Weight and dynamic flow considerations
By Mike Mavrigian
Pulling weight out of a race car is generally a good thing, since making a race car lighter liberates previously wasted horsepower and can aid in achieving handling and braking enhancements. When we consider the weight of the engine as a system, removing weight benefits overall vehicle poundage. When we also consider the weight of specific engine components, this not only affects vehicle static weight, but engine response and durability as well. Here we’ll take look at how and where reduction of weight affects both static weight and engine performance.
While lighter may always sound better, we need to recognize one critical factor: we still need to balance the crank and to maintain crank stiffness/strength. If we remove too much weight from the counterweights (more than our bobweight factor), we’ll need to reintroduce weight (via Mallory/tungsten metal) to the crank, which defeats the purpose of initially achieving a lighter component. Lightening any crank must revolve around the balancing issue.
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While several methods of weight removal are available, each method has it’s own effect. Drilling a hole through the centerline of the journals (most commonly referred to as gun-drilling or rifle-drilling) serves two purposes: this removes static weight from the engine and race car, and it promotes vacuum and scavenging in the crankcase. For example, if an oil pan features a scraper system, this may isolate the crankcase cylinder-to-cylinder. Gun drilling can help to equalize crank case pressure and vacuum, transferring air back and forth to improve the scavenging. This is applicable to both wet and dry sump layouts. As an example, Tom Lieb at Scat told us that when GM was developing the LS1, the case was so excessively pressurized that initially uncontrollable external leaks occurred at numerous locations. As a result of Scat’s involvement with the program, they were able to solve the problem by strategically gun drilling the crank.
The rod pins can be drilled as well (which reduces overall weight and aids in matching bobweights when using light rods and pistons). Pin drilling is done with regard to oil passage locations, which often require specific angle drilling to clear oil holes. This makes it easier to balance the crank (the weight removed by drilling the rod pins can generally equal one or two slugs of heavy metal). Gun drilling can, depending on crank stroke, remove 3-4 pounds on a “typical” small block Chevy, or 4-6 pounds on a big block Chevy, and without reducing crank strength.
Undercutting (machining the counterweights thinner at predetermined locations) and profiling the counterweights via knife edging and bullnosing are also effective ways to reduce weight, again, keeping in mind the final balancing scenario. According to Tom Lieb, when billet cranks are produced, since the entire crank will be CNC machined anyway, Scat profiles the counterweights in the process. When a forged crank is produced, “excess” material exists as a natural by-product of the forging process in order to remove the crank from the forging die. If an end-user simply rounds off the counterweight edges (in an effort to bullnose), this might result in a weight savings of .75 to 1 pound If the counterweight radius is reduced (on a lathe), plus profiling by knife-edging, the weight savings can translate into removal of 4-5 pounds. In terms of balancing, as opposed to drilling holes in the counterweights, opposite a rod throw position, the same result can be accomplished by instead reducing the radius of the counterweight. Again, the amount of weight removed must be driven by the bobweight factor.
“A lot of guys spend big money for a lightweight crankshaft,” notes Lieb, “and then turn around and install an 8″ diameter, 12 pound damper, defeating the purpose of buying the lighter rotating mass crank. A handy formula to note is that rotating mass is equal to the square of the distance from the centerline multiplied by weight.
For instance, using an 8″ damper that weighs 12 pounds as an example, the damper’s radius (distance from the centerline) is 4″. This radius squared (4 x 4) equals 16. When you multiply 16 x 12 (the damper weighs 12 pounds), you find that the damper features a rotating mass of 192 pounds. Simply by moving to a 6″ diameter damper that weighs about 8 pounds, you reduce this rotating mass to 72 pounds.”
Mason Smaaladen of Bryant Racing Crankshafts noted that “we (Bryant) square, taper, knife or round both edges upon request. Most commonly the leading edge is knife-edged to help it flow more freely. Grooving the counterweights is an additional option to save weight, however it is either uncommon or not practical for certain parts.”
A critical benefit of using lighter rods and pistons (aside from quicker acceleration) is that you impose less stress on the crankshaft. In a “typical” engine with a redline of 6500 RPM, the piston may effectively generate a force equal to almost 12,000 pounds when rod tries to stop it at the end of the exhaust stroke. As piston and rod weight is reduced, this weight goes down, resulting in less stress on the crank. And anytime you can remove stress from a racing crank, you’ve made huge strides in crank longevity and reliability.
CENTER WEIGHTS
As a result of an increasing momentum in power output, today’s performance/racing engines are spitting out incredible levels of power due to advancements in engine design, availability of technologically improved components and power enhancements such as sophisticated fuel injection and forced induction. As engine performance continues to evolve in terms of higher loads, horsepower, torque and RPM, some V8 engine crankshafts available today feature 8 counterweights where weights are added at the center, as opposed to the traditional 6 counterweights (three front and three rear). Also referred to as fully counterweighted, this addition of center weights at each side of the center main serve to both simplify the balancing job and most notably, to help dampen the flex of the crank at higher RPM, addressing the additional torsional forces that the engine may experience at high loads/high RPM. Depending on the supplier, this may be offered as either a standard feature or as an option. Center weights serve to reduce crank flex/whip in engines that will experience extreme levels of both power output and engine speed.
The addition of center weights is increasingly recommended for long-stroke applications. As you increase crank stroke, heavier counterweights may be needed in order to compensate for the weight of the assembly. Instead of using external weights to compensate, the addition of center weights allows you to retain internal balancing. This has become increasingly important due to an increase in crank load due to longer strokes and new levels of insane boost as the use of forced induction has increased dramatically in popularity. The addition of center weights serve to enhance crankshaft longevity.
COUNTERWEIGHT PROFILES
While counterweights are essential to offset the inertia of the pistons and connecting rods, in an effort to maximize the reductions of windage and parasitic losses as the crank rotates, it’s common for cranks to feature various counterweight shapes at the leading and trailing edges of each counterweight. The leading edge may feature either a tapered face (commonly referred to as “knife edging”) or a rounded, radiused shape. The trailing edge may also feature a tapered “knife edge” face. The theory is that this creates, or simulates, an airplane wing airfoil shape, to increase efficiency as the crank spins through the crankcase air/oil film atmosphere. The goal is to reduce windage and parasitic losses. Under-cutting, where material is removed from the sides of the counterweights represents another means of reducing mass when needed. However, be aware that certain race sanctioning body rules don’t allow under-cutting or knife edging.
Counterweight shapes and mass represent a compromise between location, shape and the ability to achieve balance.
SURFACE TREATMENTS
Enhancing the counterweight surfaces also aids in reducing energy losses created by windage by making the surfaces smoother/more slippery. This is done via polishing or application of specific surface treatments/coatings, resulting in a “chrome-like” surface. Certain coatings may also be applied to journal surfaces, resulting in increased hardness and filling/eliminating any microscopic imperfections that are left from the machining process, improving bearing life by enhancing Ra. Numerous options with regard to surface enhancement are available as options through various crank makers as well as from certain specialty coating services.
Read this article with all images in the digital issue of Engine Professional magazine https://engineprofessional.com/2024EPQ2/#p=56