I was wonder which one tends to crack faster stainless steel or cast iron?

 

cast iron will crack. Stainless steel will not.

 

they can both crack if not properly supported and braced. if a stainless manifold cracks its usually at the welds due to a contamination of the weld and lack of penetration in the weld. stainless is a trick to weld. i got some pics of a Fmax stainless manifold that looks really nice but is full of crappy welds.