No load revving

[Karl_Childers]

I would like to know what the real deal story is on revving up a 2-stroke with no load. What are the tolerances? What is the failure/ consequence for over revving. Is it all just B.S.? I have heard all kinds of horror stories about catastrophic engine failures due to someone over revving it in the driveway. WHAT IS THE TRUTH???

Rob

 

[walt-oxie1]

Re: No load revving

Over revving ANY engine is bad especially with no load or cold. With any internal combustion engine, while running it is trying to fly apart. On a 2 stroke, they are obviously lubed through the gas/oil mix. It takes a little bit for that lubrication to start working properly, much like starting a car and it taking a second or 3 for the oil pressure to come up. Most people that have failures with 2 strokes either rev them up as soon as they fire off which is very bad or decide to rev them up and over do it. Since there is no load present, they continue to rev even after they let off the throttle and the RPMs go to high. The 2 major problems are little or no lubrication present or the Maximum RPMs get exceeded, neither of which are a good thing.

 

[Karl_Childers]

Re: No load revving

Thanks for the reply man, I really appreciate the insight, but what does load have to do with it? What is the difference if I have the prop in the water pushing the boat at 5500 RPM or if I am on the hose in the drive way in neutral at 5500 RPM. I don't dispute that this is a bad idea, I am just trying to figure out what the difference is?? What does Load have to do with it? I have heard that with no load the rod bearings take more abuse (which makes no sence to me) and just today I was told that with no load and revving the engine too much over 2000rpm on the hose in neutral in the driveway will cause ring failure, which makes no sence to me. Please elaborate.

Rob

 

[walt-oxie1]

Re: No load revving

The biggest problem with no load conditions is harmonic vibrations. The intensify greatly with no load present to dampen them. The harmonic vibrations cause the internals to shake to the outer ends of their tollerences and cause them to hammer a little. The excessive hammering is what causes the damage/failure.

 

[Jeff_G]

Re: No load revving

A couple things can happen depending on the engine.
Some engine's ignition is rpm dependent and can cause a run away condition.
The main problem though is the loading, unloading of the engine/bearings, crankshaft etc. The loading/unloading of these components can cause excessive wear and even damage to the rotating parts such as the bearings, very common with no load revving is bearing chatter, this in itself can lead to bearing and crank damage. If the damage and or heat generated is bad enough failure of parts can occur. Another problem is unrestrained rpm where as it actually exceeds the safe tolerances of the engine. This too can cause all kinds of problems such as the crank failing or even the flywheel!
Running under load evens out the internal forces/load on the components keeping the parts from chattering, bouncing etc.

Years ago I had a neighbor who delighted in revving up his motor on the hose in his driveway after a days use. I talked to him about it to no avail. The third week as I sat on the front porch one Sunday his motor abruptly stopped. I walked over to is house. He had put a rod through the block when the rod bearing failed.

In most cases, when an engine is in PERFECT shape no damage will occur, after all the engines run high rpm most of the time, and are under compression load. But the chances of damage are greatly increased when revving without a load.
Dang I hope that was understandable!

 

[corm]

Re: No load revving

Hi,

When you over rev a merc with finger reeds the reeds will start to flutter coming down on the tips causing chipping or splitting. The broken reed pieces
end up stuck between the piston and the cylinder wall causing major problems.
Also 2 strokes are designed to be under load on the power stroke so you could almost run them without a rod cap. When they are wound up and going wing-ding-ding it is putting undo stress on the rod cap, bolts and crank. This can result in a chirping engine from the bearings being hammered into the shaft and rod or just a broken rod cap. This is less of a problem in the newer engines do to better rod and bearing designs. The newer reed designs are less likely to break too. Having everything up to operating temperature is a good idea too.
Then your bearing & piston tolerances are where they should be.

 

[walt-oxie1]

Re: No load revving

I forgot to mention 95% of 2 stroke engines use needle bearings (2 stroke diesels being the exception). The hammering on needle bearings causes them to flatten out. You know as well as I do, you cannot roll something that is flat.

 

[JB]

Re: No load revving

Well, the mechanical consequences are outlined above, but the real cause is thermal runaway.

In thermal runaway a 2 stroke goes into a diesel-like cycle in which closing the throttle is of no use because it can suck enough fuel through the idle circuits to over-rev with no load and turning off the ignition is of no use because it is firing on a diesel cycle.

The only ways to stop an engine in TR is to choke it to death or let it rev high enough to break.

Not every 2 stroke will run away if you rev it a bit, just like not every rattler will bite you if you tread near it, but once you have had one run away you will NEVER do it again.

 

[Karl_Childers]

Re: No load revving

Thanks to everyone for trying to help me wrap my head around this. I can't disput anything any of you have said, because everyone clearly knows more than me on the subject. That being said, thermal runaway sounds about as likely to happen as a meteor hitting my house tonight. How can an engine "diesel" if you close the throttle butterflies? Fire needs air. I can see how it could sputter and have a "run-on" effect, but I just can't see how it could continue to increase until it reaches a catastrophic failure. But then again, I'm no mechanic and certainly no engineer. I just like to fish, and want to understand my engine better. Thanks for the input, I'd love to hear any more opinions/facts/theories.

Rob

 

[corm]

Re: No load revving

Hi,

thermal runaway sounds about as likely to happen as a meteor hitting my house tonight.

HaHa, You'll freak the first time when either one happens to you. Then you never do either again.

 

[emckelvy]

Re: No load revving

Just disconnect the vacuum-operated anti-overevving switch on a 50's-vintage Evinrude or Johnson Big Twin (30, 35, 40), crank open the throttle in Neutral, slam it shut, and let the fun begin as it runs away! You better run too, it's real scary!!!!!

The switch operates on the high vacuum developed from slamming the throttle shut and shorts out one set of points in an effort to reduce rpm to a controllable level. I always wondered if anything would really happen if the switch wasn't working and MAN! did I find out!!

The old V4's with big pistons and long, skinny rods didn't like to be revved up like that, either and would do nasty things like break.

The OP is right, once you see one of these do this you'll never wanna see it again!

Cheers........ed

 

[j_martin]

Re: No load revving

Thanks to everyone for trying to help me wrap my head around this. I can't disput anything any of you have said, because everyone clearly knows more than me on the subject. That being said, thermal runaway sounds about as likely to happen as a meteor hitting my house tonight. How can an engine "diesel" if you close the throttle butterflies? Fire needs air. I can see how it could sputter and have a "run-on" effect, but I just can't see how it could continue to increase until it reaches a catastrophic failure. But then again, I'm no mechanic and certainly no engineer. I just like to fish, and want to understand my engine better. Thanks for the input, I'd love to hear any more opinions/facts/theories.

Rob

Well, at least on my XR4, the throttle plates are completely closed at idle, and the timing is way after TDC. There is either enough bypass built into the butterflies, or other air circuits to supply enough to run the engine.

The biggest item I see is that these hot rods don't have much mass for the torque they produce, so revving them no load they can go off the top pretty easily. They are designed to run pretty rich and wet, so when you wind them up, then close the throttle, they will dry out long before they slow down. All in all, it sure can't be good for them.

I like to fish, too, and I like my fishin' motor too much to risk wrecking it just to try to prove the experts wrong by revving it up on the cuffs.

 

[Dukedog]

Re: No load revving

It all depends on tha guy, that tha hand operating tha throttle, is attached too

 

[Namenottaken]

Re: No load revving

Thermal runaway?? Are you talking about preignition or over-reving??
Over-reving anything is just dumb and you deserve what you get..

 

[Karl_Childers]

Re: No load revving

Please re-read my original post. I, in no way ever advocated over revving an engine, and I actually resent the implication. I was really just trying to understand the theory behind why it will destroy your engine, and what happens when it does. My question has been answered pretty well, and for that I am grateful.

There are a lot of shade tree mechanics out there that will boast seemingly outrageous things and back them up with half cracked facts. Due largely to the fact that these guys are so adamant about their claims, their claims seem to become generally accepted. I couldn't care less about "what some guy told you once" or "what you heard" I care about scientific facts, and experience and this seems like great place to learn just that. If it seems like I was disregarding thermal runaway, I am sorry, I was not. I do appreciate any comments or answers to my question.

thank you

Rob