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#31
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Quote:
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1980 240d , chain elongation, cam marks reference: http://www.peachparts.com/shopforum/diesel-discussion/10414-help-i-need-check-stretch.html http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html evap fin cleaning: http://www.peachparts.com/shopforum/diesel-discussion/156207-photo-step-step-post-showing-w123-evaporator-removal-1983-240d-1982-300td.html?highlight=evaporator A/C thread http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html |
#32
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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.
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1990 300e 230k (old reliable) sold 1987 300D 230K sold 1987 300D 232K 5 speed 1998 E300 140K Last edited by ajnorris; 06-20-2011 at 08:41 PM. |
#33
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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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1980 240d , chain elongation, cam marks reference: http://www.peachparts.com/shopforum/showthread.php?threadid=10414 http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html evap fin cleaning: http://www.peachparts.com/shopforum/diesel-discussion/156207-photo-step-step-post-showing-w123-evaporator-removal-1983-240d-1982-300td.html?highlight=evaporator A/C thread http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html |
#34
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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. |
#35
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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:
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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1990 300e 230k (old reliable) sold 1987 300D 230K sold 1987 300D 232K 5 speed 1998 E300 140K |
#36
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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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1980 240d , chain elongation, cam marks reference: http://www.peachparts.com/shopforum/showthread.php?threadid=10414 http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html evap fin cleaning: http://www.peachparts.com/shopforum/diesel-discussion/156207-photo-step-step-post-showing-w123-evaporator-removal-1983-240d-1982-300td.html?highlight=evaporator A/C thread http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html |
#37
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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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1990 300e 230k (old reliable) sold 1987 300D 230K sold 1987 300D 232K 5 speed 1998 E300 140K |
#38
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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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1980 240d , chain elongation, cam marks reference: http://www.peachparts.com/shopforum/showthread.php?threadid=10414 http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html evap fin cleaning: http://www.peachparts.com/shopforum/diesel-discussion/156207-photo-step-step-post-showing-w123-evaporator-removal-1983-240d-1982-300td.html?highlight=evaporator A/C thread http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html |
#39
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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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1990 300e 230k (old reliable) sold 1987 300D 230K sold 1987 300D 232K 5 speed 1998 E300 140K |
#40
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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 Sixto 87 300D |
#41
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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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1990 300e 230k (old reliable) sold 1987 300D 230K sold 1987 300D 232K 5 speed 1998 E300 140K |
#42
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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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85 190E 2.3(SOLD) 86 230E (-->300D) sold 87 300D (-->300TD) sold 68 250S w/ a 615 and manual tranny (RIP) 87 300TD (SOLD) 95 S280 "The KRAKEN" (Turbo 2.9 602 transplant) traded 86 190E 2.3... current project |
#43
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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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'84 300SD sold 124.128 |
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