I think the more I try to explain things, the more I need to explain my explanation. I should quit while I'm still behind.
The "force" does not change. It's the load that is there or not there. And yes, that analogy needs to make the assumption that the "subject" keeps the force the same but does not allow the "rate" to change when there is no "load" as in revving an engine up to high rpm and holding it there for a few seconds.
The "clearances" I speak of are the normal ones built into every engine. The crank bearing to crank has to have a specified clearance (so a film of oil can get in there), as does the the crank to rod big end, as does the rod to wrist pin. Then there are all those gaps between shaft splines, gear teeth, etc., although these are not so much of target concern.
ps. I'm not an engineer. That's why I am unable or willing to venture into more complex explanations.

 

jlangton,

Love the "smart" chart. I saved it and set it as my wallpaper. Might have to use that occasionally!


To everyone else:

Please cut the malarcky or I'll have to Dumpsterize this baby. You know who you are.

 

my two bits:

for rev matching - while the throttle is on the increasing pressure on the internals keeps a (moderate) load on, no?

The pistons that aren't firing on the power stroke are pushing out gas (pressure), taking in gas (vaccuum - which will pull on the piston, slightly?) and compression (lotsa pressure)

My understanding is continual high revving is not the best thing, and actually the deceleration of the motor under closed throttle is where the most "slap" occurs. I've read of motorsports engine builders doing this very thing when breaking in a motor to seat the valve retainers firmly.

who knew piston slap was so complex? Lotta variables involved....

I'm done. I think we'll all be okay.... except one of us.... you know who you are....

 

[QUOTE]Originally posted by xviper
[B]The "clearances" I speak of are the normal ones built into every engine.

 

[QUOTE]Originally posted by xviper
[B]
You are right ----- sorta.

 

Here's one more bit of info and AGAIN, it's an "explanation" NOT a "warning".
Just had a talk with another mechanic friend of mine and he added this:
It has to do with timing chain/belt slack. With no load, the engine can rev up high very quickly. In doing so, the tensioner may not be able to keep the chain/belt taute (sp?). This could throw off the valve timing enough in an interference engine to cause a problem. With load, even if it's through the drivetrain on sheer ice, the rate of rpm rise is sufficiently controlled that the tensioner can do its job.
Take it for what it's worth and DON'T GET WORKED UP ABOUT IT! Note the words "may" and "could".
And what "Former" said was not too far off. My friend also said that in engines with hydraulic lifters, revving it up quickly (like with no load) may not allow the oil pressure to build up fast enough to get the lifters fully stiffened up.
It is a complex scenario.
So, if you have a direct gear driven cams, rev away!

 

Ok I am going to take a stab at this one since xviper is having so much fun. The acceleration and deceleration of an engine happens every time you step on and off the throttle but what happens when there is no load is the shock factor that happens between the 2 when there is no load. It USE TO BE a problem with older engine designs that required more clearances in the motor not so much these days with the tighter clearances the motors of today have. To ATTEMPT to make this simple lets use a 2" crank journal and a 2.25" connecting rod. If you center the connecting rod on the crank you have .125" clearance around the crank. ( I know this is exaggerated but its only for example). Now the .125" clearance is filled with oil to lubricate the rod on the crank. When a force is applied to the rod the clearance on the side the force is being applied will reduce and the opposite side will increase. The second the force is withdrawn from the rod and a pulling effect is applied the clearances will reverse and this is where damage can occur and led to engine failures. There is a slapping effect that occurs when the forces are switched rapidly. When there is no load the slapping effect increases kind of like falling off a cliff with a rope when you hit the end of the rope there is going to be a pretty big jerk then if you fell off a cliff with a bungee cord which will ease the force transition. I hope this help explain this effect because now my head hurts.

 

SanMarinoCpe, not a bad way to put it at all.
ps. My head hurt on page 1.

 

 

But this reversal happens whether the engine is under load or not. For instance, in the power stroke, the piston is driving the crank downwards, and in the compression stroke the piston is still fighting to push the crank down. But in the intake stroke the crank is _pulling_ the piston down, which as you say is going to pull the other way on the bearing. The only difference is the increased load under load, which seems like it would make the whole situation worse. It's falling off a cliff with a rope either way, except in one scenario you're kate moss and in the other you're rosie o donnell

The timing chain and hydraulic lifter explanations makes perfect sense.