"The V8 can be completely balanced, requiring crankshaft counterweights.
Source of this information?"
Bosch Automotive Handbook has this information. My edition was from back around 1980, though new editions are currently available.
Also from
http://craig.backfire.ca/pages/autos/v8-engines :
"The crossplane V8 would have first-order imbalance because of how the pistons are moved by the crankshaft, but the use of full-weight counterweights prevents this."
"Full-weight counterweights on the crankshaft are shaped and weighted in such a way that they counter the forces from the pistons and connecting rods on the nearest pin at the same time. Regular counterweights only counter the forces from the connecting rods and the crank pin. Only an engine with a 90° v-angle can utilize full-weight counterweights."
"Full-weight counterweights are placed on both ends of the crankshaft to oppose the rocking motion that would come from the forward most pistons moving in the opposite direction to the rearmost pistons. The middle two crank pins do not need full-weight counterweights because the piston motion forces are somewhat cancelled by the pistons on the other side of the engine. These counterweights are still very large compared to regular counterweights, however."
"The crossplane V8 has second-order balance, and that fact is the main reason for its existence. The odd-looking crankshaft moves the pistons in such a way that the net velocity of all of the pistons is always equal to zero, meaning that no second-order vibrations will be present."
An engine is generally considered balanced if the net primary and secondary forces and moments are zero, four conditions.
In actual applications, crankshaft flex and uneven firing orders on a cylinder bank may result in some vibration.
