Part 8 - Rotating Assembly Balancing - Honing Harmonic Balancer Bob Weight Math and Setup -In Depth Performance Engine Build 620+ Horse Power Street/Strip Pump Gas 454 Big Block Chevy
you did a good job getting the weight down on those stock rods but they're still kinda heavy compared to scat i beams. doing the work yourself helps justify buying those expensive wrist pins.
I am a curious sort of person. I wanted to derive for myself where the rule of thumb "100% of the rotating plus 50% of the reciprocating" came from for the bob weights. I set up an incredibly detailed spreadsheet with all of the rotating assembly components' masses, positions, and velocities during a revolution. What I discovered in the end, was that the 100/50 rule of thumb only truly applied when the center of mass of each connecting rod was exactly one-third of the distance from the big end to the small end. Yet, I have never heard any engine builder mention this. Is it possible the Engineers at the factory knew this, and roughly designed the engine with this in mind, and it somehow never translated down to the mechanics/aftermarket?
If I'm currently Selecting Crank, Connecting Rod, Pistons and considering a Super Light Manley or Lanati Crank, to keep rotating Assembly as Light as Possibly, Seems to be Smart to Calculate Rods n Piston weight (As You have shown) to Match Crank Bob Weights as Closley as Possible, If Not, Might make more sence to buy Normal Weight Crank rather that adding Mallory ? Thoughts
Also, is this considered external balance? Am I wrong that internal balance would NOT have weights on the flex plate or the harmonic balancer, that these would be "neutral balance items"?
why divide by 2 to get half bob weight only to multiply by 2 again. total bob weight will be the reciprocating weight plus rotating weight always right? 1014.6 + 1262.2 = 2276.8. why not just stop at that step?
Newbie to this so....possible dumb question. What would be difference with new parts (scat forged crank and h-beam rods) instead of used? Same lightening, resizing of rods or just whats taken in the balancing process? Would you still do all the work on a new scat crank, or again, just whats needed for balancing? BTW...small block stroker motor, street strip, looking for approx 520-550hp. any help would be greatly appreciated. Am enjoying the videos, learning alot. Thanks.
Unless it was specially ordered, the balance will be farther off on a new crank. The crank can be manufactured to a "close" balance, but calculating the bobweight and correcting the column is the only way to finish balance the crankshaft. Even rod length changes the balance. Scat sells the 383 crank in a 6" rod or a 5.7" rod version (different counterweights). Your H-beam rods are typically within 2 grams/set. SBC factory rods from engine/engine can be more than 30-35 grams.
