loader

Spartan

Quote from: ladylake on December 13, 2014, 03:20:08 AM
With a short bar and not leaning on it very hard you might by with a lean tuned saw without burning it up, long bars take torque to turn and need more fuel.   Steve

Running a saw rich does not make more power in my experience, running the correct mixture makes more power.  Just saying when I strap on the long bar, I have to lean it a little to bring it to correct mixture again. 

ehp

Your missing the point , with a longer bar there is more drag and with more drag comes more heat in the motor so you need to set the motor richer because of more heat , the hotter a motor runs the leaner it becomes so you need to set the carb richer to help or its going to lean burn , like I said before  lets say take a 660/066 stihl that runs and set for a 20 inch bar and make 10 cuts back to back in something hard like say hard maple that the bar just cuts threw , now tach your engine to see what rpm its running at , write that number down, now without changing anything other than the bar and chain to a 36 inch setup and make 10 cuts back to back in hard maple that this bar just cuts threw and tach your saw and see what the numbers are , Your saw will be turning way higher numbers with this longer bar setup and be so lean it could ruin the topend of your saw . The reason I say a saw like a 066/660 is because its not coil limited on rpms so you can really see what the saw is set at, at the Stihl dealer here all 660's that are running a 36 inch bar are set at 12,000 rpms , the dealer has no wood to tune the saw once it gets hot but I have done this tons of times , set the saw at 12,000 rpms , make 10 cuts and see what its set at once it gets hot , most times 13,200 to 13,400 rpms is what the saw it turning for rpms so that is set right where it should be 

Spartan

Yeah I guess I am missing the point, or misunderstanding you.  But what you say does not make sense to me.  when I put on a longer bar, it runs richer.  rpms go down.  I don't see how they could go up with more drag.  I have to take fuel out or I lose power.  I understand about adding fuel to cool, I have tuned fuel injection systems.  But you also have to understand that the hotter the air gets the less dense it gets and the afr goes down.  Maybe thats why I have to pull fuel if the air is getting hotter.  But you can't have a saw that does not perform well either.  a hot engine should not make lean condition.  I can understand heat generating because the engine has to work harder pulling more weight.  I just am not getting why the opposite of what you are saying is happening to me.  I don't have a tach but when it goes in the wood its rich.  I have run saws enough to know what rich and lean are as well as knowing when my saw is at a higher rpm.  So you are right, I am missing your point.

Andyshine77

When an engine is under more load it heats up, when an engine heats up it requires more fuel. If you don't provide the engine with more fuel, the engine will get hotter and hotter, and the no load rpm's will keep going up, until the piston starts to melt or something breaks. 

We are not talking about ambient air temperature. Ambient air temperature does effects intake air density, but it's a separate subject than engine/combustion temperature. No load rpm's almost mean nothing, in the cut rpm's is what you look for. When the engine is under load and it starts to overheat, the rpm's will go down. You need fuel to make power!!   
Andre.

Spartan

All I can say is you may be right but my experience is different than your explanations.

HolmenTree

I think we may be comparing apples to oranges here......factory stock saws to modified woods ported saws.
My Stihl 090 here cutting 54" diameter kraft paper rolls with a 60" .404 harvester chain at WOT for 5 minutes straight, is set no different when it's on the 36" Alaskan sawmill.
And I have cut hundreds of these rolls. To date the cylinder still has never been removed from my 090....and always  had the governor disabled since brand new. ;)



 
Making a living with a saw since age 16.

ehp

your 090 is not even close to how a saw today runs, I have 1 090 left but had lots of them, totally different design of motor and not very well design as far as power goes , your motor turned 7000 rpms , new saws today are alot closer to 9300 to 9600 rpms in the wood stock so alot closer to melting the piston , I'm talking a stock motor not ported saws . The reason Spartan seen less rpms once putting the longer bar on is because of drag from the chain on the longer bar . Guys I have spent weeks and weeks on engine dynos running two stroke motors  and have ran just about every test you could think of and alot of test most would not but its up to you, like the saying goes you can take a horse to water but you cannot make him drink

Cornishman

ehp   
I think it would be helpfull if you could explain why a saw gets leaner when it gets hotter.

HolmenTree

Quote from: ehp on December 14, 2014, 04:54:57 PM
your 090 is not even close to how a saw today runs, I have 1 090 left but had lots of them, totally different design of motor and not very well design as far as power goes , your motor turned 7000 rpms , new saws today are alot closer to 9300 to 9600 rpms in the wood stock so alot closer to melting the piston , I'm talking a stock motor not ported saws .
I think you read as bad as you write :D
My 090 runs way higher then 7000 in the wood ....like I said in my last post, I had it's wind vane governor disabled since new.
Making a living with a saw since age 16.

ZeroJunk

Quote from: Cornishman on December 14, 2014, 05:35:27 PM
ehp   
I think it would be helpfull if you could explain why a saw gets leaner when it gets hotter.

They will lean out when they warm up. That is the reason why you never tune a cold saw.

But, I always thought it was burning off excess fuel in the crankcase from starting and sitting rather than having anything to do with the actual temperature.

Andyshine77

Quote from: Cornishman on December 14, 2014, 05:35:27 PM
ehp   
I think it would be helpfull if you could explain why a saw gets leaner when it gets hotter.

A hot engine is capable of proper combustion of the charge. The hotter it gets the more fuel the engine can burn, so you must compensate, if not the temperature will keep climbing. That is unless you have an AutoTune saw. :) A cold engine doesn't combust the charge completely. Ask yourself this. Why does a saw 4 stroke without any load? The 4 stocking sound is a weak burn/miss, do to incomplete combustion. When the engine is placed under load, the combustion temps quickly rise, now you have proper combustion. The temperature also drops quickly when you go back to idle. After making really long cuts it's best to let the saw idle a bit to cool down. 

If you pay attention to the exhaust in this video, at around 35 seconds you can see a hotspot on the exhaust getting larger with each cut. The same thing happens in the combustion chamber. The saw was a basic port job without any added compression. The more compression you have, the higher the head temperature. http://youtu.be/dxz8oIWv-_Q?list=UUDzYN6LbndM64YHQp_JOb4Q

Guys this is really basic stuff. Telling someone like Ed he's wrong is simply ignorant. Questions are always fine however :)     
Andre.

Andyshine77

Quote from: HolmenTree on December 14, 2014, 09:55:44 PM
Quote from: ehp on December 14, 2014, 04:54:57 PM
your 090 is not even close to how a saw today runs, I have 1 090 left but had lots of them, totally different design of motor and not very well design as far as power goes , your motor turned 7000 rpms , new saws today are alot closer to 9300 to 9600 rpms in the wood stock so alot closer to melting the piston , I'm talking a stock motor not ported saws .
I think you read as bad as you write :D
My 090 runs way higher then 7000 in the wood ....like I said in my last post, I had it's wind vane governor disabled since new.

It has to do with energy. The 090 has a large displacement engine, with low relative power/energy output. A lot of energy an internal combustion engine produces, is in the form of heat. Now the heat a 090 does produce is taken care of with mass, a large amount of charge coming into the engine, and a large flywheel moving air over the cylinder. And yes a lot lower rpm's. 
Andre.

HolmenTree

Quote from: Andyshine77 on December 15, 2014, 12:24:09 AM
Quote from: HolmenTree on December 14, 2014, 09:55:44 PM
Quote from: ehp on December 14, 2014, 04:54:57 PM
your 090 is not even close to how a saw today runs, I have 1 090 left but had lots of them, totally different design of motor and not very well design as far as power goes , your motor turned 7000 rpms , new saws today are alot closer to 9300 to 9600 rpms in the wood stock so alot closer to melting the piston , I'm talking a stock motor not ported saws .
I think you read as bad as you write :D
My 090 runs way higher then 7000 in the wood ....like I said in my last post, I had it's wind vane governor disabled since new.

It has to do with energy. The 090 has a large displacement engine, with low relative power/energy output. A lot of energy an internal combustion engine produces, is in the form of heat. Now the heat a 090 does produce is taken care of with mass, a large amount of charge coming into the engine, and a large flywheel moving air over the cylinder. And yes a lot lower rpm's.
Don't forget it's aluminum cast muffler also transferring the heat away from the exhaust port and the cylinder was never milled to raise compression.
But to ehp who says my 090 runs 7000 rpm after I explained earlier that I disabled the governor kind of ticked me off. If you're going to answer a statement  at least get your facts right and "listen".
 
The amount of  reliability my 090 has produced over the years has surpassed any expectations I had of the saw......nothing is tougher then cutting 54" kraft paper rolls one after the other , up to 45 in a day. That includes touching up the chain after each cut and burning 5 gallons of fuel mix and a gallon of chain oil day in day out.
So as ehp says it doesn't have a good design for power wise....... shows the mentality of a performance cookie cutter modifier.
Well I can say my reliable  090's cylinder is still pumping since it was first installed at the factory way back then.............. and that's with its governor disabled since new.

Andre you did a good explanation BTW of engine heat transfer with your prior post. ;) 
Making a living with a saw since age 16.

Andyshine77

Think of the 090 as a tractor, a modern saw like the 562 is a Ferrari.
Andre.

Cornishman

There seems to be a lot of confusion here
Does any one disagree with the following
Saws are designed to run richer at higher rpm.This is /was used to limit max rpm.
4 stroking is a sign of running rich. At WOT an engine should 4 stroke then when in the cut will slow down, the mixture will weaken to the correct level and the engine note will clean up.
Fitting a longer bar will increase the load and slow the rpm resulting in a weaker AFR so the saw will need to be retuned richer to compensate. A richer mixture will also help cooling as the saw will be working harder with the longer bar.
At idle a saw will cool especially if set a little rich
Once a saw has reached normal working temp and burnt off the crankcase residue it will
run leaner because it is then only using fuel supplied through the carb.
I do not see how the carb characteristics would change with heat after this point so can
anyone can explain the mechanism of this.

Andyshine77

Quote from: Cornishman on December 15, 2014, 06:04:31 PM
There seems to be a lot of confusion here
Does any one disagree with the following
Saws are designed to run richer at higher rpm.This is /was used to limit max rpm.

The carb's jetting can and does limit rpm.

Quote from: Cornishman on December 15, 2014, 06:04:31 PM4 stroking is a sign of running rich. At WOT an engine should 4 stroke then when in the cut will slow down, the mixture will weaken to the correct level and the engine note will clean up.

It's load that raises the temperature, thus burning the charge completely.   

Quote from: Cornishman on December 15, 2014, 06:04:31 PMFitting a longer bar will increase the load and slow the rpm resulting in a weaker AFR so the saw will need to be retuned richer to compensate. A richer mixture will also help cooling as the saw will be working harder with the longer bar.
At idle a saw will cool especially if set a little rich

Yes a saw will cool down at idle, because there is no load and plenty fuel.

Quote from: Cornishman on December 15, 2014, 06:04:31 PMOnce a saw has reached normal working temp and burnt off the crankcase residue it will
run leaner because it is then only using fuel supplied through the carb.

There really isn't that much residue in the case, it really has to do with incomplete combustion do to low engine temperature.   


Quote from: Cornishman on December 15, 2014, 06:04:31 PMI do not see how the carb characteristics would change with heat after this point so can
anyone can explain the mechanism of this.

If you're talking about ambient air. Cold air has has density. Density= Mass/ Volume. Hot air is less dense , so it has less Mass/Volume. The more air you have the more fuel you need, and the more power you'll have.

If you're talking about why an engine needs more fuel the hotter it gets because of load, it was explained in my previous posts. 
Andre.

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