Cast manifolds tend to be cast from well, cast iron.
Cast iron manifold
Pros:
-can be cast into many many forms
-take alot of heat
-can usually be fit into a compact area (ie 350Z)
-cheap to produce
Cons:
-cast iron radiates heat very fast, probably one of the poorest choices for a turbo manifold in terms of material but again it is easy to cast and cheap to use, hence the use of it
-Can crack over time
-Usually offer poor flow characteristics due to the casting process and compact and odd shapes they are cast in.
-generally lend to more back pressure and as a result higher EGTs…inefficient in terms of performance and should never be considered in an all out performance turbo application
Tubular manifolds:
When contructed from the proper materials and designed properly these offer the best potential for horsepower. Keep in mind that the material you construct these out of is crucial as this manifold is supporting the weight of a turbocharger and has to deal with engine movement in the engine bay and and exhaust hanging off of it acting like a lever arm.
Pros:
-Generally constructed from stainless steel, this material is a poor conducter of heat, important in that you want to keep as much heat inside the manifold to keep the exhaust hot and flowing.
-Can be designed to flow properly and merge thus offering less backpressure and ultimatly more useable horsepower.
-Stainless steel tends to be a much more forgiving material to the amount of heat soak, flex and general wear on joints etc when welded properly as compared to cast iron or mild steel (which should not be used)
Cons:
-Stainless steel has an expansion rate of about 50 times that greater than cast iron or mild steel
-If not braced properly taking into consideration where the weight is acting on the manifold and how it is going to move due to expansion, the manifold will crack with time
-Not the most compact designs ever… depending on how you look at this or the application that could be a good thing and it could be a bad thing
One thing to note about the use of stainless steel is that the thicker the material you use (sch40 or sch10 etc) the faster the heat will be transfered from it to the air. Thinner materials hold more heat (sounds illogical but its true) however need to be braced considerablly to deal with mechanical stresses.
Focusinprogress, if you are looking to make as much horsepower as humanly possible out of this engine or you are looking to run anything higher than 14lbs of boost then I would look into using a properly designed tubular manifold but if you are going to be running sub 1 bar, you could probably get away with a Cast Iron turbo manifold, you may want to spend the extra buck and send it for jet hot or swaintech coating.