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So we clearly understand why the cats fail, too close to the manifolds themselves causing them to heat up way to fast and burn up, but why do the manifolds crack so quickly?
 

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I think you've got your cause-effect backward. The manifolds crack because of the heat-cool cycles. The crack makes the engine think it's running lean, and so the computer richens the mixture as much as possible. That unburned fuel destroys the cats.
 

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Already answered in your other post by LubeCakes: Cast steel can only take so many hot cold hot cold cycles before it fails.....And they can go from below 0 to hundreds of degrees in a minute.
They will fail.

Most guys that claim they have 150k+ on their manifolds with no issues have put those miles on in a very short time (in, say, 4 years) so that means they have a lot of extended driving time. It's not the heat that does it, it's the constant heat up, cool down, heat up, cool down cycles that cause stress on the metal. The metal expands every time it's heated and then contracts every time it cools. Eventually it's going to stress enough to crack.
 
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