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Same manifold on two different setups can crack or not crack due to stress/support.
temperature causes metal to expand, and when something tries to expand and cannot, a force is developed internally.
Add to that a shear force from weight/gravity of whatever is hanging on it.
Add to that the force of bending, to the uniform axial distribution of normal forces,
Many forces at work. All accountable for by engineers who design these materials and know how much stress the metal can take before it yields.
So whats the problem then?
The setup itself, the weight and location of what is hanging off the manifold, is different on every car. Sometimes a downpipe has no support and the whole weight hangs on the manifold. Sometimes the engine twists and this causes stress to form where the manifold/turbo/downpipe resists the motion. bouncing/jostling in unrestrained setups also causes bending/torsion/normal forces to form instantly then disappear.
The solution is a realization that the cracking of a manifold goes beyond it's construction, and into/complies with the rest of the setup which hangs on it. You need to account for engine twisting, downpipe bouncing/jostling, the weight of everything, the temperature of everything involved, as it all comes together to result with stress.
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