Apparently T304 stainless steel doesn't rust but becomes brittle at very high temperatures. This is why stainless steel manifolds on mk2 gti's are not recommended because they tend to crack.
Mild steel is what comes as standard. They rust and they don't look as nice.
I have a stainless steel system with twin 2.5" DTM's....it sounds incredible, improved my fuel economy and I'll never have to buy another for my current car!
