Tech: Choosing The Right Crankshaft With Scat

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A crankshaft serves a vital function within any engine, converting the force created by an engine’s pistons traveling up and down, into a force that moves the wheels of a car in a circular motion, so the car actually moves. We spoke with our friends at Scat Enterprises Inc. to learn more about crankshafts, and how to choose the right one for a specific application.

What Is A Crankshaft?

Located within a car’s engine, the crankshaft is connected to each of the connecting rods and pistons, as well as the flywheel. A car’s engine creates motion by generating timed, controlled explosions within each cylinder. As pressure in each cylinder increases from the combustion (explosion), the piston in that cylinder is pushed down. This is known as the power stroke.

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A crankshaft, connecting rods, and pistons (rotating assembly), prior to assembly.

As the crankshaft rotates, each piston moves up and down in sequence. When the crankshaft rotates, it also turns the flywheel in a circular motion. This in turn spins the transmission, sending power to the rearend. Essentially, it is a shaft that transmits power developed by the engine to various parts of an automobile, and the crankshaft is the primary link between the input and output of car.

Forming A Crankshaft

According to Tom Lieb at Scat Enterprises, the heat-treating process for any crankshaft is a lengthy process. “The foundation of the steel to be used, all depends on what you put into the pot. This will determine the structure of the steel and its strength capabilities,” said Lieb. “The metal goes into the oven, which we heat to a specific temperature, based on the alloy content. We then drop the piece of steel into the quenching liquid, which consists of glycol, shocking all of the grain in the metal into place. After being machined, it goes through a stress-relief process, taking away the memory of all of the stress the metal went through.”

There are different cranks, each based on cost, strength, and whether there is any tooling available for them.
– Tom Lieb, Scat Enterprises

“Cast crankshafts are manufactured using the most basic production methods,” Lieb said. “Molten metal is poured into a mold, then machined into the final shape of the crankshaft. This method is the most inexpensive, but the least durable.”

Making forged crankshafts involves molten metal being poured into ingots, and then transferring them into a rolling mill. “The metal is red hot and moves like jelly, but it won’t pour just yet,” Lieb said. “Forging takes the basic dough (or in this case steel), and forms a grain within the metal, so that the dots can connect. When the metal is rolled into a bar, the dots roll into unison from the compression forces that squeeze the molecules together.”

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Scat has spent years perfecting the crankshaft, adapting to advances in technology.

Lieb provided us with a definitive example to explain constructing a crankshaft. “If you’re aiming to build a forged small-block Chevy crankshaft with a four-inch stroke, the process begins with a round bar of metal that measures 4 3/4 inches in diameter,” said Lieb. “Following the forging process, the diameter of the crankshaft stretches to 6 3/4 inches. The grain that previously flowed parallel trough the crankshaft, has been forced to turn 90 degrees at different angles, offsetting the centerline on the crankshaft.”

What Is Windage?

  • Windage is a force created on an object by friction when there’s relative movement between air and the object itself.
  • Windage is the parasitic drag on the crankshaft created by sump oil splashing onto the crankshaft during rough or spirited driving.
  • As a result, there is dissipated energy in turbulence from the crankshaft, moving the crankcase gas and oil mist at high RPM.
  • Windage can create lubrication problems, thus bringing the development of special scrapers to remove excess oil from the counterweights and screens, creating a barrier between the crankshaft and oil sump.
  • Information courtesy of Crank Scrapers
In a billet crankshaft, the metal is forged, with its grain organized in a linear structure. “Everything is uniform with no stress areas,” Lieb said. “The only point where strength is lost, is where the crankshaft gets thin. A billet crankshaft is formed from a round bar of refined, forged-steel, that measures a large diameter. With the metal blanks weighing twice as much as a standard forged crankshaft, the product in the end is stronger since it’s put under almost no stress.”

Crankshafts also come in different sizes: small, medium, and large. “There are different crankshafts, each based on cost, strength, and whether there is any tooling available for them,” said Lieb.

In regards to build quality, there is a specific hierarchy, depending on your specific application. “Horsepower-wise, cast crankshafts are great to a certain extent,” said Lieb. “Forged crankshafts take over after that point, followed by billet crankshafts, which are utilized on Funny cars. Basically, it comes down to horsepower, cost, and overall reliability.”

Lieb said that a crankshaft is driven by weight, and that it should be as light as possible. “There are various ways to go about lightening the crankshaft,” he said. “An engine accelerates quicker with a lighter crankshaft, since the engine doesn’t have to work as hard to spin it. The counterweights are profiled and shaped, based on weight and balancing. The shapes are based off the windage inside, which all help to facilitate the motion of the crankshaft.”

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Looking closely, you can see the linear grain structure in this crankshaft.

Choosing A Crankshaft For A SB Chevy

Lieb also explained various options for small-block Chevy configurations. He said it all boils down to what you are expecting the engine to reliably accomplish. “You have to look at it from a specific standpoint,” he said. “For example, if you’re building a street rod with a small-block 350 to 383 cubic-inch engine, a cast crankshaft will do just fine. Cast is essentially for mild-performance applications. Stepping up to a small-block in a dirt modified or sprint car, will require a stronger crankshaft, since cast won’t be able to handle the horsepower.”

Crankshafts are made for specific applications that range from mild performance to full-blown Funny cars.

Lieb said the only major difference between a cast and billet crankshaft is the material, citing weight as the major factor. “Small-block engines have progressed over the years with different components,” he said. “The ability for us to make anything the customer wants gives us flexibility. We have 30 CNC machines that let the computer do the work.”

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Crankshafts are a unique piece of automotive machinery, serving to handle the energy delivered during combustion.

“Finding the intended objective with the customer comes first,” said Lieb. “The weight of the connecting rod and piston, determines how light we can make the crankshaft. If we know the application and horsepower level, we simply take the crankshaft and design around it.”

The strength of the crankshaft is determined by the diameter of the main bearing-journal and the stroke. Altering the diameter can either add or reduce a crankshaft’s strength. Small-block Chevy engines have three different main bearing sizes, causing Scat to mix and match throws to achieve maximum strength. “Choosing the right main-bearing diameter can help to reduce friction,” Lieb said. “Reducing friction and maximizing strength equates to getting the most power and reliability from the crankshaft.”

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The horsepower level of a specific application will determine whether a cast, forged, or billet crankshaft is the correct component for your engine.

Double-Keyed Crankshafts

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A crankshaft in the process of being double-keyed to handle more stress.

The foundation of the steel all depends on what you put into the pot, which will determine the structure of the steel and its strength capabilities.

Typically, a big-block nose and double-keyway is used for an engine with a dry sump oil system, or a supercharger setup. “Without double-keying and adding a big-block nose, it can’t operate without it,” said Lieb. “Using the standard small-block diameter nose for this application will lead to the nose breaking off.”

Double-keying the crankshaft serves as an insurance policy to handle the high horsepower load being put on the end of the crankshaft from actions such as backfiring or any kind of heavy stress put on the engine during use.

The Importance Of Counterweights

Counterweights play a vital role by helping to balance the crankshaft. Additionally, they are in place to also make loading more efficient, thus saving energy. Most importantly, counterweights provide a smoother operating engine, while also allowing an engine to reach a higher RPM with less effort.

The connecting rods and bearings have a huge influence on the crankshaft. “The counterweights essentially work against the force of the throw side,” Lieb said. “Center counterweights help to prevent the crankshaft from flexing.” If the weight of the counterweights is not sufficient enough to balance the crankshaft, an external balance is required. External balancing means that a weight is placed on the flywheel to add to the weight of the counterweight on the crankshaft.

External balancing allows a crankshaft to be properly balanced. Late-model, one-piece-rear-main seal, Chevy small-block 350 cubic-inch engines use a combination of both, as it is internally balanced at the front of the crankshaft, and externally balanced at the rear.

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Center counterweights are put in place to prevent the crankshaft from twisting or flexing during operation.

Uniqueness And Thrust

“We’ve been designing and building crankshafts for the past 50 years,” said Lieb. “We’ve built prototype crankshafts for GM, and hold a resource of information and experience that is tough to beat.”

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Oil travels through the crankshaft, and each bearing is lubricated through these holes while it is in motion.

Lieb left us with one piece of wisdom, “Quality control, the manufacturing process, and good materials lead to a crankshaft that is what you want, is affordable, and above all performs.”

So whether you’re working with a mild-performance application, street rod, or a Funny car, Scat has the ability to adapt to your needs.

We can’t recommend any specific product for your engine, since all engines are built to a different spec and are used for different purposes ranging from daily driving and weekend warriors, to championship drag racing and dirt-modified applications. But, if you’re curious to see what is best for your specific application, don’t hesitate to give them a call, or check out their website, to see what options you have.

Article Sources

About the author

nicaguon

Nic Aguon is a graduate from San Jose State University with a Bachelor of Science degree in journalism and mass communications and holds a passion for sleek and unique automobiles. Serving as a Staff Writer for multiple publications at Power Automedia, Nic pledges to bring readers clean and concise storytelling that hits the nail right on the head. A jack of all trades, his interests range from imports to American muscle, hot rods, and Kustoms.
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