Myth or truth? Revving in neutral

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Originally Posted By: mechtech2
Punisher -
Letting off the gas in gear and free revving an engine do not compare. Everybody lets off the gas a the end of the drag strip.
I see far more engine blow when accelerating down the track.

Max RPM are tough on any engine, in gear or not. But I still can't see why free revving is worse.


Well, free revving isn't good, but leaving your transmission in gear, slamming the throttle closed at the end of the quarter (or straightaway for a roundy pounder) is worse. The worst, absolute worst is having a stupid driver who hangs in the high groove/high bank during yellows, but I digress.

At WOT I worry about detonation or preignition killing a piston. Rods, not so much unless I am running some massive boost, or nitromethane. Big boost or nitro, I worry about more than just rods.

The original post asked about rods and stresses, and IMHO high revs in combination with low absolute manifold vacs can pull a rod/cap/bolt apart quicker than the same RPM fully loaded. If your rod is borderline a "X" RPM under load, it is in real trouble at "X" RPM unloaded. Any condition that reduces the net force on the rod is a good thing.
 
Rubbish, at absolute vacuum on one side, there's an extra 175 lb of force when the throttle is "slammed closed".

For that to cause engine failure means that it was already unlikely to make it back to the pits, and certainly not down it's next pass, due to accumulated fatigue.
 
I had a car with a sticky carb right while on a cold startup especially in the winter. In order to get engine to idle smooth after the startup, I would rev the engine(cold)til the choke unstuck. I didn't start having engine troubles til ~175,000 miles. I did this for years!

No, I don't think reving an engine(cold or warm) in "P" or "N" is the best thing to do. I just did it with this car.
 
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A test for loose main bearings is to run a hot engine under load, such as pulling a long hill, not necessarily really high rpm, just highway cruise, then suddenly close the throttle. If the oil pressure jumps more than a few psi it indicates looser mains. Think it's something like when under load the bearings are hunkered down pushing an oil wedge, and when you suddenly close the throttle, the bearing centers in it's journal bore causing the jump in pressure. Anyway, somehow this seems to relate to the discussion and one of you oil geniuses on the site can perhaps expain the connection--if any.
 
When a car has a noticeable bad rod, revving it to around 2000 RPM [hot] will make it audible. But releasing the throttle quickly does make the knock louder.
 
chrysler made some race cars in the early 60s. a note came with the car, kind of like this, dont rev the engine at top rpm longer than 15 seconds.
 
those were for The Max Wedge Motors. Big block RB motors turning 6500 to 7000 rpm. I'm not sure why they said that, but I know they did. If anything I'm sure it was for the rod bearings or Maybe? the main bearings. also maybe it was heat or a oiling weakness??????????

Max Wedge Central
 
Originally Posted By: Shannow
Originally Posted By: meep
Metals handle compression better than tension.


10 out of 10 cranes would dispute that statement.

Concrete would agree however.


LOL... that actually cracked me up.

but we aren't comparing metal to concrete. and nobody builds concrete cranes! (so we can't compare that either).

Take a coat hanger. bend it until it breaks. look at how it breaks. does it tear at the outside of the bend? or does it crumble on the inside? I can't explain it any easier...?

a little reference, just the first thing I saw.
http://www.newton.dep.anl.gov/askasci/mats05/mats05199.htm


M
 
Meep -
Tighten a guitar string and it will break . And you don't have to tighten it too much.
But put it in a vise, and you'll never break it.
So in general, I agree with you.
 
Originally Posted By: meep
Originally Posted By: Shannow
Originally Posted By: meep
Metals handle compression better than tension.


10 out of 10 cranes would dispute that statement.

Concrete would agree however.


LOL... that actually cracked me up.

but we aren't comparing metal to concrete. and nobody builds concrete cranes! (so we can't compare that either).

Take a coat hanger. bend it until it breaks. look at how it breaks. does it tear at the outside of the bend? or does it crumble on the inside? I can't explain it any easier...?

a little reference, just the first thing I saw.
http://www.newton.dep.anl.gov/askasci/mats05/mats05199.htm


M


Next time you look at a crane, compare the elements in tension (the cable), to the elements in compression (the jib).

See which one has the largest cross sectional area, given that both are made out of steel.

UTS (Ultimate Tensile Stress) isn't the only design factor, columnar buckling is generally the most worried about.

That's when you put the guitar string longitudinally in the vice and get effectively zero load on it before it fails (not breaks, fails).
 
you clearly know a lot on the subject-- and I'm not an expert.

In healthy fashion-- here's my thoughts--- follow my thoughts... because if I don't understand, I'd like to.

the cable and the jib are different materials and designed to handle different forces. Jib has to be rigid and strong and must resist buckling, because any flexing, any at all creates a compression/tension scenario between the inside and outside of the bend, and with those kinds of pressure would create failure. And the jib may also be carrying ~2x the compression, since there is also often an upper cable "above" the jib forming 2 right triangles over a central pivot to the rear of the jib. adds rigidity, but also doubles the compression the jib sees. So yes, I guess there would be greater cross-sectional area of metal in the jib, but it also must remain absolutely rigid, since any deflection would cause a failure. What this adds to the discussion, then, is shape. shape plays a part in design.

The cable must be flexible, and it is designed to handle the tension. I would think it's also a different material?

You and I both make points, and we both clutter our examples. In this single instance of a conrod, it is one material in one shape. It's an I-beam or a rectangular cross section, depending on the rod. The "I" structure provides some buckling resistance, the total volume of cross-section material supports tension. So it may almost come down to the design of the conrod in a specific application, ie., the specific conrod used in the engine may determine whether or not it is more prone loaded or unloaded failure.

BRB lemme see if I can find something on AL tension vs compression.

But now that I've thought about it--- even if some engineering spec says "forged Al is stronger in compression (or tension)", it's moot because that is only one factor in the forces it's subjected to in a conrod application.

(see, now neither of us can really say one way or the other??)

Mike
 
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meep, you are doing good.

Buckling is a material/design failure, while not the same as a tensile failure, it is still failure of the component. That's where I take umbrage at the steel is "better" in compression...so looking at the crane, they have to use more steel, and spread more around to resist buckling failure than the tensile loaded rope.

My concrete note is because when designing for concrete, you assume that the concrete has zero tensile strength (you actually assume it's already fractured). The tensile load is calculated purely on the reo bars, and the compressive as if there's no reo. (bridges etc. work on pre-tensioning the reo, and leaving the concrete in compression, even when the loads are tensile - imagine your paper clip bending, with a filament down the middle holding the ends together)

Bringing in Al, can also bring in fatigue, as a steel rod can be designed to "never" fatigue, Al will always fatigue given enough cycles.
 
Originally Posted By: Shannow
meep, you are doing good.

Buckling is a material/design failure, while not the same as a tensile failure, it is still failure of the component. That's where I take umbrage at the steel is "better" in compression...so looking at the crane, they have to use more steel, and spread more around to resist buckling failure than the tensile loaded rope.

My concrete note is because when designing for concrete, you assume that the concrete has zero tensile strength (you actually assume it's already fractured). The tensile load is calculated purely on the reo bars, and the compressive as if there's no reo. (bridges etc. work on pre-tensioning the reo, and leaving the concrete in compression, even when the loads are tensile - imagine your paper clip bending, with a filament down the middle holding the ends together)

Bringing in Al, can also bring in fatigue, as a steel rod can be designed to "never" fatigue, Al will always fatigue given enough cycles.


That's a good point! Al connecting rods have a finite life expectancy and have to be replaced after a given period of time in engine hours, passes....etc.
 
Originally Posted By: CBR.worm
I have never seen Aluminum rods - is that common today? I used (Pankle?) titanium ones once, but that ended in disaster.


Only common in race cars.
 
Originally Posted By: Shannow
meep, you are doing good.

Buckling is a material/design failure, while not the same as a tensile failure, it is still failure of the component. That's where I take umbrage at the steel is "better" in compression...so looking at the crane, they have to use more steel, and spread more around to resist buckling failure than the tensile loaded rope.

My concrete note is because when designing for concrete, you assume that the concrete has zero tensile strength (you actually assume it's already fractured). The tensile load is calculated purely on the reo bars, and the compressive as if there's no reo. (bridges etc. work on pre-tensioning the reo, and leaving the concrete in compression, even when the loads are tensile - imagine your paper clip bending, with a filament down the middle holding the ends together)

Bringing in Al, can also bring in fatigue, as a steel rod can be designed to "never" fatigue, Al will always fatigue given enough cycles.


Ok. Thank you for actually reading my replies and /teaching/ rather than flaming. I've always understood the difference between concrete and rebar but I only had enuf M.E. classes to be dangerous, and that was 20 years ago.

So, is what you are saying that it's really the deformation of the conrod, i.e, buckling that becomes it's failure mode, not compression persay? And I am then correct in my thoughts that shape will play a primary factor in its resistance to buckling?

If so it gets kinda stupid-simple-- an "overbuilt" conrod won't fail from compression, but then an overbuilt conrod won't fail from tension either. but weight is optimized for performance... and different piston weight, stroke and associated velocities, rod shape and material specs, all play a part in this--- so it all comes down to specific design-- ie., no one-size-fits-all answer... right?

wow.

Again, thanks Shannow. I appreciate you sticking with me on this one and not just calling me a DA ... I appreciate the learning.
 
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