Cast iron is TOUGH.

 

technically when using correct engineering definition of toughness, it's equal to the area under a stress-strain curve and cast iron is less "tough" than carbon steels... cast iron is more capable under compression, but very brittle under tension and has a lower ultimate tensile strength (around 200 MPa), it is very ductile, vs carbon steels which are better under tension, but are not very ductile, but have higher UTS (around 450 MPa for ASTM A-36), but take less strain (strain being the % elongation or ductility of a material), so the areas under the curves are about equivalent (cast having slightly less area under the stress strain curve)... (think of it as long thin rectangle (17x1) v a fatter square (4x4), roughly same areas, different dimensions. toughness in that sense is its ability to resist fracture under stress...

in other words, hit it hard enough, it doesn't crack v hit it hard and it cracks, but very rarely does your manifold take a hit that would fracture steel or cast iron or even martensitic steel (which is probably the most brittle type, and would be a pre heat treated tool steel)

the fact that cast iron is good under compression is why it's used in engine blocks, it takes a lot to deform a cast iron block even in high temperature and compression, the problem with them is the thin parts of the cylinder walls where there is potential tension (normally the end cylinders), this area gets too thin and the block cracks.

i really would assume they use cast iron due to the increase in exhaust temperatures, mating a cast iron block to a steel exhaust manifold would result in a cracked exhaust manifold because the block expands more than the manifold...

based on a roughly 10.8 or so coefficient of thermal expansion (alpha) for carbon steels, and a 12 or so for cast irons, the formula for thermal expansion being alpha (delta T), so higher the temp difference, the more it expands.. this normally results in the manifold cracking or tearing at the bolt holes because it expands less than the block

typically when you firmly mount metals together using bolts or welding, you want them to have similar material properties so as to avoid failures of that nature

and that gentleman... is the first time i've used my degree since graduating college

(editting for spelling)

 

well....i only understood about half...but thank you... that was extremly helpfull.

 

I think maybe by steel you were refering to tubular manifolds? Cast manifolds are made of iron and run very abruptly into the turbo's exhaust housing. This creates for a low rpm spool, but increasing backpressure as rpm increases. As rpm increases, the amount of exhaust exiting the engine increases. If the manifold can't keep up with this demand, backpressure builds within the manifold. By a high rpm (say 5k+), a lot of backpressure has been created within a log style manifold, which results in loses of power. The greater the backpressure, the more power lost. Enter tubular manifolds. These are just pipes welded together, usually made of stainless steel. They are designed by varying pipe (or tube) diameter and length to make power (flow) within a specific rpm range (high rpms). All race applications with use a tubular style manifold.