It's just as easy to compress metal as it is to compress fluid. No can do, so, unless it's a porous casting,the weight of a cast and forged object with the same displacement should be the same. I'm as far of a scientist as you can get, but still I have a theory about cast and forged cranks differencies. Has anyone really weighed them? Because, my first tehory is that since the cast crank is weaker, they have to make the crankpin size heavier duty than in a forged crank, and since the counterweight sizes are the same, the crank needs external balancing. So, it's actually heavier, not lighter, than the forged crank (yes, haven't weighed for example a 383 forged and cast crank). The second theory is that because of the porousity, the counterweights of the cast crank are lighter...