Is there a way of knowing the effective results of a lighter bob weight, Example// A engine with a 1800g bob weight turns 7500 vs the same engine with a 1600g bob weight turns 8500/ before failure

I was leaning toward crank failure, It seems they measure crank rpm by piston speed in the calcs, but no mention of bob weights, I was curious of how they do this

Post deleted by polyspheric

Given the engine with the lighter bob weight has proper balancing for the weight change, is there a way of measuring the lighter rotating assembly and express it in max rpm change?

All of the scientific stuffs sounds interesting but what about the HUGE bob weights of the hemis screaming past 8000 rpm`s back in the day.

Thankx Mike, that makes perfect sense but this doesn't, small block chevy 434, 650 hp with a cast steel crank and a bad rod ratio, alu rods with light pistons, crank cut down to 40 lbs, this motor sees 8k for the last 4 years, I have drove this rail and seen the trap rpm so I know he is not full of it, and the crazy part is it was a used crank when he started, This goes against everything I have read about crank shafts, He has also built several engines for other racers the same way and they have not failed , Is this just dumb luck or is there more to bob weight than meets the eye? I read the cast stroker post the other day and have seen all the failures and opinions from the guys on here and tend to agree with the reasoning that cast cranks don't belong in race engines, But it seems to work in chevies?

Wow, I just spent a couple of hours reading related material to try and answer my question, It still remains basically unanswered, but now I know why, deep subject Thankx to all who replied, you all had a valid piece of it I gota go cause my head hurts

My $hit weights a tonne . There is a price and physical reality for horsepower
990 pins need not enter here.

572" Hemi has a 2389 Bob weight..it will see 7000-7200 rpms..

My 557" wedge has a Bob weight of right under 2300 grams..I turned it 8K once or twice...LOL

have you looked at Wallace racing site? They have a Piston Inertia calculator, and a piston speed calc also..
http://www.wallaceracing.com/Calculators.htm

Remember freewheeling (no load) inertia of the crank is one thing, acceleration under load is the sum total of ALL the rotating masses (trans/shafts, R&P, axles) and bearing and of course the frictional losses (to the road and the rings/bearings).