Balancing an engine

Engine balancing is about reducing harmonics to as low as possible. There are no uncontrolled explosions unless the engine is pinging or detonating. It's a controlled burn with a comparatively slow pressure rise and conversion of that energy to mechanical force. Cranks twist and "un-twist" with every firing event, compression event, and change of direction of the piston and small end of rod. The force of a tiny amount of unbalance goes up exponentially with rpm. My math may be a litle off, but looking at formulas for centrifugal force.... A .1 gram out of balance of the big end of a rod on a 4" stroke small block, becomes close to 85 grams on a 2" radius at 5K rpm. now picture some rods being equal to the engineered tolerance, some being as much as 18 grams off... Then couple that with a crank whose casting left heavy spots... It ends up being a lot of stress the crank and block don't have to deal with. Like Chief says, any energy not being spent on the tires, is being absorbed by the other parts and wearing them faster. That's why even cast crank externally balanced factory displacement engines get internally balanced when I do them.