Engine builders are often conditioned into selecting crankshafts based solely on the expected horsepower output of the engine, or at least that often-optimistic number carries the most weight in the decision process. But savvy builders — whether assembling an honest street engine, rogue weekend warrior or a savage race-only bullet — will recognize the importance of analyzing other factors before choosing between cast iron, forged steel or billet steel.
When it comes to race engines and uber-street-performance, crankshaft selection is generally narrowed down to a choice between forged and billet materials; most of this story will focus on that decision. However, cast cranks warrant discussion simply because more engine builders will decide between cast or forged than those who must choose between forged or billet. Either way, expert advice will always be useful.
“The only real factor is the intended purpose of the vehicle,” says Alan Davis of Eagle Specialty Products.
“There are many factors,” adds Kirk Peters of Lunati. (See video below on the manufacturing of Lunati crankshafts) “Weight of the cars, time spent at maximum RPM and duration of heat cycles.”
“First thing you ask is what do you want to use the crank for,” sums up Tom Lieb of Scat Crankshafts. “Then you ask how much money do you want to spend.”
Horsepower levels help decide
Aftermarket crank suppliers may post maximum intended or suggested horsepower numbers along with the crank specs for cast or forged models. They are good starting points for the discussion with a company’s tech rep as most owners have a good idea of their engine’s intended horsepower. But, again, exercise discretion, provide accurate engine and vehicle information in addition to asking plenty of questions.
“For instance, we recommend our small-block cast cranks for use up to 500 horsepower. That does not mean if you make 501 horsepower everything will explode,” explains Davis. “That also does not mean that in any application as long as you make under 500 horsepower, you are fine. I would not use a cast crank in a blower engine – even if it is under 500 horsepower, due to the stress the blower belt system puts on the snout of the crank.”
Cast-iron crankshafts will work fine in a majority of street applications, especially restorations. In fact, most OEM crankshafts are made from cast nodular iron or cast steel, so there is a measure of durability and strength in the construction method. Also, many race-sanctioning bodies require cast cranks to keep costs down.
The casting process is basically limited to certain iron or low-grade steel alloys. In other words, there’s a diminishing point of return when introducing high-quality steel into a casting.
“The whole purpose of offering a cast crank is a cheap alternative to a stock part,” says Davis. “When you start using high-end steels—such as 4130, 4140, 4340—and more expensive manufacturing processes, you are taking the crank out of its target market.”
“The key to a cast crank is the heat treating. Castings require a different type of heat treating than a forging,” says Lieb, adding that shopping for used cast cranks presents its own set of warnings. (Watch the video below to see how Scat crankshafts are made) “Look for a brand name. On the low end of the spectrum, cast cranks are made in China from dozens of manufacturers, but only a handful are credible.”
Look at forgings first
Most performance engine builders today have their eye on forged crankshafts, based on a strong reputation at the track and increased validation in OEM engines. Also, aftermarket forgings now cover a wider cross section of the engine market, and pricing has become more competitive. The move to the more expensive billet crankshaft depends on a variety of factors that usually involve intricate designs and more expensive materials.
“Forgings should always be the first consideration when looking for an aftermarket crankshaft,” advises Greg Hill, process engineer at Callies Performance Products.
“The process of pressing a bar of steel into the rough crankshaft form gives the forging a superior grain structure and flow that increases strength. The tipping point for moving to a billet is typically requiring a stronger material and needing geometry outside the standard forging shape.”
As noted above, any discussion about crankshaft selection will include a debate over strength differences between forged and billet.
There is no industry-wide consensus but manufacturers will make strong pro-active arguments for many points of view.
“A forging is not as strong as a billet,” says Lieb. “Categorically the grain structure of a forging is such that it is not as strong.”
“Billet has no limitations, and are custom-made one at a time. Forgings are only feasible if a very large quantity is to be produced,” counters Davis. “All other variables being the same, a forged crank will be stronger than a billet crank.”
“The drawback for aftermarket forgings is that they’re made for wide spreads of strokes,” says Sonny Bryant, founder and owner of Bryant Racing Crankshafts. “Most people in years past thought there was an advantage because of grain flow, but there’s really no grain flow on forging because you cut all that out by running either a shorter stroke or longer stroke [than the forging was pressed].”
The fatigue factor
If anything, the spirited debate over strength differences has basically endorsed both crankshaft styles for performance engines, and there is no formula or guidelines set in stone for selecting either over the other—certainly not using horsepower levels as a deciding factor. A 900-horsepower NASCAR engine will run a billet crank because of the intricate design needs and grueling endurance requirements; while a 1,500-horsepower BBC in a Comp dragster is comfortable with an off-the-shelf forged crank. The key to proper selection is understanding the demands on the crankshaft for a particular application.
“The fatigue factor is one of the first things that determines what kind of crank,” explains Lieb. “The crank is constantly flexing. If you’re running a NASCAR engine for 500 miles, a forging won’t last. It doesn’t have the fatigue life that a billet has. If you have a Comp car, the fatigue life is any given weekend. It’s a totally different situation. But when you get into Top Fuel and you’re running a blower and pumping nitro into it, then you’re talking about fatigue life from the impact of the combustion pressures. It takes a lot of abuse in that situation and you can’t get away with a forged crank.”
“The NASCAR scenario is a perfect example of the necessity of billets,” adds Hill. (Note the video below which demonstrates machining of a billet crank at Callies.) “Their engine development is constantly evolving, and the required changes to a forging die or material to accommodate these changes aren’t realistic from a time or financial perspective.”
The most distinct difference between forged and billet is location of manufacturing facilities. While most of the crankshafts found in North American-built OEM engines are cast or forged in the US, a significant number of aftermarket cast and forged crankshafts start out in China–although some companies like Callies will also source raw forgings from Japan and locally in the US. But unlike the early Chinese imports from the early ‘90s that were quickly denounced due to appalling quality and durability, today’s cranks are forged or cast in a number of foundries with excellent production standards and access to quality steel.
The distinctions in aftermarket offerings lie in material choices, designs, machining, heat-treating and other finishing chores, much of which is conducted in the US by some of the companies. Others prefer to let the supplier handle the finished product but under their tight supervision and quality control standards. There is no advantage to sourcing billet crankshaft production to China since they’re mostly one-off custom models and delivery costs and delays would negate any labor or materials cost benefits.
4340 is like chocolate
In the forging process, a piece of rolled steel is heated to a near molten state, placed in a die, then pounded into the shape of a crankshaft using a heavy press. Compared to a cast crank, considerably more machining is needed to finish a forged crank. The terms twist and non-twist forging often surface when discussing cranks. When looking at a typical V8 crank, the throws are arranged at 90-degree angles to each other. In the twist method, the crank forging is reheated and twisted to form each throw. However, modern tooling has all but eliminated twisted forgings from the aftermarket, and all quality crankshafts are manufactured with the non-twist method.
The most popular aftermarket forged cranks boast 4340 steel material. There’s no need for a full metallurgy lesson when selecting crankshafts. Often, you don’t have a choice from the manufacturer. Steel is simply iron mixed with a few alloying elements such as carbon, chromium, manganese, nickel and silicon. To identify the different alloys, engineers and professional organizations such as SAE and AISI have adopted a 4-digit classification system for steel. Carbon steels are those alloys comprised of primarily of iron and carbon along with small amounts of manganese and silicon. Traces of sulfur and phosphorous can also find their way into the formula. Most OEM cranks are manufactured from carbon steel, such as 1010, 1045 and 1053.
“The first two numbers represent the primary and secondary alloying elements,” explains Davis. “1010 has 0.10 percent carbon, 1045 having 0.45 percent and so on. Properties of these steels are very similar and they are very inexpensive due to the lack of complexity.”
More complex formulas used in crankshaft production include 5140, 4130 and the previously mentioned 4340. 5140 adds chromium to the mixture, which is why it’s often referred to as chromium steel. Just for reference, 4130 steel (and also 4140 – the difference, of course, being the amount of carbon) is better known as chromoly steel and comprises carbon, chromium and molybdenum. Adding nickel to the mixture then produces 4340 steel.
“It might as well be gold due to what it does to the cost, but it’s very important,” says Davis. “4340 steel is alloyed with 1.65 to 2.00 percent nickel. There is a surprising jump in both strength and cost due to the addition of nickel to the alloy.”
“4340 is generic term for alloy steel,” adds Lieb. “It’s like chocolate, what kind do you like: German, white, Danish? Within the 4340 family, there’s probably a 10 to 15 percent difference between all the alloys. The key is designing your heat treating around the alloy.”
See how Callies tests the fatigue of its crankshafts
Putting heat into the crank
Also note that some crank manufacturers may list the steel composition starting with EN, such as EN24, EN30 or EN40. Those are European classifications and relate to alloy steels. For example, EN40 is similar to SAE 4340.
Heat Treating Basics
- Following rough machining, the cranks are baked to over 1,000 degrees F, then rapid cooled or quenched in water, oil or some type of polymer quenchant.
- Now the cranks are stronger but have little ductility and impact resistance. Stress relieving or tempering involves heating the crank again to a moderate temperature around 600 to 700 degrees F, then the cooling process is controlled according to the properties of the steel.
- The final step after all the machining is hardening of the journals, usually with a process called nitriding. This involves heating the crank in a closed-chamber oven and introducing ammonia and nitrogen gas, which reacts with the carbon on the surface of the metal and hardens approximately .010-inch deep. Nitriding is said to basically double the surface hardness of the journals.
“The additional nickel will add to short term durability,” says Bryant. “They don’t get a lot of cycles. For the most part the materials are close. Both are 30 carbon and hardened close to same level.”
Heat-treating is another crankshaft topic that needs attention. Again, however, the specifics, such as temperatures and quench methods, will rarely enter into a typical engine builder’s decision when selecting a crankshaft. The manufacturer will have already determined the proper heat treatment, not only from experience but also working closely with metal suppliers who have seasoned metallurgists and lab facilities to determine heat-treating specifics. Most US aftermarket crankshaft companies either have their own in-house heat-treat operation or farm it out to a nearby facility where quality control measures can be observed.
“When we do our own heat treating, we’re effectively testing the crankshafts,” says Lieb. “Because if something is wrong with the metal, the cranks will come back cracked, misshaped or swollen. Something will happen to the crank that we can spot.”
Some shops will also subject the cranks to a cryogenic treatment or rapid cooling to around -300 degrees F right after the first heat treatment. The cryo step helps complete the quenching process and make the steel more dense.
Counterweight design considerations
While engine builders rarely have any say on the forging process or heat treatment of any crankshaft, they do have some choices when it comes to counterweight design and weight of the crankshaft. Here’s where close consultation with the manufacturer’s rep is crucial to ensure proper selection for a given applications.
“Counterweight design is a compromise,” explains Bryant. “There are two bullet points: ease of balancing and correct main bearing loading. One doesn’t necessarily follow the other.”
The main bearing loading is calculated through simulation, but sometimes the correct weight presents a burden on the balance.
“You don’t want the correct main bearing loading and the crank needs 10 sticks of mallory,” says Bryant. “You’d like to modify the counterweights to reduce the mallory and still function properly in the application.”
“Counterweight shape is somewhat of a compromise between location, shape, bearing loads and balance,” echoes Hill. “Ideally a counterweight would be directly opposite of the journal with a 180-degree fan, blending tangentially into the side of the pin arm with a 1.000-inch or larger radius. As strokes increase, balancing becomes more influential in counterweight shape and location.”
Callies recently completed a 2-year project on LSX pin arm and counterweight redesign, reducing the heavy-metal requirements by 50 percent. The result was a reduction in overall weight of the crank.
“The success of this type of project was made possible by proper gathering of data, a program written in house to analyze balance data with input from our solid models, and utilizing our FEA software,” says Hill. “After putting this system in place for the LSX, we’ve been able to improve the rest of the engine families with a much shorter time investment.”
Rethinking lightweight crankshafts
Weight reduction is another crankshaft consideration that has drawn critical review in only the past few years. Conventional wisdom promised that a lighter crankshaft would rev quicker. But just as valvetrain design moved towards heavier but stronger pushrods, a similar shift is happening at the crank shop.
“We’ve learned over the years that stiffness is much more important than weight,” says Bryant. “Probably less than five percent of the cranks we produce have a target weight to hit. It wasn’t always that way. Fifteen years ago we were making a concerted effort to reduce weight because we thought there was something there. But when the smoke cleared, you were better off to have the stiffness to make more power and be happier in the engine block than something that the driver thought was faster because it revved up quicker.
“The trend with the top engine builders now is that stiffer is better than lighter,” continues Bryant. “When you make a crank stiffer you add durability.”
One of the last options that engine builders may have with crank selection is journal size. The most popular applications will offer large and small journals, depending on the customer’s needs—and there are pros and cons to each.
“It’s friction area versus strength,” explains Peters. “When the rod journal is cut down, the friction area is reduced. But the cross section between the rod journal and mains has been decreased, taking strength out of the crankshaft.”
“You have to take into consideration the cross section between the throw and the main,” adds Lieb. “The last thing you want is a Honda journal (1.888-inch) on a 350 Chevy 4-inch stroke crank. You have no overlap between the main and the throw.”
So, what’s your pick? There are crankshafts to fit any performance engine and application. The key to selecting the right one is working closely with the tech reps at the manufacturer and being honest about your needs. No doubt the aftermarket has stepped up to provide quality forgings for street and race applications that feature many options developed on the track. And stepping up to billet offers more options in design and materials. It’s just a matter of being honest about the application and working with the tech reps to select the right crack.
“What a crank looks like when it’s produced is all a balancing act between weight, strength, improving performance, intended application, cost, availability of supporting parts and production feasibility,” sums up Davis.