The short version for ‘factor changes’ is that engine balancing on V engines is not linear in regards to the rpm.  Rotational (dynamic) forces change within the engine as the rpm goes up which in turn dictates a change in balancing parameters on the crankshaft itself when balancing for specific rpm ranges.   And then special conditions come into the fray such as compression ratios, boost pressures, rod lengths, etc. which in turn must be compensated for in an application specific balance job.  Although the percentage of reciprocating mass is the value actually being altered in these types of circumstances, the compensating value for these is typically within the oil number on the balance card.  And due to the proprietary nature of some of the formulas dealing with ‘overbalance’, don’t be surprised if a balance card lacking the finished bobweight value is handed back to the customer if a card is handed back at all.

Dynamic balancing is performed when a rotating mass (ie. crankshaft) can have the potential for a number of rotational imbalances in varying degrees at different planes throughout its length while in motion.  On the short end of the scale is a flywheel which will exhibit mostly static imbalance and very little in the way of dynamic balance characteristics.  A crankshaft is on the longer end of the scale and can exhibit both static and dynamic imbalances.  A static imbalance can typically be felt outside the confines of the engine while dynamic imbalance can feel to be vibration free but instead is being couterproductive in power output to the flywheel and being destructive to the engine internals.

Here’s a couple of pictures which may help to clarify the visual difference between dynamic and static imbalance.

And here’s the link to the balancing article which goes into much more detail.

http://www.eatonbalancing.com/blog/2007/11/21/engine-balancing-part-1/

Lorena, Texas (South of Waco)

       If you guys read what he said here and in the link, you'll know more about the subject than 98% of your bar buddies. You'll also never again question the value of balancing dynamically. The balanced motors last longer, make more power, get better mileage and are better dancers!

Frank/Rebop

Bristol, In ( by Elkhart) 

Thanks for the explanation and the picture, you are much more eloquent than am I, now when Mr. Burns reads this he will know why his "race truck" crank was balanced with a 52% reciprocating factor instead of the normal 50% and also why I like to balance the flywheel sperate from the crankshaft, except in the case of an externally balanced engine.

Just to add; the internal/external point on Teds Blog brings to mind my dislike for the 454 scrub. I see a lot of these with broken crankshafts, which I have attributed to the external balance, but a bigger problem I think is that GM had an engine that was not built strong enough for the power that they want it to put out, compounded with the fact that so many motor homes are built on chassis that have this engine in them. Our shop is close to I-5 and I see a large number of burnt up and over worked 454's that give up out on this main north/south thoroughfare.

   God Bless.   Tim                           http://yblockguy.com/

350ci Y-Block FED "Elwood", 301ci Y-Block Unibody LSR "Jake", 312ci Y-Block '58 F-100, 338ci Y-Block powered Model A Tudor

tim@yblockguy.com  Visalia, California    Just west of the Sequoias

I have your rods and will ship the balanced set on Monday. Thanks for the extra, It is greatly appreciated.

   God Bless.   Tim                           http://yblockguy.com/

350ci Y-Block FED "Elwood", 301ci Y-Block Unibody LSR "Jake", 312ci Y-Block '58 F-100, 338ci Y-Block powered Model A Tudor

tim@yblockguy.com  Visalia, California    Just west of the Sequoias