In the old days, crankshafts got surprisingly little consideration in a typical high-performance engine buildup. Typically, the same crank that came out of the core motor was reused, with crank prep generally limited to a polish or grind job to clean any rash off the journals. For a really trick engine, sometimes a factory-forged crank would be hunted down to fill the bottom end, if the factory offered such a piece for the engine type. Sure, billet race cranks, welded strokers, and the like have been around for decades, but for average street hacks, this stuff was as exotic as aluminum heads once were. The aftermarket has changed with time, and exotica like custom cranks and aluminum heads are no longer out of reach of the power-hungry masses, and that's a good thing, because the crankshaft is the backbone of an engine, and these days we aren't building spineless jellyfish.
Crank choices today run the gamut from the traditional run-what-ya-got approach to starting fresh with a custom billet-steel aftermarket crank. The decision comes down to matching the needs of the application to the available choices and the budget of the project. With the horsepower and torque so easily made with today's cylinder-head and cam technology, it pays to match the output with sufficient bottom end beef. Here's a guide to selecting the right crankshaft for the job.
This partially machined blank at Scat crankshafts is a non-twist forging, which can be machined to accommodate a range of strokes. Aftermarket forged cranks are typically made from high-quality grades of steel, and also gain strength by the design of the tooling to accommodate a non-twist manufacturing process. Such tooling is more practical for the comparatively smaller production runs of the performance aftermarket, as opposed to the mammoth production runs for the OE market.
Cast vs. Forged
A crankshaft is a pretty substantial chunk of metal arranged in a highly contorted shape. There are a couple of different ways to arrive at the basic shape, and this forms the basis of whether the crank is a forged or cast piece. In casting, a mold is made and molten crank material, usually cast iron, is simply poured in to create the raw casting. Casting is cheap, the tooling is long lasting, and the raw casting springs from the mold very close to the required final shape, minimizing the final machining requirements. All of these attributes are endearing enough to make cast cranks the overwhelming favorite for OEM and mild performance applications.
In creating a forged crank, an entirely different process of metal forming is used, aptly referred to as the forging process. In forging, a hot chunk of rolled steel is placed between heavy dies having the pattern of a crankshaft. Under extreme pressure supplied by a forging press, the metal is squeezed into the crank's basic shape. The simplest crank forging dies are arranged in a single plane, which produces a crank forging that has all the crankpins in one plane. To index the crank throws at 90 degrees, the raw forging is twisted to offset the journals in two planes to create the final raw crank blank.
An improved forging process involves forging the crank in two planes, so that all the journals are pressed into their final configuration, eliminating the need to twist the crank to index the journals. The result is fewer internal stresses in the forging, as well as an improved grain flow in the metal. Cranks made with this type of tooling are referred to as non-twist forgings. Tooling for a non-twist forging is considerably more complex and less durable than that for a simple flat forging, and there is typically more excess material to be machined from such a blank to create a finished crankshaft. Manufacturers producing crank forgings in huge volumes naturally gravitated to the lower cost and higher tooling life of a flat forging. In the aftermarket, with smaller production runs and an emphasis on durability for high-end cranks, non-twist forgings are available for many popular engines.