Dangers of No load wide open throttle.

This has been mentioned lots in regard to tuning and exceeding Factory Max Rpm. I can understand that lean condition saw will readily over rev and the danger lies in scoring piston/cylinder, but providing the mixture is not too lean, what dangers lie in running no load w/o trottle. Bearing damage? Why? Flywheel exploding? Piston pin retaining clips dislodging? What physically is the problem. Some here have experience building and running modified saws that if held full throttle out of the cut will blow. Does the con rod break or stretch from the piston and rod momentum. What makes it tick, or more to the point, stop ticking?

Frank

In all Stihl owners manuals it states not to w/o throttle no load until you've ran a few tanks to break it in. I've never blown a saw up from running it no load so I don't what will happen.

One time a guy I know was trying to get an 021 to put out some bar oil. Instead of figuring out what was wrong, he took off the chain cover and bar, started it up and pegged the throttle. He was hoping the extra speed would clear out whatever was clogging it, I guess.
Anyway, the clutch blew apart at max rpm. One of the weights glanced off his forehead and flew straight up in the air. He shut the saw off and cursed, wiped some blood off his head with his sleeve and was standing there for a while when the weight came slamming back down to earth a few feet away. It was up about a mile and a half.
The fuel tank got damaged and these are throw away saws, so that was it for the saw, and we all learned a lesson.

I dont know that running a saw at WOT out of the cut will hurt it, I do it all the time, with the clutch cover on it of course. If anything, the manufacturers are trying to help you by not putting unneccessary wear on your engine. I could be and probably am wrong but thats my opinion. IF you make a saw rev to something like 17K rpm the saw almost surely will literally FLY apart. McCulloch made a saw for a very short time that had so high a max rpm that it was feared the saw would disintegrate if the governer failed. I think it was designated model BP-1. then again, Sachs Dolmar had the wankel powered KMS4 that if you modified the engine right, would rev to around 28K RPM redline.

I cant help but laugh mike.

Only time i run a saw with no bar and chain is when im working on the oiler, then I only blip the throttle or hold it just off idle to check if oiler is working...

Originally posted by oldsaw-addict
I dont know that running a saw at WOT out of the cut will hurt it, I do it all the time, with the clutch cover on it of course. If anything, the manufacturers are trying to help you by not putting unneccessary wear on your engine. I could be and probably am wrong but thats my opinion. IF you make a saw rev to something like 17K rpm the saw almost surely will literally FLY apart. McCulloch made a saw for a very short time that had so high a max rpm that it was feared the saw would disintegrate if the governer failed. I think it was designated model BP-1. then again, Sachs Dolmar had the wankel powered KMS4 that if you modified the engine right, would rev to around 28K RPM redline.

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I got an old 009 at work that i put together from parts that is a pile.
I should pin the trigger with it off and see what happens

use a zip tie on the trigger so i can get away from it.

Mike,
How about a picture of the scar.

........on your "Friend's" head of course.

max

I alwys keep the clutch turned away from my head. I had a clutch spin off an 056 super at WOT. Bout chased it to the next town.

The stihl engine failure analysis book says something about very high speeds allowing the rings to "clip" the ports. The theory is that the rings distort and dont have time to return to the ring land to clear the ports. (that ought'a start a sh1t storm)

clip

Clipped rings are a sure sign of an overspeed. 056 was the worst forthat problem, don't see it quite so much now. I think the governors in the ignition have helped a bit.

Uh oh, I see VERY dark clouds coming, And I think the are heading northeast!

The theory is that the rings distort and dont have time to return to the ring land to clear the ports.

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That is Stihls way of dodging responsibility for inadequate port chamfers. During normal operration the ring bulges into the port when it passes over it. Its the chamfers job to ease it back into the cylinder with out chipping the plating or damaging the ring.

That doesn't sound right to me, Ben.

What does'nt sound right, Mike?

Mike. Check this thread out. it has a little back ground info on "ring bulge" http://www.dootalk.com/forums/index.php?showtopic=11623

I've got to agree with Ben on this one. Ken Dunn has mentioned this as one of the potential problems with stihls.

What didn't sound right was that there was enough time for the ring to bulge in that instant it flies by the port. I'm just having a little trouble getting my head around that concept.
I'm also trying to figure out if the light really goes off when I close the refigerator door.

Interesting stuff Ben Walker

Mike, 2 stroke rings have little initial outward pressure but are energised by prsssure bleed from combution into the ring groove. As the exhaust port is cleared pressure on the outside of ring drops but pressure is still trapped under ring forcing it to bulge into unsupported area of port. Higher rpm- less time to bleed off I suppose.
This is exactly the kind of discussion I hoped would arise.

Tip Recommended to place ends of piston pin retaining clips at bottom of piston not at sides. Ends can flutter at high rpm and cause failure.

Frank

That makes sense to me, the ring expands into the exhaust port at high rpms then in the stihls it is just slammed back into place which will cause unavoidable damage to the piston rings somewhere or another along the line. If there is a light and gentle chamfer at the top of the port to ease the ring back in place it will last longer and work better. Is that about right? I hope I translated this into simple english for the non technincally inclined here, no offense guys.

The chamfer then, has to be steep but not deep because the chrome cylinder lining is only so thick and you don't want the ring contacting any of that soft metal behind it.
Got an image of a proper and/or improper chamfer?
And what about compression ring chamfer?

Tip Recommended to place ends of piston pin retaining clips at bottom of piston not at sides. Ends can flutter at high rpm and cause failure.

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I always do that when I replace a piston.

The chamfer then, has to be steep but not deep because the chrome cylinder lining is only so thick and you don't want the ring contacting any of that soft metal behind it.

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Mike, To do it right you chamfer the jug before its plated, then apply the plating. After the plating is in place you clean up your work. Factory Nicicsil(not Chrome) coatings are thin , but you could still get a decent chamfer with out replating. Sorry I do not have a picture for you.

BTW often this is the the reason that engines that have been ported by good tuners last longer. Its mucheasier on a cylinder and piston ring set when the ring is not slamiing into the ring groove as it passes over the ports.