Crankshaft Tech - Crank Call

Twist vs. Non-Twist Forging
Forged cranks are pressed into place on a die, but there are two different techniques used to accomplish this. The simplest method is to forge one of the crank throws at a time in a flat forging die. The crank is then twisted, and the die forges the next throw. Conversely, in a non-twist forging, all four throws are forged simultaneously, which requires a more complex die. Non-twist forgings are said to reduce internal crankshaft stresses during the manufacturing process, but not everyone's buying it. "If all variables are controlled properly during the forging process, there's little if any difference between twist and non-twist forgings," opines James Humphries of Lunati. "Most aftermarket cranks these days are non-twist forged anyways, so there's no sense in arguing either way. It's more of a marketing thing."

Heat Treating
In addition to materials, and casting or forging techniques, heat treating can greatly impact the strength of a crankshaft. Nitriding is the most prevalent method of heat treating used in aftermarket cranks, where ionized nitrogen is vacuum deposited onto the crank surface in an oven. By penetrating .010 to .012 inch into the metal surface and changing the microstructure of the steel, surface hardness is doubled from 30 to 60 on the Rockwell scale, and fatigue life is increased by 25 percent. The OEs usually favor induction hardening over nitriding, which results in deeper penetration into the metal surface (.050 to .060 inch). This process uses a magnetic field to heat the surface. "There are pros and cons of both methods, but nitriding is most common in the aftermarket," explains Humphries. "Induction hardening is more localized, whereas nitriding treats the entire crank at once. However, induction hardening penetrates more deeply, which enables turning down the journals once or twice during rebuilds before having to heat treat the crank again."

Knife-Edging
Does knife-edging a crank's counterweights really reduce windage and increase power? Not everyone thinks so. "Knife-edging was developed more for ease of balancing than power, and won't do much on a street motor," explains Callies' Dwayne Boes. "Like a snow plow, oil hits a knife edge and gets thrown all over the place when it should ideally land on the nose and flow off to the side. A bull-nose rounded leading edge is the most efficient, like the bow of a ship."

Overlap
Just as the term implies, journal overlap is simply how much of a crank's main and rod journal diameters overlap each other. As stroke is increased, moving the rod journals farther away from the main journals reduces overlap and compromises strength and durability. Likewise, smaller rod and main journals reduce bearing speed and friction, but also reduce overlap. "The reason why GM increased the size of the mains to 2.65 inches on a 400 SBC compared to 2.45 inches on a 350 was to maintain journal overlap with the longer 3.75-inch stroke," explains Judson Massingill of the School of Automotive Machinists.

Billet Or Forged?
Although we've clearly outlined the hierarchy of the various grades of castings and forgings, we haven't declared whether billet cranks or their forged counterparts offer the ultimate in strength. Quite frankly, we don't know the answer, and we won't even attempt to make an educated guess. There are compelling arguments for each from a host of credible sources, so we'll print their words, and let you decide.

Alan Davis Of Eagle:
"People think billet is stronger than a forging, but that's not true. Billet got that reputation from back when forged aftermarket cranks weren't readily available, and billet was the only way to go for a performance crank. With a forged crank, the forging process creates an interwoven grain structure. With a billet crank, the grain structure just runs parallel with the crank. Billet is a better option if you need a custom one-off crank since it doesn't require expensive tooling equipment. On the other hand, the 200-ton presses required for forgings cost at least six figures, so they're more suited for large production runs."

Tom Lieb Of Scat:
"A forging is not as strong as billet because the forging process stretches and shears the grain structure. A forging starts out as round bar of metal and gets twisted and turned to make the rod throws. What used to be centerline of the bar is now offset, and the grains get stretched, traumatized, and weakened, although some sections of it are substantially stronger than in a casting. With billet, there are no stress riser areas because the grain structure runs parallel to the length of the entire crank. Forgings are stronger than billet in bolts and axles because the metal isn't being stretched and sheared. There isn't a single Top Fuel, Funny Car, Nextel Cup, or F1 team that uses forged cranks, so you have to ask yourself why."

Dwayne Boes Of Callies
"If the exact same material is used for both, a forging is stronger than billet because the grain flow is upset and relocated. However, it's much easier to get special alloys in a billet material."

Judson Massingill Of The SAM:
"Up to 600 to 700 hp, forgings are every bit as good as billet cranks, given adequate journal overlap. However, when you start reducing the overlap with long strokes and small rod journals to reduce bearing speed, billet comes out on top. In our motors, billet lets us get away with less journal overlap."