Engine rpm's\ Chain speed

[Knotdodger]

A quick question, I have a 20in bar on my 361. I just got an es 18in.
Will that make much difference if any on a tach??? I dont use a tack to
adjust the carb, just by ear. Or is the engine speed different from the chain
rpm's??? Just wondering. The more I think about it the worse off I am.
So If a man were to go from a 25in bar, carb perfectly tuned. Too a 18in bar
would he be running lean??? Oh, I dont have a 25in bar, I would never use one. Just curious.
Thanks fella's:Eye: :Eye: :Eye:

Rob Boursaw

 

[ShoerFast]

I don't even think about checking my 361 with out a tach, and regularly run it anywhere from 6000' - 9500' .

There just hard to hear the burble , and really go to town when to lean.

From the hip, optimum would maybe 14,000 rpm's with the 20" bar and 14,200 with the 18",,,,IOW, not much change in were you want your tune at.

 

[bump_r]

I'm no expert by any means, but I can parrot what has been said here by experts.

You are correct in your thinking. You will want to tune the saw with the cutting system it will wear in the field. The parasitic losses vary enough system-to-system that the state of tune will change slightly (sometimes significantly) when the system is altered. And yes - when downsizing, the parasitic drag will decrease and you will need to richen the mix to keep engine speed down to spec.

Some folk swear that you need to re-tune every day, sometimes more than once per day to compensate for temp, humidity, altitude, and barometric pressure (and weather you have cherry or grape Kool-Aid in your cooler, I guess ), but I'd rather upsize my saw choice a notch than tip-toe that close to the ragged edge of having enough saw for the job.

 

[sdnomad]

Maybe from what some are saying I should get a 20" bar and call it good, instead of swapping back and forth between an 18 and 24 inch bar. I don't care to spend the money and store a second saw around the house for those few times I need a 24" bar.

 

[IchWarriorMkII]

You could, or if you could tune it for the 18" bar, and slap the 24" bar on. If it pulls the 24" bar like you think it should then don't worry about retuning and fine tuning to eek out all the juice.

 

[romeo]

richem up for longer bars

Not much difference in an 18" and a 20" bar for tuning. If you are going to a shorter bar you will probably be a little on the rich side so I wouldn't worry about being to lean. I never tune off a tach anyway, being at over 9000 in elevation, if I tune them to where they say they just feel too lean and loose to much power.

 

[troutfisher]

Not much difference in an 18" and a 20" bar for tuning. If you are going to a shorter bar you will probably be a little on the rich side so I wouldn't worry about being to lean. I never tune off a tach anyway, being at over 9000 in elevation, if I tune them to where they say they just feel too lean and loose to much power.

Yup, I switch from a 20" to a 28" on my 372xp, I don't re-tune.

I adjust the carb with a short bar on the saw, Tuning with a long bar on will get you lean.

 

[SawTroll]

... I adjust the carb with a short bar on the saw, Tuning with a long bar on will get you lean.

Right, if you don't want to retune when swapping bars, tune with the shortest bar you use, and you will be slightly rich with the longer ones.

The alternative is much worse.......umpkin2:

 

[Austin1]

Right, if you don't want to retune when swapping bars, tune with the shortest bar you use, and you will be slightly rich with the longer ones.

The alternative is much worse.......umpkin2:

Good point I thought of that as I use a 15'' on my 2159 a lot as the tree's at high elevation are all small anyway! Then when I go to the 18'' and working in bigger wood I don't fear a lean condition even though I am cutting at a lower elevation I have to cut much bigger wood. Better safe than sorry!

 

[Cedarkerf]

A quick question, I have a 20in bar on my 361. I just got an es 18in.
Will that make much difference if any on a tach??? I dont use a tack to
adjust the carb, just by ear. Or is the engine speed different from the chain
rpm's??? Just wondering. The more I think about it the worse off I am.
So If a man were to go from a 25in bar, carb perfectly tuned. Too a 18in bar
would he be running lean??? Oh, I dont have a 25in bar, I would never use one. Just curious.
Thanks fella's:Eye: :Eye: :Eye:

Rob Boursaw

Youd have to get a permit after a background check and waiting period to have a bar that big that far east. Just get a thirteen inch bar and cut twice:chainsawguy:

 

[Dan Forsh]

ShoerFast
and
bump_r

I've gotta ask? Are you the front end and back end of the same person?

I'm not mad, look at posts 2 and 3.

 

[Lakeside53]

remember....

There is only one "correct mixture" irrespective of the bar length...

Setting WOT with a particular bar is only a crude way to approximate the correct mixture.

 

[Crofter]

There is only one "correct mixture" irrespective of the bar length...

Setting WOT with a particular bar is only a crude way to approximate the correct mixture.

Yep! your hands and your ears are going to load the saw to what the individual operator feels is correct sound in the cut. You wont have to push as hard on the longer bar of course but the engine will be taking the same load regardless of bar length. Unless you are just using the long bar for reach, you will likely be making longer duration cuts in bigger wood so your peak cylinder temp could be higher with a longer bar. I think the proper mixture is one rich enough to give a fair bit of leaway. As BUMP'R mentions earlier, why tune that close to the line that you have to change your carb setting if the sun goes behind a cloud! :chainsawguy:

 

[SawTroll]

Youd have to get a permit after a background check and waiting period to have a bar that big that far east. Just get a thirteen inch bar and cut twice:chainsawguy:

Cool, will try to remember this post when I get another "permit" to rep you! :bang:

 

[outdoortype]

longer duration cuts vs. chain speed

I think Crofter is on to something reguarding the longer duration cuts accompanied with longer bar length. I say this because I tuned my saw with a 16" bar. I switched to an 18" bar (for a bigger tree)and the plug looked much cleaner than when I was using a 16" bar. I was really suprised at first because I expected it to look "richer" due to the decrease in RPMs. Before it had more brown residue with a hint of black. This time it had just a slight red brown haze.

 

[ShoerFast]

I think Crofter is on to something reguarding the longer duration cuts accompanied with longer bar length. I say this because I tuned my saw with a 16" bar. I switched to an 18" bar (for a bigger tree)and the plug looked much cleaner than when I was using a 16" bar. I was really suprised at first because I expected it to look "richer" due to the decrease in RPMs. Before it had more brown residue with a hint of black. This time it had just a slight red brown haze.

Being careful to just grab one worm to fish with at a time, and not tip the can.

Just as steam-engines have the highest TQ at zero RPM's , gas engines have a higher cylinder pressure with longer bars and in harder cuts, the sharpness of chain can also effect the firing pressure in the cylinder.

What your adjusting with the "H" screw is the absolute ratio that the saw will run at. A slightly rich saw, with a longer bar , duller chain or harder cut will have a higher cylinder pressure, and a hotter burning temperature. That alone will use the fuel better and burn more completely.

Your plug is showing a saw that could use slightly more ignition timing with the 16" bar and subsequent cylinder pressures, but has an over-all tune that excepts the parameters of the 18" bar.

IOW: Your saw could stand a little more DG* removed with the 16" bar as a final measure of "fine" tuning. (also, but I wouldn't touch it, your saw could use a lower octane fuel with your 16" bar)

Bottom line, what would be wrong with setting the "H" screw-fuel ratio to fastest timed cuts, then checking that setting with a tach. at WOT free-speed as a means of duplicating that ratio?

*DG, Depth Gauge, Some call them rakers, the amount of "cut" each cutter is allowed into the wood. My 361 likes a fairly grabby .035 @ 35 degrees with round filed chisel.

 

[ShoerFast]

ShoerFast
and
bump_r

I've gotta ask? Are you the front end and back end of the same person?

I'm not mad, look at posts 2 and 3.

Dan

I made up the "shoerfast" nick-name when I noticed there were a lot of very sharp people here, sort of an attempt to ballance things out a little, we all need a good laught!

Bump_r

 

[outdoortype]

wow, what a wealth of tech info!!!

ShoerFast, that makes a lot sense. Some of your post was little "over my head" but I think I get the meaning-Leave it where it is and I should be OK w/16" setup but optimal with the 18". Also, I was cutting in above normal temps for me (70F) and leaning it out may cause problems when I cut in colder temps this fall.

 

[Crofter]

Shoerfast; I am just quibbling here; I dont feel there should be higher cylinder pressures in the saw with the longer bar if the operator is doing his job!. If he lays on the saw, sure the longer chain will drag the saw down more. That is lugging it down and it will run hotter and have higher cylinder temps and pressures because of a lower piston speed in relation to expansion rate of the burning charge. No disagreement there. I just dont think a good operator should let the bar length dictate the operating rpm of the saw. That is his job. Adjusting depth guides accordingly for differnt chain lengths or engine size does make that easier.
That is why I dont think different bar length requires a different mixture. The proper mixture will give a different no load wide open throttle tach reading though, but as Lakeside mentions that is a crude way of arriving at approximately something.

 

[Knotdodger]

Well, thanks for the info fella's.. I don't have and will never need a 25in
bar. The 20 I have on the saw now, barely notice's the maple. ( Thanks to Mr. Neiger) The 18in don't know It is cutting anything.

Hmmmmm,,, I wonder about a 8 tooth sproket.. Op' never mind
it's just a work saw after all... Thanks guy's!