By Mike Mavrigian
While the “traditional” approach to crank balancing is to consider 100% of the rotating weight and 50% of the reciprocating weight, there are slight advantages in deviating from this by slightly increasing or decreasing this balance factor ratio. For instance, a high-revving race engine may experience an out-of-balance (vibration) at a certain engine speed; while crankshaft balance may be totally acceptable in the 2000 to 4000 RPM range, but might experience a dynamic imbalance at, say, 6000 or 8000 RPM. In order to “tailor” the balance to best suit the critical anticipated operating range of the engine, we may slightly increase the reciprocating weight factor to a higher percentage of reciprocating weight.
Basically, the goal (if a problematic engine vibration does exist) is to optimize crank balance to accommodate the engine’s specific application. If a drag racer is spinning his engine to, say, 8000 RPM, and he experiences an out-of-balance condition at say, the 7000 – 8000 RPM range, slightly overbalancing can be utilized to balance the crank for that area of RPM (possibly experiencing a vibration at a much lower RPM, where he doesn’t care). A road racer or short track engine is called upon to pull in a wider range of RPM, so a happy medium needs to be found in terms of crank balance that will suit the application. In such a case where acceleration out of corners is a priority, utilizing a slight underbalance may allow the engine to more quickly reach the desired peak RPM.
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Unless you already have experience with a specific engine combination and application that you know requires an over or underbalance, probably the best/safest move is to consider 50% of combined rotating and reciprocating weight (one half of total weight for each side of each bobweight) and then dyno and/or track test. Quite often, finding that “sweet spot” for crank balance in a racing situation requires experimentation.
If a race engine is built to run on a fairly consistent high RPM, for example at a sustained 8,000+ RPM, if the crank was balanced at 50% as would be normal for most V8 engine, we are now faced with the effects of combustion forces (high compression) that create additional forces that may result in dynamic imbalance conditions at those engine speeds. This is often compensated for by slightly overbalancing the crank. Instead of factoring in 50%, we may increase the factor to as much as 51 or 52%. While this approach may result in a slight imbalance condition at low RPM, since the race engine won’t be operated at lower engine speed, this doesn’t present an issue. The goal is to achieve a neutral balance at the operation range of a specific engine application.
While the majority of V8 engine crankshafts are balanced using a 50% bobweight factor, a slight overbalance is preferred by some builders in order to not only better maintain balance specifically at high RPM use, but (theoretically) to serve as a boost to the combustion process with engines that feature high combustion pressures (say, 14:1 or more). The theory is that a slightly heavier weight factor of, say, 51.5% will aid in achieving compression, acting as a “helping hand” so to speak, providing an extra nit of energy to aid in compressing the air/fuel mixture.
An analogy that comes to mind (at least to my mind) relates to fishing. When using a bait casting rig, you may be slinging a relatively heavy lure. In addition to the build-up of power due to rod flex storing and releasing energy, the mass of the lure makes it easier to cast. When fly fishing, the lure is usually very light, which, on it’s own, does not allow you to cast it out. With a fly fishing rig, the main line provides the mass, so you’re actually casting the line…the fly is simply along for the ride. The weight of the fly line provides the boost or “helping hand”.
Underbalancing, in theory, may provide slightly quicker acceleration, allowing the engine to reach it’s peak operating RPM faster.
Granted, the use of over or under balancing has both supporters and detractors, and remains a debatable issue. Some claim that it helps and others claim that there is no benefit, with claims on both sides made by some very knowledgeable and reputable builders. Some proponents of overbalancing, for example, note that their experiences have been positive, with their engines out-performing competitors. Some even admit that they overbalance “because it makes them feel better” with regard to performance and durability attributes. In the end, experiment….if a specific balance weight percentage works for you and causes no bearing issues, stick with it. In the end, what works for some may not work for everyone.
HEAVY METAL (TUNGSTEN)
If weight must be added to crankshaft counterweights, one or more holes are drilled into the counterweight, followed by the installation of a heavy metal slug. (tungsten alloy is also referred to as heavy metal, mallory metal or densalloy.) Comparing weight slugs of the same diameter and length, tungsten is heavier than steel… about 2-1/3 times heavier than steel and about 1-1/2 times heavier than lead. Tungsten alloy typically used for crank balancing, is an alloy consisting primarily of tungsten with the addition of nickel and iron. It’s an extremely dense material. For example, if a hole is drilled that removes 120 grams of steel weight, a tungsten slug that weighs 280 grams is installed, thus resulting in adding 160 grams of weight to the counterweight. The modern balancing machine will provide information regarding the need to add or remove weight, in addition to where the weight modification is needed, along with the diameter and depth of the required hole for weight removal, and the amount of weight to be added, if necessary. Typically, a tungsten slug is installed at an interference fit of about 0.002″.
If a tungsten weight slug is installed at the outer face of a counterweight, in addition to the press fit of about 0.002″, a tack weld is required to prevent the potential centrifugal release of the slug during high RPM operation. A superior approach is to place the slug laterally through a crank’s counterweight by drilling a hole through the counterweight from front to rear and press-fitting the slug. This eliminates the potential of the slug moving or being released during crankshaft rotation.
The chart below is an example of tungsten slugs (diameter, length and weight), comparing the weight of tungsten to steel. Weight values are shown in grams; slug diameter and length are shown in inches. (Courtesy ABS Products)
Bear in mind that tungsten slugs are somewhat costly, with prices ranging from about $0.22 to about $0.38 per gram. Approximate price examples are as follows:
.500″ x 1″ @ $22.00
.500″ x 1.200″ @ $25.00
.625″ x 1.200″ @ $36.00
.750″ x 1.200″ @ $42.00
.875″ x 1.200″ @ $37.00
1.00″ x 1.200″ @ $45.00
Depending on how many slugs are required and the weight of each slug, the use of heavy metal slugs can potentially add as much as $200 or more to a customer’s balancing job. Again, the increased cost of using tungsten metal is dependent on the amount of material required and the number of installed locations. While an initially quoted crankshaft balancing job may be, say $250, the cost will jump if heavy metal is required. The need to use heavy metal will be determined once the crankshaft with bobweights installed has been spun on the balancing machine. The need to add weight might increase the cost by a mere $50, $100, or $200 or more. You won’t find out until the crank is balance-checked. The customer simply needs to be aware of the possibility.
Read this article with all images in the digital issue of Engine Professional magazine https://engineprofessional.com/2025EPQ1/#p=54