Originally Posted by 67-4-fun
are the stock 400 sbc crankshafts a full cast or cast/steel? I think they are full cast but i wanted to ask just to be sure. also what are there rpm limits?
Calling this steel is something of a stretch, it is ASTM A536 spec nodular iron.
That said they will take some pretty good abuse. This spec covers stuff with tensile strength limits for about 60 to 100,000 pounds/in^2 depending on the spec. There are those who think the 9000 cast steel shaft is 80-60-06 grade, that I can't confirm.The 80 is the minimum tensile tensile strength in the grade, that's not to say the alloy can't exceed that Not up to the typical GM 1053 forging where the material is rated at 110,000. The middle number is the minimum yield strength which would be where it deforms. The 06 spot is the elongation, small numbers here mean a brittle material , big numbers a less brittle material. What this tells you is that the casting will stretch very little before it hits its ultimate strength and breaks. Where a large number like 22% in the case of the 1053 forging tells you it will stretch quite a ways before it lets go. Years ago Pontiac and Buick actually used this nodular cast stuff quite successfully for connecting rods, they called it Arma Steel, but it was nodular iron just the same.
All of these numbers are theory of course, the design, manufacturing process with its variability, and environmental exposure one to the next can greatly influence just how deep into these theoretical numbers you can push any particular part.
A good balance job and a good damper does a lot to stretch the load limits with a cast crank. Typically about 500 horses and 6000 RPM is about the upper limit, you can trade horses for RPM, say 400 at 7000 would be a reasonable max. The other element is gear changing and wheel spinning starts. With a stick gear box the shock loads on the crank are pretty high so the power needs to back off 50 or the revs 500.