When you strip it down to basics, the concept of balancing a crankshaft is simple: You’re essentially offsetting a load on one end with another.
For a crankshaft, often referred to as the engine’s “backbone” and arguably its hardest-working component, achieving balance is critical. Converting the reciprocating (up-and-down) motion of the connecting rods and pistons into rotating motion subjects the crank to harmful forces that can cause unwanted mechanical activity and prevent the engine from efficiently transferring power to the wheels.
In fact, as RPMs increase, so do those parasitic forces that can rob your engine of usable horsepower. Balancing helps optimize the engine’s output by minimizing the vibrations, deformation, and potential wear and damage caused by unbalanced forces (i.e., the constant pounding from your rods and pistons).
It’s important to understand that while balancing does not directly increase or add horsepower, it does reduce the forces that can hinder performance. So, it’s more about unlocking or freeing up existing power than actually producing it.
To put it simply: Balancing produces greater stability, and the more stable the crankshaft, the better it can perform as intended.
About those Counterweights
When engines are manufactured, they go through a process of balancing to ensure that all components are shaped and weighted correctly. The crankshaft is no exception, and it’s designed with counterweights whose job is “literally to counteract the weight on the opposing side–the pistons, rods, etc.–and help bring the crankshaft into balance,” notes Manley Performance President Trip Manley.
Most of your stock V8 and everyday grocery getter crankshafts have six counterweights–with no counterweights off the webs on the center of the crank. Why six? A few reasons.
First, placing counterweights at the ends of the crank provides more than adequate stability at lower RPMs. The lighter weight also lets you rev quicker and provides better throttle response. What’s more, six-counterweight designs cost less to manufacture, since they use less material.
But it also traces back to the fact that engines built only a few decades ago ran at lower RPMs compared to modern powerplants. These older engines had a higher tolerance for unbalanced weight, since running at a relatively low RPM band is less disruptive. The six-counterweight configuration is largely a carryover from those older designs that weren’t meant for high-horsepower or high-RPM applications.
Now, even though many older crankshafts have performed admirably under demanding conditions, eight-counterweight cranks are generally better equipped to handle the increased load and twisting forces of a racing or high-performance application. Also called center counterweighted (CCW) cranks, these units include a counterweight for each rod journal and are designed for greater strength and stability at higher RPMs.
Making Adjustments
Counterweights actually have two main functions: balancing out the weight of the rods and pistons, and dampening the twisting forces from the engine’s firing sequence. As noted, a CCW design has the advantages of greater strength, less bending movement, and reduced harmonics versus a six-counterweight crank.
But regardless of the number of counterweights, both types of crankshafts still need to be properly balanced.
OEM-made crankshafts come out of the factory pre-balanced to their stock piston-and-rod combinations. Which is great. But that all flies out the window when you’re talking about aftermarket cranks. For example, any number of variables–lighter pistons, longer-stroke engines, etc.–must be accounted for, and that means adjusting the counterweights.
The bottom line here is that no engine turning high RPMs will last very long if the crank is out of balance. And crankshafts should always be balanced to their specific piston-and-rod combo, which is done by adjusting the counterweights or adding weight externally. And that brings us to…
Internal vs. External Balance
You’ve probably heard another set of terms related to balancing: internal and external. So, what do they mean, and how are they different?
Basically, an engine that’s internally balanced means all weight adjustments are made on the counterweights. A common method of adding weight involves drilling and adding mallory metal–a tungsten alloy about twice as heavy as steel–into the counterweights at strategic locations. Conversely, removing weight typically involves precision-drilling holes into the counterweights.
For externally balanced engines, weight is added to the balancer, flexplate, or flywheel. You’ll also need to externally balance some engines with longer strokes, bigger pistons, or larger displacements, since they may not have enough room in the crankcase for counterweights that are big enough to offset those forces.
For internally balanced engines, it’s worth noting that external parts like the harmonic damper and flexplate/flywheel have a neutral balance, meaning they won’t impact the other rotating parts and can be balanced separately. However, for engines that are externally balanced, the damper and flywheel need to be mounted on the crank before balancing.
One more thing: All crankshafts are listed by the manufacturer as either internal or external balance. For example, Manley Performance’s line of Big Block Chevrolet Crankshafts are marked as “4340 Non-Twist Forgings, Designed for Internal Balance.” This means they’ve been balanced within 1% of a published bobweight calculated using Manley’s rods, pistons, etc. However, they have not been precision balanced to within 5 grams of the actual bobweight of the exact parts being used in the build. What do we mean by bobweight? Glad you asked.
What About Bobweight
Since you can’t run a crankshaft on the balancing machine with the actual rods, pistons, etc. attached, separate weights that equal their mass need to be measured out and bolted on. This simulated weight of the components opposite the counterweight, or the total weight of one cylinder’s worth of components, is referred to as the “bobweight.”
How do you determine it? Figure you’ve got your rotating weight, which includes the big end of the connecting rod, rod bolts, and rod bearings, plus a small amount–usually about 2 to 3 grams–to simulate the oil between the crank journals and bearings. And then you’ve got your reciprocating weight, which is the small end of the rod, the piston, piston pins, piston rings (and groove lock spacer, if applicable), plus a few more grams for the oil that hangs around those moving parts.
Once those component weights have been equalized, that’s when you calculate the bobweight using a relatively simple formula: rotating components x2 plus reciprocating components x1.
Take It For a Spin
Now that you’ve determined the bobweight and attached your weights to the rod journals to simulate the mass of the rods and pistons, a balancing machine spins the crank–750 RPM is the industry standard, also preferred by Manley–to determine both the position and magnitude of unbalance.
“The balancer tells you where and how much weight to add or remove,” explains Frank Ten Broeck, Manley’s Operations Supervisor. “Our balancer has a drill press head mounted to it, so if the balancer calls for weight to be removed, you move the drill over the counterweight while it’s still in the balancer and drill to the depth the balancer calls for.
“If you have to add weight,” he continues, “then the crank has to come out of the balancer and into a fixture, and you drill basically through the side (or face) of the counterweight and install a heavy metal slug in the hole.”
Ten Broeck does point out, however, that in this process “the dial on the balancer will move while drilling, so before drilling any additional holes, you need to cycle the balancer before drilling where the dial indicates.”
Something else to keep in mind: Maintaining the integrity of the crankshaft and its materials is critical. As you drill into the counterweights–especially off-center–they can become unstable. The higher the RPMs, the more unstable your counterweight gets; it not only moves back and forth at speed, it also begins to oscillate, which weakens it.
That said, Ten Broeck notes that at Manley, “our cranks come in to us from our supplier balanced to the published bobweight we call out.”
The Bottom Line
Balancing the rotating assembly means making sure the crankshaft is the right weight to balance out the forces from the rods and pistons. If the crankshaft is too heavy or too light, it can cause the engine to vibrate or otherwise restrict performance and/or longevity. With today’s lightweight, high-revving engines, achieving that balance is critical to operating at peak efficiency.
For more information on crankshaft balancing, visit Manley Performance at manleyperformance.com or call (732) 905-3366.
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