Stock displacement is so '80s. The days of miniscule and primitive displacement gains through offset crank grinding are a more distant memory than Fritz Mondale's presidential campaign. Due to the rise of affordable aftermarket stroker crankshafts in the last decade, cubic inches are cheaper than ever. Concurrently, cylinder head technology has been forced to keep pace with the demands of incessantly swelling displacement, and horsepower has proliferated to obscene proportions. As our hobby collectively frolics in a golden age of horsepower, no single engine component, save for the cylinder head, has advanced the cause as much as the modern crankshaft. So unless your sanctioning body forbids it, if you're building a motor, you're probably in the market for a stroker crank.
Nonetheless, the choices are many, and not all cranks are created equal. Should you settle for a cast-steel piece, or step up to a forging? What's the difference among 5140, 4130, and 4340 steel? Do you really need to step up to forged in all instances? Does billet live up to its hallowed reputation? How do you distinguish between marketing hype and actual product quality? And most importantly, what's the right crankshaft for your application? Fortunately, the misinformation stops here. We've networked with the top crankshaft manufacturers in the country to definitively answer all the aforementioned questions, including insight into metallurgy and various manufacturing techniques. Don't panic if you're into less mainstream makes, because we have Buicks, Olds, and Pontiacs covered as well. While some of the information won't jive with common public perception, the facts aren't always easy to digest. We have the truth, but can you handle it?
Cast vs. Forged vs. Billet
Manufacturing techniques play a substantial role in the ultimate strength of a crankshaft. Casting and forging are the two most common manufacturing methods, and each has benefits and drawbacks. Cast cranks start life as liquid iron or steel, and are poured into a mold. This allows the raw casting to closely resemble its final shape, which reduces the amount of final machining. Combined with the fact that the equipment necessary to produce castings is relatively inexpensive, it's obvious why cast cranks are the predominant choice of the OEs. Aftermarket cast cranks offer significant improvements in strength, and can be had for as little as $200.
Weighing in at a scant 66 lbs, Eagle's lightweight 4340 forged cranks for big-block Chevys
In contrast, the forging process requires heavy duty presses and more extensive final machining operations. Forging involves heating up a cylindrical slug of metal to a molten state, then pounding it into shape with presses and dies. It is this compressing action that creates an inherently stronger end product over a casting. "In a casting, the grain structure looks like beach sand," explains Tom Lieb of Scat. "In a forging, the force of the press compresses the grain together so it becomes one uniform grain flow. As the space between the molecules is compressed, each molecule is forced to 'hold hands' with the next molecule." Compared to a cast crank, the drawback of a forging is cost. The heavy duty hydraulic presses used in the forging process are extremely expensive, which leads to a costlier product. Expect prices to start at $500 for more popular engine makes.
Think of billet cranks as an offshoot of forged cranks. Like a forging, a billet crank starts out as a large cylindrical ingot of steel. However, while a forged crank is compressed during the forging process, the steel ingot used in a billet crank is already forged, albeit not quite as compressed as in a forged crank. The key difference between the two is how the ingots are shaped into cranks. "The metal bar used to make a forged 4.000-inch SBC crank measures about 4.75-inches in diameter and the crank's total width ends up being 6.75 inches when the forging process is complete," Lieb says. "The metal bar used in a billet crank of the same stroke is much larger, at roughly 8 inches, weighing 350 lbs compared to 150 lbs in a forged crank. Instead of twisting and pounding the metal in different directions as with a forging, a billet crank is made by whittling away the metal so the grain structure runs parallel throughout the entire length of the crank." Due to the increase in materials and labor over a forged crank, billet cranks are the most expensive of them all. Custom one-offs carry price tags in the neighborhood of $3,000. As for whether a billet crank is stronger than a forging, because there is no consensus in the industry, the various manufacturers will duke it out later in the story and we'll let you make the call.
Before delving into the specifics of metallurgy, there are strength characteristics universal to all castings and forgings worth nothing. In a lab, metal is tested for strength by pulling a one-inch round bar apart until it breaks. Tensile strength relates to the amount of force required to start stretching the bar. Yield strength describes the force needed to continue to pull the bar apart. The difference between tensile and yield strength between castings and forgings is significant. "With a casting, you only have to reduce the cross section of the bar by six percent before it breaks," Lieb explains. "With a forging, the cross section can be reduced by 20 percent before the bar breaks."
Crank overlap is simply the portions of the rod and main journals that overlap each other.
As an alloy consisting primarily of iron, the small quantities of metal added to that iron are what determines the differences in strength among various grades of steel. A set of standards established by the American Society for Metals (ASM) determines the content of metal grades in addition to their nomenclature. "For entry-level cast cranks, increasing the carbon content in proportion to iron improves strength," says Alan Davis of Eagle Specialty Products. The most basic cranks are cast iron, which typically have a tensile strength of about 70,000 to 80,000 psi. Slightly increasing the carbon content of iron produces nodular iron, resulting in a tensile strength of roughly 95,000 psi. Both materials are used extensively by the OEs, but won't quite cut it for more serious aftermarket stroker crank applications. Commonly used in entry-level aftermarket crankshafts, cast steel has greater carbon content than nodular iron, and a tensile strength of about 105,000 psi. "In a typical small-block, a cast-steel crank can easily handle 500 hp. Although we've seen them pushed to much high power levels, we generally recommend a forged crank at anything above that power level."
Moving up the totem pole, factory forged cranks are made from steel alloys such as 1010, 1045, and 1053. While their tensile strengths are similar to that of a cast-steel crank, their elongation rating is more than three times greater. This translates to a far less brittle material. Nonetheless, they're a far cry from the ultimate durability of an aftermarket steel crank. "Factory forged steel cranks have high carbon content, but they lack the chrome and nickel content of the premium alloys used in aftermarket cranks," Scat's Lieb explains. "In these types of alloys, chrome and nickel are what make them stronger. There are other materials involved, but they are used to make sure everything mixes together properly and don't impact strength."
The most basic aftermarket-grade steel is 5140, which boasts a tensile strength of about 115,000 psi. This material used to be-and to some extent still is-an excellent choice for racers on a budget, but is less common than in years past due to the increasing affordability of premium alloy cranks. These include 4130 and 4340 forgings, which have tensile strength ratings of approximately 125,000 psi and 145,000 psi, respectively. Engine builders and crankshaft manufacturers universally accept 4340 as the ideal alloy for strength and durability. Because aftermarket 4340 cranks start between $500 and $600 for common engine platforms, the lesser grades of steel are dwindling in popularity. "We have plenty of customers pushing 1,500 hp through a 4340 forged steel crank," Eagle's Davis says.