They say that the heart of the engine is the camshaft, since it is one of the key components that dictates the engine's power level, power band, idle quality, and other characteristics. If the cam is the heart, then the crankshaft is the spine. The crankshaft also dictates power and powerband, but in a much more ambivalent way (through its stroke which, along with the bore size, dictates the engine's cubic-inch displacement). The crank is what transfers the up and down reciprocating movement of the piston and rod into the rotating motion required to drive the transmission. It carries the weight of all eight rods and pistons, and must deal with the shock loads of the combustion process. A stock crank does this fine...in a stock engine. But when power levels start to climb, that stock crank will eventually give under the tremendous loads imposed upon it.