How to Build Killer Chevy Small-Block Engines. Mike Mavrigian
with a bullnosed leading edge, this approaches the aerodynamic effect similar to that of an airplane wing and provides less windage drag during revolution and faster evacuation of parasitic oil.
This closeup shows the extreme scalloped profile of the Callies Performance Products Magnum XL. Unneeded counterweight material is removed, providing a drastic weight reduction while maintaining a high strength-to-weight ratio. It is designed for lighter-weight rods and pistons, in terms of balancing accommodation. (Photo Courtesy Callies Performance Products)
Aftermarket cranks commonly feature generous fillet radii that eliminate potential stress risers. (Photo Courtesy Lunati)
Finish-grinding journals is performed with a high degree of precision on state-of-the-art grinders that are constantly checked for calibration.
The leading edge of this counterweight is bullnosed to reduce drag, providing less resistance than a square-cut edge.
Unlike mass-production factory cranks of years gone by, today’s aftermarket crank journals commonly feature very precise and consistently machined journal dimensions. It’s rare to find a crank made by a reputable manufacturer that features out-of-tolerance diameters and taper. While measuring journals and installed bearing dimensions for fit and oil clearance is always necessary to verify, it’s rare that corrections will be needed.
Note the star-cut flange for reduced weight. This is a feature offered by several crank makers. (Photo Courtesy Lunati)
This is an example of a forged steel crankshaft from Scat. Standard-weight cranks feature full-size counterweights, while gun drilling provides a level of weight reduction. Unless extreme lightening is required for certain racing applications, a standard-weight crank is an excellent choice for all high-performance street and a wide range of racing applications as well. The critical elements include precision machining and journal hardness that are offered by leading crank makers.
An example of extreme lightening of a racing crankshaft is shown. The Magnum XL series features lightening profiles where counterweight material has been machined from non-stress areas, greatly reducing parasitic material and weight. This minimizes windage within the crankcase. Oil control is improved through the elimination of disruptive undercuts, resulting in smooth-sided, free-flowing counterweights. Each main and rod journal is gun drilled for additional weight reduction and improved throttle response. Magnum XL cranks are shipped fully balanced to the builder’s exact assembly weight, requiring the builder to supply bobweight information. (Photo Courtesy Callies Performance Products)
Examples of the Lunati Signature Series are shown. They are made from a non-twist 4340 forging and are rated at handling over 1,500 hp. Features include gun-drilled mains, lightened rod journals, micropolished journals, and windage-reducing, contoured-wing counterweights. Signature Series blower applications are also available and are designed for use with Roots-type superchargers. Features include 0.125-inch fillet radii on rods and mains, duel keyways, larger nose bolt threads, and enlarged flexplate flange threads. (Photo Courtesy Lunati)
Bore and Stroke Combinations
Determining a bore and stroke combination for cubic-inch displacement involves a very simple formula:
Bore x Bore x Stroke x 0.7854 x number of cylinders.
Example: Let’s say that the planned cylinder bore size for an 8-cylinder engine is 4.125 inches and your planned crankshaft stroke is 4.000 inches.
4.125 x 4.125 x 4.000 x 0.7854 x 8 = 427.6503
Selecting Crankshaft Stroke
Too many enthusiasts tend to choose the bigger option as the best choice for just about everything—a bigger cam, bigger heads, a bigger carb, etc. Increasing crankshaft stroke provides greater torque down low. Shorter strokes allow the engine to rev higher. It all depends on where you want the peak power and torque. Especially for forced induction engines that utilize supercharging or turbocharging, a longer stroke simply isn’t needed because the increased dynamic compression under boost is making the power.
Consider the relationship of cylinder bore diameter and crankshaft stroke, wherein we refer to the “square” of the engine. If the bore diameter and stroke are equal, for example if bore size is 4.000 inches and stroke is 4.000 inches, the engine is square. If the bore diameter is greater than crank stroke, the engine is referred to as over-square. If the bore size is less than stroke, it’s under-square.
With the bore size as a constant, increasing the crankshaft stroke tends to produce greater torque and low-RPM power but is more limited in engine RPM. Going to a shorter stroke allows increased engine speed and moves power higher in the RPM band. If your goal is to obtain more torque for street driving, building the engine square or under-square is preferable. If you’re planning a road racing or oval track build, moving to an over-square platform is likely the better choice.
There’s no magic formula to determine which stroke and/or bore size is ideal because other factors, such as cylinder head flow, valve size, valve angle, camshaft profile, etc., influence the final outcome. However, speaking in very basic terms, when selecting the crankshaft stroke, longer strokes suit higher-torque requirements, while shorter strokes are better suited for higher engine RPM. Many street builds call for maximum displacement and maximum torque, which is why many opt for the biggest bores and longest strokes that will fit into the confines of a specific block package.
A very generic view of stroke selection is that a longer stroke provides increased torque with the powerband moved toward the lower RPM range, while a shorter stroke provides the capability of higher revs with the powerband moved into the higher RPM range.
Forgings and Billet
Steel forged crankshafts can be made using either a twist or non-twist method. A twist forging takes a raw forged crankshaft and, while heated and malleable, twists the forging to orient the rod throws in the proper clock position. A non-twist forging forges the crank with rod throws already in the proper clock positions. The difference is that a non-twist forging has a more uniform grain structure and is therefore stronger.
High-performance forged cranks intended for extreme applications are made using the non-twist method. This is followed by finish-machining, heat treating for strength and stability, and nitriding for increased surface hardness. Examples of makers include Callies Performance Products, Crower, Scat, Winberg, Eagle, and Lunati.
Billet crankshafts are, as the term implies, CNC machined from a solid blank of high-quality dense steel. Billet crankshafts are, not surprisingly, more expensive due to the material waste and the increased CNC machining time. Billet cranks are available from several manufacturers, including Scat, Callies Performance Products, Winberg, and Bryant Racing, to name a few.
The advantage of choosing a billet crankshaft is twofold: strength and custom application. The molecular structure is consistent because the process begins with a dense forged billet. Depending