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  #31  
Old 06-20-2011, 08:20 PM
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Quote:
Originally Posted by engatwork View Post
Too much power on the power stroke (at operating temperature) for the rods to be able to handle?
UH... Isn't that a little obvious for a theory ?

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  #32  
Old 06-20-2011, 08:30 PM
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No offense taken, but I don't think anyone has seriously thought through my explanation. oh well. I give up, not that I'm convinced that I'm wrong yet though. Old rodbender theories range from simple head gasket failure, to chaotic combustion, to just plain weak rods. My favorite being the rock theory. I'm personally not buying any of those since they don't explain the whole story (why just 1 and 6, why not the 2.9L, why didn't the new rods always solve the problem. etc). A head gasket that is ~.030" (correct me if I'm wrong, I don't know this measurement off hand) will not take up .013" thermal expansion especially considering the head bolts get tighter at temperature. But point taken, I'll drop it.

Quote:
I do not understand that concept at all....
If you have a main bearing between EACH inline bore.. that is the best of all possible worlds... that supports the crank the most number of times possible with 6 inline bores... X+1 is the formula.....
This is because in a 4 main bearing set up you have more cross sectional area away from the rotating axis making the design inherently more torsionally rigid. It's like how a hollow rod with the same cross sectional area is more torsionally rigid than a solid one. Basically the lever rule. I agree it seems sort of counterintuitive.
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Last edited by ajnorris; 06-20-2011 at 08:41 PM.
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  #33  
Old 06-20-2011, 08:51 PM
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Originally Posted by ajnorris View Post
No offense taken, but I don't think anyone has seriously thought through my explanation. oh well. I give up, not that I'm convinced that I'm wrong yet though.
This is because in a 4 main bearing set up you have more cross sectional area away from the rotating axis making the design inherently more torsionally rigid. It's like how a hollow rod with the same cross sectional area is more torsionally rigid than a solid one. Basically the lever rule. I agree it seems sort of counterintuitive.
LOL, it seems counterintuitive because you have it backwards.
You are just making stuff up... no offense... it just does not follow the physics of the situation... Having 7 main bearings in an inline 6 is the best of all possible worlds... the most support you can give the crank ..... which is always good...
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  #34  
Old 06-20-2011, 09:01 PM
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Quote:
Originally Posted by ajnorris View Post
No offense taken, but I don't think anyone has seriously thought through my explanation. oh well. I give up, not that I'm convinced that I'm wrong yet though. Old rodbender theories range from simple head gasket failure, to chaotic combustion, to just plain weak rods. My favorite being the rock theory. I'm personally not buying any of those since they don't explain the whole story (why just 1 and 6, why not the 2.9L, why didn't the new rods always solve the problem. etc). A head gasket that is ~.030" (correct me if I'm wrong, I don't know this measurement off hand) will not take up .013" thermal expansion especially considering the head bolts get tighter at temperature. But point taken, I'll drop it.

I have thought through it. I also convinced myself that I had figured it out by this same theory. Then I thought about it for a while: "how could the engineers have missed college level physics and introductory material science?"

I realized, they knew something I didn't.

And the HG is .063" and is not a solid across its profile. It is a layer of perforated steel coated in "god knows what" on each side. And you can bet it is designed to become more responsive as it heats.

I am not telling you to 'drop it'... I think its good exercise. But thats all it is.
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  #35  
Old 06-21-2011, 12:17 AM
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Leathermang,
Respectfully, I assure you I'm not making that up. I don't have a better reference other than drawing the free body diagram, but this is from Wikepedia (I know, I know, not a real reference)
Quote:
Crankshafts on six cylinder engines generally have either four or seven main bearings. Larger engines and diesels tend to use the latter because of high loadings and to avoid crankshaft flex. Because of the six cylinder engine's smooth characteristic, there is a tendency for a driver to load the engine at low engine speeds. This can produce crankshaft flex in four main bearing designs where the crank spans the distance of two cylinders between main bearings. This distance is longer than the distance between two adjacent main bearings on a V6 with four mains, because the V6 has cylinder bores on opposite banks which overlap significantly; the overlap may be as high as 100%, minus the width of one connecting rod (1.00" or so). In addition, modern high-compression engines subject the crankshaft to greater bending loads from higher peak gas pressures, requiring the crankthrows to have greater support from adjacent bearings, so it is now customary to design straight-sixes with seven main bearings.[7]
Many of the more sporty high-performance engines use the four bearing design because of better torsional stiffness (e.g., BMW small straight 6's, Ford's Zephyr 6). The accumulated length of main bearing journals gives a relatively torsionally flexible crankshaft. The four main bearing design has only six crank throws and four main journals, so is much stiffer in the torsional domain. At high engine speeds, the lack of torsional stiffness can make the seven main bearing design susceptible to torsional flex and potential breakage. Note that a V12 engine can be made with the same number of crank throws as the seven main bearing straight-six, although each throw must be wide enough to accept the connecting rods of both opposing cylinders requiring either that each rod be far narrower, or that the crankshaft length be extended. Another factor affecting large straight-six engines is the front mounted timing chain which connects any camshafts to the crankshaft. The camshafts are also quite long and subject to torsional flex as they in turn operate valves alternately near the front of the engine and near the rear. At high engine speeds, camshafts can flex torsionally in addition to the crankshaft, contributing to valve timing for the rear most cylinders becoming inaccurate and erratic, losing power, and in extreme cases resulting in mechanical interference between valve and piston — with catastrophic results. Some designers have experimented with installing the timing chain/gears in the middle of the engine (between cylinders 3 and 4) or adding a second timing chain at the rear of the engine. Either method can solve the problem at the cost of additional complexity.
Anyway point being cylinders 1 and 6 might be off by 1-2 degrees when the IP is dead on, not serious but something to think about at least.

JT20,
I can see how that thick of a head gasket would take more deflection. On the other hand you would be surprised some of the basic materials stuff some mechanical engineers get wrong sometimes. After all the same engineers did design a head that cracked from thermo-mechanical fatigue. (granted they didn't have complex finite element models back then). That being said you are probably right. I just am curious as to the real reason for the failure and figured I'd take a stab at it.

Horse is dogfood now.
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  #36  
Old 06-21-2011, 01:28 AM
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""Crankshafts on six cylinder engines generally have either four or seven . Larger engines and diesels tend to use the latter because of high loadings and to avoid crankshaft flex. Because of the six cylinder engine's smooth characteristic, there is a tendency for a driver to load the engine at low engine speeds.""

PURE BS about ' tendency for a driver to load the engine at low speeds'.... or the implication that that has excessive effect on the crankshaft in a properly designed engine.
The sentence before that... that big diesel 6 inlines usually have 7 main bearings.... there is a reason for that... you just did not know which part to accept as fact.....Is your car an automatic ? If YES... then tell me how you would ' load it at low speed ' ?
The only way to ' load it ' is to shift at too early a speed..... and some designs have a natural tendency to resist that anyway...
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  #37  
Old 06-21-2011, 06:10 AM
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Leathermang,
Point is, which I previosly said, 7 main bearings is better for off axis flexing, but worse in torsion. Diesels have much higher off axis bending loads than gas engines so they have 7 mains but the result is there is less torsional stiffness to the crank. Harmonics modes are a completely different story. All I'm saying is that a straight 6 with 7 mains is going to have the most angular deflection out of any configuration (other than a straight 8 or 12 etc.). This motor having a greater lever arm and bigger bore (not to mention more fuel at low RPM) exerts more torsional stress on the crank than a .960. That was my only point. I'm not touting the rest if the wikededia explanation (which obviously some of it is probably conjecture or holds true only for certain situations), but I couldn't find a real source, without citing many page crankshaft stiffness analysis papers which nobody wants to or cares to read. Kindly don't call my physics explanation for the torsion flexure BS until you work it out on paper first.
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  #38  
Old 06-21-2011, 08:39 AM
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Much of what you are claiming assumes the crankshaft is not designed or made well enough to deal with whatever is happening in that engine...
If crankshafts are not breaking your whole exercise is moot.
and crankshafts break/crack/develop stress fractures,etc the least on inline 6 designs with 7 main bearings supporting them.
That is all I am saying. So faulting having 7 main bearings on an inline 6 engine makes you look irrational.
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  #39  
Old 06-21-2011, 08:31 PM
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I'm not faulting a 7 main straight 6 just saying there are consequences of the design (ie, it has more crank torsional displacement than a v8 or v6 etc.)What I was saying is that the crank could elastically twist in torsion and put the cylinders out of time in relation to one another. This crank has the same main bearing diameter as the m103 but way more loading (both reciprocating mass and compression). In any case agree to disagree.

Check this out if you like.(good bed time reading)

http://bc.biblos.pk.edu.pl/bc/resources/CT/CzasopismoTechniczne_8M_2008/MitianiecW/TorsionalVibration/pdf/MitianiecW_TorsionalVibration.pdf
Sort of interesting that a straight 6 has the first torsional mode well within the rev range (and quite a large magnitude compared to actual engine torque) at that the conclusion is that the damping is mainly acheived by cylinder wall friction. Of course our motors have a harmonic balancer so point is probably mute anyway. Sorry about the tangent. Doesn't have much to do with my original theory.
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  #40  
Old 06-21-2011, 09:13 PM
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I don't follow torsional elasticity within the application. I agree that in general the longer the crank the easier it is to twist the ends. Within the application, as #1 starts combustion, #5 is compressing but #4 is in its power stroke. So #4 buffers #1 from the resistance of #5. #6 is going through an intake cycle. It's counterbalanced mass contributes more torsional load than it's operation. I'd argue that an inline 4 has greater instantaneous torsional loading than an inline 6.

The next issue is being out of time or phase - relative to what? The cam and IP are driven off the front of the engine so #6 is most likely to be out of phase because of torsional elasticity. Why is #1 the more likely victim?

Mind you, this is an intuitive rather than technical rebuttal. I'm often guilty of not believing things simply because I don't understand them

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  #41  
Old 06-21-2011, 09:33 PM
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Sixto,
I think the reason a straight 6 has that issue as opposed to a 4 is that the natural mode is within the rev range of the motor where in a 4 cylinder it is probably much higher. I'm not sure as it depends on allot of variables. This article explains the modes for a v8 and gives some good background info but I concede torsional harmonics get over my head pretty quick, not my area of specialty.
http://racingarticles.com/files/general-damper-article-2.pdf

Yeah, I got nothing other than my original theory to explain #1.

Didn't mean to get on the crankshaft torsion tangent.
Thanks,
Ashley
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  #42  
Old 06-21-2011, 09:35 PM
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Wow! I actually understood most of the points in this thread... I feel smarter already!
No really... it's a hell of a mental/visualization ride!
I'm following Sixto's logic though...
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  #43  
Old 06-22-2011, 03:50 AM
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Ocam's Razor

1.Over-Bored Block with Cylinders Too close to each other
(Compared to O.E. design) and all that implies... Heat,Etc.
2.Weak Rods
3.MAYBE,a problem with Head design or Alloy composition.

Crux is Mercedes ability to NOT EVER tell us what Really was wrong.

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