Log Style vs Equal Lenth Turbo Mani
Turbines cannot create a vacuum on the exhaust system. The only way for that to happen would be if the turbine was spinning faster than the gases were coming into the housing. The only way the turbine would spin faster is if the compressor wheel was doing the driving of the unit by having the intaken air push the compressor wheel fast enough to make the turbine on the other end of the shaft spin faster. We know this is not the case, however, as the turbine does the driving, and the compressor provides the resistance by forcing air under pressure. If the siuation was reversed, the turbo would produce no psi whatsoever. John
Stainless steel mani's crack because SS has an expansion rate 50% greater than mild steel. Most often, though, SS mani's are bolted to cast iron and aluminum parts, secured with steel fasteners that will all expand at the same temperature to varying sizes based upon the therodynamics of the material from which they are constructed. Only the best engineering can produce a SS exhaust manifold of any type that will survive the constant expansion and contraction. Don't get me wrong, it is an exceptional material for longevity, but it also requires more technical expertise to use successfully for a quality part that must suffer like an exhaust mani. And as for guiding gases to maximum velocity inside a twisty pipe, not gonna happen. Consider a winding road: can you top your car out on one? Didn't think so. You need wide open space for that. Cosworth Formula 1 mani's are designed to correct for angle, pipe diameter, and equalization of length in each primary pipe. This is how the turbo gets its exhaust gases efficiently, and how a mani lives a longer life. Believe me, there are PLENTY of space constraints placed upon every single aspect of of a Formula 1 car, most especially the engine components. Ehaust gas temperature is rarely the worst culprit in manifold failure, there are scores of other contributors to that: torque loads, vibration/frequency, time elements, rapid cooling, and so on. Johngeorge wrote:look at the cosworth formula 1 turbo manifolds...note the design. they have unrestricted space constrants and the manis are long and twisty guiding the exhaust gases to maximum velocity. lenght is very much a factor as is diameter and material. these are all contributing factors to optimum mani design. often certain mani designs crack even when made of high quality stainless steel. heat is not the culprit but some thing called sonic boom. it causes violent movement of the exhaust gases which cause the mani to crack.
i will have to disagree, heat is the main reasons that manifols crack. look at a mani on a dyno...it glows red which means expansion and then cools. contraction. the blades dont have to spin faster to cuz a vacum effect, however small, by the pushing of the exhaust blades on the exiting gases. and as for the manifold pipe diamter, equalization and pulse frequency in a cosworth tuners apply the same principle in street el manifolds. and as for topping out my car on a twisty road...depends on the road most likely yes.
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king_johnthegreat
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If the mani's cracked while glowing red, then yes heat would be the cause (Along with pressure, and other factors I have already listed), but they do not. They crack while cooling, making conductive properties the culprit. The material possesses too much elasticity at high temperatures to begin cracking, instead it would spawl. Spawling is a term applied to materials that heat up too rapidly, or unevenly due to imperfections in the material. Mani's don't usually spawl, either, though. Typically manifolds are constructed from high ductile iron used for casting, or medium to high carbon mild steel and T304 Stainless tubing that has been mandrel bent and MIG or TIG welded. These materials are usually cultured for well homogenized molecules and fewer imperfections. Other factors that are often times not at all addressed are things like ceramic coatings and metal stress. Ceramic coating may be good for other underhood components, but can wreak havoc on the part itself. Weaker designs and materials may not be able to handle the added heat ceramic coating places on the part. Compounding this issue are things like metal stress. Investment casting and sand casting usually do not suffer from metal stress the way welded flanges and tubes do, but really all means of creating a manifold will possess some inherent imperfection leaving a weakness behind. Vibratory relaxing (Metalax) and magnafluxing the manifold may rid the piece of any weakness, and betray any weakness trying to stay hidden. These things are designed for heat, though, and usually never even reach the metal's specific smelting (Not a typo) point. Believe me, I have been welding for about eight years; engineering for two, and have studied PLENTY of advanced thermodynamics. Beyond that, my family has more than its fair share of proffesional race car drivers (some of whom you could catch on ESPN, TNN, and SPEED). You don't do this kind of stuff without knowledge. And no, still no vacuum by the turbo. John
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TrunkMonkey
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now that that's out of the way, i'd like to readdress the "sonic boom" thingy.
a sonic boom in a exhaust manifold/header doesn't exist.
if the manifold is too long the gases accelerate to the speed of sound and create a sort of sonic boom within the manifold.
in the real world a sonic boom occurs when an object (most likely a plane, but some cars have done it too) exceeds the speed of sound (@750mph).
raise your hand if your exhaust travels at 750mph.
a sonic boom in a exhaust manifold/header doesn't exist.
if the manifold is too long the gases accelerate to the speed of sound and create a sort of sonic boom within the manifold.
in the real world a sonic boom occurs when an object (most likely a plane, but some cars have done it too) exceeds the speed of sound (@750mph).
raise your hand if your exhaust travels at 750mph.
the conductive properties dont cuz it to crack though...its the constant heating up and cooling. just becuase the mani conducts heat does that mean it would crack without heat? no of course not. the fact that the mani heats up and cools and that causes the crack is an indication that heat is a large factor.
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SSDwellah
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The reason mild-steel manifolds crack (generally at the welds) is that it will expand more than the ductile iron manifolds. The downside to the iron manifolds is that they will oxidize (rust) rapidly due to the high heat. If there was a good way to cast an equal length stainless steel (not mild-steel) manifold there would be less chance of cracking because it's all one piece.
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TrunkMonkey
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king_johnthegreat
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The importnat part of the statement that basically contradicts the idea that heat is the factor for failure is "heats up AND COOLS." They don't crack until they have cooled, and too quickly for that matter; making the dissapatory property the one at fault, not the opperating teperature. IF the manifold doesn't cool TOO QUICKLY, it is highly UNlikely it will crack. That is why certain applications demand a certain amout of material in density and thickness. If a wall is too thin it will cool too quickly, inside and out. A thicker wall retains higher core temperatures longer, causing the manifold to take longer cooling down. Stainless is the best of the three materials we have discussed for self insulation, and is inherently better for making a manifold that doesn't require such a thick wall, or for mani's that must suffer from higher heat, such as in a turbo app. Again, the greatest cause for breakage is stress fractures resulting from poor design, improper or insufficient amount of material for controlled dissapation and cooling, and inherent metal stresses. I have seen metal that was so hot, it was actually burning. I mean, it was emanating flames sustained by the gases released by the metal itself; no other source for ignition but self combustion. When it cooled, it did not crack. Explain how that might of happened, considering it was a lesser grade iron than typically used for manifolds. And, for the sake of being technical, EXACTLY where does the exhaust gas attain the speed of sound (i.e.: the combustion chamber, the exhaust port, the primary, the collector, the down pipe, etc.,)? John
ok lets get technical. what is heat? its the energy molecules or atoms that make up a given substance. when a mani heats up the molecules have so much energy they shuffle away from thier neighbouring atoms. as the metal cools irrespective of velocity at which it cools the molecules shuffle and this causes cracks in the manifold.self insulation? how can it insulate itself? what stainless steel is its a poor conductor of heat thats why its a good material for manifolds also becuase of this property, as you say, a thiner wall of steel can be used which saves weight. and for obvious reasons thats good. sonic boom? appliy thoery of energy and molecular movement, compound that with an incorrectly designed manifold and you have your answer.
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TrunkMonkey
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(demcj scratches his head and raises his hand)george wrote:sonic boom? appliy thoery of energy and molecular movement, compound that with an incorrectly designed manifold and you have your answer.
what's the theory of energy and molecular movement?
why would someone knowingly use an incorrectly designed manifold?
why are they making a third Friday movie?
why don't you accept the fact that you have no clue as to what you're talking about?
ok one more time for Demcbj, the longer the runners the greater the gas velocities which results in quicker spool up. now Demcbj there is an optimum lenght for any given application and turbo set up to get the maximum possible gas velocities and exhuast pulses. if the runners are too long then the gases accelerate too much...sonic boom which is harmful. ok?
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king_johnthegreat
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Yes, let's please get technical- "what is heat? its the energy molecules or atoms that make up a given substance." Energy has absolutely no mass, whatsoever. No molecule OR atom is WITHOUT mass. So right there, the statement is false. The atoms do not move away from each other when heat expansion occurs; but yes, the molecules do expand in almost every direction. They rub each other, they swell and create pressure on oneanother, and whatnot. stress fractures and cracking begins when cooling has nearly completed (Which, by the way, I would be interested to know how anything cools with "Velocity") and the bond between some molecules is stronger than others, and begins breaking the weaker bonds. Stainless "Self insulates" by its very nature, being as such it is a poorer conductor of heat than cast iron and mild steel. In grade school we learned that if it is a poor conductor, it is a good insulator. If SS is a poorer conductor of its own heat than the other two, then it retains its heat longer than the other two, making it a better insulator than the other two. The density of SS is directly related to its specific gravity (Or weight, as it is known for by all you non physics majors). The molecular density of SS is greater than mild steel, in large part due to its nickel content (And the reason it does not rust as readily). Nickel is also not the greatest conductor of all metals, so its contibutions to our application are two fold. You would have to use considerably thinner wall SS tubing in order to get a lighter mani, perhaps thinner than is safe to use for a turbo app., but fine for a N/A motor. I was asking seriously, also, where the sonic boom is becoming present in the system. I would really like to know at what point this occurs, where the gas accelerates to the speed of sound. John
Atoms DO move away from each other when they are heated...the heat breaks the polar attraction between them. above you stated " if the manifold doesnt cool too quickly" velocity if speed and a given object can cool at a given spee or rate. heat is energy give an atom enough heat and if will break the molecular attraction at srt. becuase of the heat and thus the movement of the steel atoms the atoms DO shuffle and when they cool down this causes cracks. as i had mentioned before. self insulates is not correct, the term is poor conductor as you mentioned. also poor conductor doesnt mean it doesnt conduct heat throughout the whole manifold but rather it conducts heat at a slower pace. we agree that the cracks occur when the cooling has taken place and thats becuase, as i have mentioned before, heat allows the molecules to move and shuffle. as for the SB see above John i have explained it above.
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TrunkMonkey
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you know, i started typing a nice long explanation as to how an equal length manifold allows exhaust gases to flow smoother than a log manifold. i was going to state how an equal length manifold allows exhaust gas pulses to arrive at the collector/turbo at regular intervals instead of colliding with each other like in a log manifold. i was also going to state how a log manifold can suffer more heat abuse and expansion than an equal length manifold because of this. i was going to state how it's impossible for exhaust gases to reach the speed of sound because there are too many restrictions in the system and an engine cannot pump out more air than what it takes in, but then i thought...
what's the point?
what's the point?
I guess my question is, if heat cracks manifolds, why do equal length manifolds that have not been braced properly have a greater tendency to crack, over those which have been braced?Anyway, if you are going to get an Equal length manifold, and it's thin walled, i would suggest you (the starter of this thread) to brace it to the block, as well as run a piece of flex pipe in your downpipe.
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