[DavidNJ]

I sorry, but you're wrong. The mass on the crank is at most at radius of 3-4". Most of it inside of 2". In fact most of it in the main journals, if they large and not gundrilled as on an FE. There is very little inertia in a crank. May 3-6 lb-ft^2 in the heaviest big block. That is maybe 3-5lbf-ft of torque under aggressive acceleration. Most of the rotational mass of the engine assembly is in the flywheel and clutch which is at much larger radius.

Second, iron and steel are approximately the same density. Less than a 10% difference at most. And that is usually made up by needing extra metal to make up from the lower yield strength. In a big block crank, a lot of the mass comes from the counterweights to balance heavy piston/rod assembly.

Now, you could order the crank with gundrilled rods and mains. You could use small rod journals...even small crank journals. Those large sizes were designed for a big safety factor in 1950s engines with low end cast materials. You could build an engine with 2.25" mains and 1.771" rods, gundrilled. You could use Ti rods and short compression height, small ring, small pin pistons that would need less mass on the crank to balance. That would save weight.

Note that the effect of the inertia is dependent on the rate of acceleration. The amount of HP that translates to is dependent on engine speed. So an F1 engine at 20k rpm or a ProStock or Nascar engine at 10k rpm have a bigger interest in low inertia. Also an engine with 800hp accelerating a 1500lb car (higher rate of acceleration) makes more difference than in a 300hp engine accelerating a 4000lb car. Also note, that braking occurs at much greater rate than acceleration, and blipping the throttle to match revs even greater. So the effect is noticable there.