What causes cracking in Exhaust manifolds?

[cheekychimp]

Heat cycling I know but what makes some exhausts and turbo housings do it when others don't?

Imperfections in the castings creating hotpots?
Hairline cracks that aren't repaired and expand?
Bad tuning?

If you have an expensive cast manifold you want to use but are worried about it cracking, what can you do? I have discussed the idea of extrude honing it to remove any imperfections in the casting and smooth out possible hotspots and ceramic coating the exterior. Expensive I know, but would it help prevent damage from heat cycling or is it just inevitable?

 

[AWDPower]

It's part design and part the composition of the cast iron in the manifold. The old 1G manifolds had problems with both parts. The 2G, and successive Evo manifolds got better and better in both aspects. Tuning can play a part, but I'm not sure to what degree. porting should help, as a smooth surface should reduce hot spots and leave less places for cracks to start. I'm not sure about coatings, I think they can help, but also could hurt. Heat wrap and sheilding hurts, but protects other components. I think it mostly comes down to the quality of the manifold or turbine housing.

 

[cheekychimp]

So what about these coatings for the inside of the exhaust runners essentially keeping the heat isolated from the manifold itself?

 

[Specter]

Don't forget that many people simply don't treat it right. Your not supposed to wash the engine when the engine is at operating temperature, from a drive for example. The metal is forced to contract faster than its normal contraction rate due to cooling from water. I think many people would wash the engine bay, when they are not supposed to, without thinking about it.

 

[curtis]

Paul you can cut the flange between ports to allow movement, just a bandsaw is all thats needed. Mainly I think its time more than anything, heat removes carbon overtime and heat cycles. Nature of the beast.