ENGINE BALANCING refers to the process of balancing the rotating assembly of an internal combustion engine. This consists of the crankshaft, connecting rods, and pistons. It is considered a mandatory upgrade for the tuning of naturally aspirated engines, as it decreases vibration, thus increasing efficiency, allowing the engine to rev higher, thus producing more power. To take advantage of the higher rev limit, the cams are changed so that power is made at higher RPMs.
Balancing pistons and connecting rods is simple. First, all of the pistons (or conn rods) are weighed individually, to determine which weighs the least. Each of its fellows will be lightened by the simple expedient of drilling away material on a drill press until they are all the same weight, or at least very close to it. Pistons are balanced as assemblies including the piston, wrist-pin, ring package and the retainers that secure the wrist-pin to the piston.
Crankshaft balancing is more complicated. The crank is loaded into a balancing machine similar to that used to balance tires. "Bob weights" are then added to simulate the weight of the pistons and conn rods, and then the whole assembly is spun up. The balancing machine has a gauge which tells how far it is out of balance. At this point there are two choices; Either drilling the crank to reduce weight in some areas, or knife edging it, which is removing material from the leading edge of the lobes on the crank in such a way that the resistance of the crank is reduced, making it easier for the crank to move through the engine's oil.