Max RPM - who cares?

[twistedtree]

I've seen comments comparing saws based on their max RPMs, and I've also seen objections to rev-limiting ignition systems. I don't get it.

Other that as a tuning tool, I don't understand why it matters if one saw can do 11,000 RPM, where another can so 13,000 RPM. From the few checks I've done of test reports, saws all seem to hit max power around 9,000 to 10,000 RPM, and max torque around 6,000 to 7,000 RPM, so these are the ranges where you would get the most out of your saw when cutting. So why does it matter how fast a saw can run out of the cut?

 

[bwalker]

Rev limiters complicate tunign the saw.

 

[Lakeside53]

what Ben said...

Time in cut is all that matters. WOT is just a "max" to keep your saw from falling to pieces.

 

[THALL10326]

I've seen comments comparing saws based on their max RPMs, and I've also seen objections to rev-limiting ignition systems. I don't get it.

Other that as a tuning tool, I don't understand why it matters if one saw can do 11,000 RPM, where another can so 13,000 RPM. From the few checks I've done of test reports, saws all seem to hit max power around 9,000 to 10,000 RPM, and max torque around 6,000 to 7,000 RPM, so these are the ranges where you would get the most out of your saw when cutting. So why does it matter how fast a saw can run out of the cut?

Reason is simple. If the saw is running 14,000 rpms out of the cut and the max rpm for that saw is 13,000 rpms that means the saw is running lean all the time. Sooner or later its going to lean sieze. Always good to know exactly how fast a saw is running so you know its not running lean all the time and slowly burning up.

 

[bwalker]

IMO rev limiters are a big cause of seized saws as the invariably lead to a saw being tuned too lean.

 

[ozflea]

Rev limiters complicate tunign the saw.

Their there to stop people from over leaning the engine and seizing the saw most saws should be tuned by ear by an experianced person otherwise it gets very expensive very quickly.

Off load saws should 4 stroke but under load 2 stroke but not be stangled like a chicken to get them last few revs especially on an every day work saws.

Mc Bob.

 

[ShoerFast]

Another approch,,,,,,

TwistedTree

Another approch to your awnser,,,,

Heat builds power, to a point,,,,,

Running slightly lean allows the heat to climb in the cylinder,,, makes radical use of the air/fuel,,,, that makes power and can hit a higher WOT speed.


Using a rev limiter takes the old-school tune of setting a richer then screeming "H" jet, is just de-tuning so to speek, safer way to tune.

But a rev limiter is a great idea, if you used EGT to set the final tune, but it would need to be in a consistant cut or dyno,,,,,,, like a best HP (sharp chain) or best TQ (tuff wood bad chain) RPM.

Kevin

 

[Crofter]

I think that the rev limited carb tuning may be a bit more complicated than that, and in no way keeps you from overleaning the mixture. A mixture that is lean enough to give 15,000 (were it not for the rev limiter) would start to have the ignition cutting out and making it sound like 4 stroking at the preset 12,000 or even less on 088. the sound difference between being ignition limited or mixture limited, is a lot more subtle than the old 2 stroke/ 4 stroke transition.

 

[shadyg]

Rev limiters complicate tunign the saw.

Why so Ben?
I would think it would make things easier as you could just start out on the rich side and then just slowly lean it out until you hear it start to bounce off the limiter. ( if the limiter is actually set at the saws' max rpm )

 

[ShoerFast]

There going to need to retard the timming,,,,,

Frank

Yes, thats what it would do,,,, but I'm thinking there going to need to desing a phase sensor to taylor the hall effect to retard the timming as critical engine speed is approched. (as opposed to cutting)

If they start to bump back the timming as the engine approches the PROM speed in the chip, they would get 2 birds with one stone,,,as NOX would be limited as retarded timing would limit speed and cyl temp.

We know that there going to use a prosser in a few years as 2007 is the start of the second phase of the Clean Air Act for small off - road hand held engines.

It wouldent be hard to do, just sence the leading edge and the trailing edge of the magnet and retrip the Hall-effect via the prosser,,,,, just wait till they start to use a fiol/gel "knock" sensor in one of the crank case bolts to establish the engine timming base,,,,,,, we would need to measure EGT to tune it,,,,, but we not getting a "H" screw anyway,, as everything will be sealed, (in the near future)

Kevin

 

[Crofter]

It wouldent be hard to do, just sence the leading edge and the trailing edge of the magnet and retrip the Hall-effect via the prosser,,,,, just wait till they start to use a fiol/gel "knock" sensor in one of the crank case bolts to establish the engine timming base,,,,,,, we would need to measure EGT to tune it,,,,, but we not getting a "H" screw anyway,, as everything will be sealed, (in the near future)

Kevin

Actually, once they get the bugs out, the computerized systems do work well and can adjust for a lot of variables. Really remarkably more reliable than the old way was years ago. It creates troubles though in the in between stage when there is still some user access and he tries to outhink the computer. Limiting revs seem to be one of the ways of extending life and increasing lower rev torque without having a saw that will run away with itself. Someone just posted a recall on a saws flywheels having a blowup risk. I have run an 066 that will hit 16,000 in a blink and I don't like even thinking about that flywheel a foot or so away from some tender trinkets!

 

[ShoerFast]

Were all members of ,,,,,,

Ummm Yes!

We are all members of our own Save the Trinkets Foundation!

Kevin

 

[Tree Sling'r]

I remove my rev. limiters if they are not already. The saws I run are built for high RPM. I like the higher RPM for limbing - plus it sounds cool.

 

[Crofter]

Tree Slingr;

How do you go about removing the rev limiters?

 

[Crofter]

Have been trying to chase down some info on what the big end bearing failure mode is. Seems like it is possible to be failure of the cage from the direction reversals twice every revolution and the resulting slamming of the needles fatigues the cage and spills the needles. Any saw techs have factory theory on what causes the failure.

What is the corect way to tune by ear when you have a rev limiter saw. I would think Shadyq's method would be my approach too, but am guessing that that would leave the saw a bit rich. Admittedly I have fooled very little with these though.

 

[timberwolf]

You guys talking about removing rev limiters, and at the same time I am trying to put one on my 260. I would take a 16k rev limiter if anyone has one kicking about

Just can't keep the revs down, the move fuel I get into it the faster it wants to turn, scarrrry!

 

[Lakeside53]

Why so Ben?
I would think it would make things easier as you could just start out on the rich side and then just slowly lean it out until you hear it start to bounce off the limiter. ( if the limiter is actually set at the saws' max rpm )

Unfortunately it's really hard to tell when you are being affected by the limiters UNLESS you are under high load - not very practical to do on a saw unless connected to a dyno or something similar. For concrete cut-off saws, where blade speed limiting is a real concern, you just set the carb to std setting (1 turn out), and let the rev limiter in the ignition do it's job. Without burying the saw into something, it blows smoke and sounds like crap almost the moment you hit the throttle as the saw spins up and hits the limiter. Stick it into concrete (or the nearest block of wood ) to load up the saw, and it runs sweet. I've seen several burned up cylinder from whomever "tuned" the saw had chased a sweet sound and left the saw way too lean. Same thing happens on the newer generation weed eaters. Luckily, not too many Stihl saws have limiters. The 066 "red eye" drove stihl away from these...

 

[bwalker]

Have been trying to chase down some info on what the big end bearing failure mode is.

Detonation, dirt ingestion and inadequate lubrication are three common causes.

 

[shadyg]

I hear what you're say'n Lakeside, thanks.
I guess it's time to break out the tachometers on some of these newer saws.

 

[bwalker]

Quote:
Originally Posted by shadyg
Why so Ben?
I would think it would make things easier as you could just start out on the rich side and then just slowly lean it out until you hear it start to bounce off the limiter. ( if the limiter is actually set at the saws' max rpm )


Unfortunately it's really hard to tell when you are being affected by the limiters UNLESS you are under high load - not very practical to do on a saw unless connected to a dyno or something similar. For concrete cut-off saws, where blade speed limiting is a real concern, you just set the carb to std setting (1 turn out), and let the rev limiter in the ignition do it's job. Without burying the saw into something, it blows smoke and sounds like crap almost the moment you hit the throttle as the saw spins up and hits the limiter. Stick it into concrete (or the nearest block of wood ) to load up the saw, and it runs sweet. I've seen several burned up cylinder from whomever "tuned" the saw had chased a sweet sound and left the saw way too lean. Same thing happens on the newer generation weed eaters. Luckily, not too many Stihl saws have limiters. The 066 "red eye" drove stihl away from these...
__________________

Exactly.
Another scenereo involves the saw requires a leaner setting than the rev limiter allows for proper tuning. Forget using the traditional tune by ear method when this is the case.