How to Build Killer Chevy Small-Block Engines. Mike Mavrigian
from billet stock may be as strong or stronger than a crank that is forged and finish machined.
One of the real benefits of a billet crank is the ability to create exactly the crank you want. Since it’s being machined from a blank at the outset, all dimensions can be achieved to suit your specific needs. That includes main and rod journal diameter and width, stroke, counterweight shape, snout diameter and length, etc. For a street application or a weekend warrior build, the cost versus function makes a billet crank a bit of overkill. However, for the pro racer who requires maximum durability and custom dimensions, the higher cost is justifiable.
Forged crankshafts are produced with a steel/alloy mix, slug heated to formability, compacted to rough shape in a hydraulic press, machined, and heat treated.
Aftermarket forged cranks are a better choice compared to OEM forged cranks. The reason: OEM forged cranks tend to have a high carbon content, but aftermarket forged cranks tend to have a higher content of chrome and nickel in the formulation, and the higher alloy content provides much superior strength.
Common debates exist in regard to strength. Some note that a forged crank offers superior strength because the grain structure has been moved and compacted, resulting in a more uniform grain structure during manufacturing. Billet cranks begin life as an already-forged chunk of billet steel, which is then machined to shape. However, the grain structure tends to run more parallel to the length of the crank. Depending on who you talk to, you’ll hear that billet is stronger than a forging or that a forging is stronger than billet. We won’t get into the debate here. As far as I’m concerned, a forged or billet crankshaft made by a reputable performance aftermarket manufacturer is suitable. The major difference, in my opinion, is that choosing a billet crankshaft provides increased latitude in terms of creating a custom-dimension crank in those instances where a builder’s request simply can’t be fulfilled by an off-the-shelf forged crank.
Today’s performance aftermarket offers crankshaft features that were unheard of only a few decades ago. Thanks to ongoing development within the aftermarket, we now have a greater selection of stroke dimensions, counterweight shaping, weight reduction, journal diameter choices beyond stock sizes, vastly superior metallurgy, high-precision CNC machining, surface finishes, and more, which translate into availability of performance cranks that contribute to obtaining increased power and torque along with substantially improved durability.
Shown here is Lunati’s Voodoo lightweight crank with a substantial amount of material removed to reduce rotating weight while maintaining rigidity. The 430 is a non-twist forging and is nitride heat treated with lightening holes in the rod journals. Note the undercut counterweights for further weight reduction. (Photo Courtesy Lunati)
Crankshaft Durability Treatments
In an effort to make a crankshaft more durable, more resistant to fatigue, and to provide a hard bearing surface, a process of nitriding is commonly employed. This creates a nitrogen-infused surface treatment that creates a several thousandths of an inch thick hardness increase that allows the crank to better withstand high bearing loads. An ion plasma nitriding process produces a deep case that enhances strength while creating an extremely hard bearing wear surface.
This is not to be confused with cryogenics, which offers its own benefits. Cryogenics involves subjecting the crankshaft to sub-zero temperatures as low as -400°F and warming it back up to ambient temperature in a controlled time process. This compacts the steel/alloy steel material into a tighter, more uniform molecular “grain” to offer higher resistance to fatigue.
Another process that provides a more uniform molecular structure is vibratory stress relief, which is a non-destructive method of subjecting the part to computer-controlled harmonic frequencies that vibrate the molecules, producing a more uniform structure. Vibratory stress relief is referred to as non-destructive because the part cannot be damaged during the process, unlike cryogenics, where strict protocols must be followed to avoid making the part too brittle. Improving the grain structure and/or surface hardening a crankshaft won’t produce additional horsepower, but these processes contribute to improving the durability of the crankshaft during extreme loads and speeds.
An example of a billet crankshaft by Bryant Racing is shown. Starting with a several-hundred-pound dense steel billet, the entire crankshaft is CNC machined to finished state, followed by REM isotropic finishing. (Photo Courtesy Bryant Crankshaft)
REM Finishing
REM’s Isotropic Finishing process (ISF) has been in use in various industries for decades but has been more commonly used in performance and racing applications in recent years. In combination with a proprietary chemical treatment and a vibratory polishing process, an REM-finished crankshaft’s appearance is extremely polished and smooth. Basically, it looks as though it’s been highly polished and chrome plated. With the appearance set aside, the performance benefits are what count.
Benefits include friction reduction, increased efficiency, a horsepower increase due to reduction of parasitic friction and oil cling, lower operating temperatures, reduced lubrication requirements, and increased component durability as sharp potential stress risers are reduced. Applications for REM finishing include not only crankshafts but also connecting rods, camshafts, lifters, valve springs, rocker arms, ring and pinion assemblies, mechanical oil pumps, rack and pinion steering components, transmission gears, universal joints, etc.
The REM ISF process results in a non-directional, low-Ra surface finish, which means that the surface is extremely smooth with reduced microscopic peaks. Ra stands for roughness average. The lower the Ra number, the smoother the finish. Think of it this way: consider the difference between sanding a metal surface with 80-grit sandpaper compared to using 2000-grit paper.
As an example (and a good one at that), an accomplished engine builder and close friend recounted a tale of a customer’s race engine. The engine was built using an REM ISF treated crankshaft. My friend’s shop performed all of the machine work, and the customer assembled the engine. He installed an adjustable-height distributor that featured a slip collar. The owner of the engine forgot to tighten the slip collar. As a result, during a race, the distributor began to climb out of its location to the point where it lost contact with the oil pump drive shaft, quickly resulting in zero oil pressure. He ran the engine for another lap before returning to the pits.
After the race, he brought the engine to my friend’s shop for a teardown and inspection. Incredibly, while the crank showed signs of extreme overheating and the bearings were toast, the crank’s rod and main journals were scratch-free, looking like the day the crank was finished.
The typical cost for REM ISF processing is about $600 for a crank and a set of eight rods. When you consider what’s at stake, that’s not a bad price to pay for added peace of mind.
CHAPTER 4
CONNECTING RODS
Connecting rods are one of the most critical components for any performance build. They provide the connection between the crank and the pistons and must withstand compression force and the dynamics incurred during the transition between approaching TDC and leaving TDC. Tensile strength, rigidity, and weight considerations are key factors. For engines designed to produce high levels of both horsepower and torque, only high-quality aftermarket forged steel, aluminum, or titanium rods should be considered. Cast rods should be completely ignored.
Today’s aftermarket offers a mind-boggling selection of rods in terms of superior materials, design innovations, lengths, and bearing bore diameter sizes to accommodate any build that you desire.
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