How to Build Racing Engines: Crankshaft Selection Guide

scat 383Crankshafts live a merciless existence somewhat akin to having a group of bullies beating you up over and over again 24/7. A high-end racing engine can produce a combustion pressure of about 1,400 to 1,500 psi. For example, if you apply that pres­sure to a 4.185-inch-diameter piston, as found in a Sprint Cup engine, it translates to a force exceeding 19,000 pounds against the rod journal for every cylinder. That’s a staggering compressive load. Racing crank­shafts are pretty tough customers, but they do deflect under the inter­mittent torsional loading imparted by the engine’s continuous firing sequence. This unseen dimensional elasticity (ductility) is necessary to absorb the dynamic forces at play. Dimensionally it is typically less than the existing clearance stack in the bearings, piston clearance, and ring pack clearances.

Scat Cranks


We’re talking about forces imparted by the actual combustion process, but also must consider the reciprocating mass being flung back and forth in every cylinder. That includes not just the pistons, but the ring packs, wrist pins, pin retainers, the small end of the connecting rod, plus some amount of oil clinging to these parts. Whatever that weight may be in each cylinder, it is contin­uously being accelerated from zero velocity to maximum velocity and back to zero twice for every revolu­tion of the crankshaft. And of course that is happening in each individual cylinder in an orderly sequence that tends to absorb minute variations.

In addition to combustion pres­sures, the crankshaft is also trying to maintain order among all these vari­ous masses being flung in all direc­tions. Some pistons are coming (up to top dead center, TDC), others are going (down toward bottom dead center, BDC), and some are in tran­sit, all with their own accelerations, velocities, and force vectors depend­ing largely on the stroke, rod length, and engine speed. The reciprocat­ing mass attached to each crankpin assembly must also be opposed by the crank throws to maintain balance.

If Atlas thought it was tough holding up the world, he wouldn’t be any happier trying to hang onto eight raging race pistons at maxi­mum RPM. A 90-degree V-8 layout permits pretty complete balancing of the primary and secondary forces at play, but not so well that vibratory loading is totally eliminated. Depending on contributing factors such as component dimensions and mass, the rotating assembly goes in and out of tune at different engine speeds; it may be dead smooth at some RPM and not so smooth at others. To some degree or another, this affects crankshaft stability, ring seal, and bearing integrity and it is a primary reason that racing engines must be precisely balanced to ensure smooth predictable operation.

Crankshaft Types

Most racing applications use a steel forging or a billet steel crank­shaft. Lower classes are often restricted to cast steel or OEM-type nodular iron cranks, which per­form admirably in many sportsman classes, but are functionally unsuited for high-horsepower applications. These classes frequently place weight limits on crankshafts (such as a 50-pound minimum) to prevent the use of lighter high-dollar cranks. With a tensile strength of 65,000 to 80,000 psi, common castings are relatively brittle, but perfectly suited for general automotive use. OEM nodular iron cranks feature a tensile strength above 100,000 psi with bet­ter ductility and an elongation rating of about 3 percent. This is suitable for many heavy-duty truck applications and even some OEM performance engines. The elongation rating refers to the percentage of deformation the crank can endure repeatedly with­out failure. It is basically the differ­ence between tensile strength (the force required to initiate stretching or deflection) and yield strength (the amount of force required to perma­nently stretch or deform the part). Cast cranks perform admirably in those sportsman classes that require them, particularly crankshafts such as Scat’s 9000-series, which have a tensile strength equal to most basic forgings (105,000 psi) and an elonga­tion rating (6 percent) nearly double that of most common castings. These cranks are quite comfortable in the 400- to 450-hp range in circle track racing applications and they often hold up very well even at the 500+ hp level in high-performance street applications that only see part-time abuse.

The combination of a lightweight forged crankshaft, lightweight H-beam connect¬ing rods, and forged pistons is a recipe for success in most racing applications.

The combination of a lightweight forged crankshaft, lightweight H-beam connecting rods, and forged pistons is a surefire recipe for success in most racing applications.


Forged Cranks

Forged steel cranks come in two basic varieties: twisted and non-twisted, meaning whether the crank throws are forged in place all at one time or twisted into position dur­ing the manufacturing process. The forging process consists of heating a chunk of billet to 2,650 to 2,750 degrees F and then pounding it into shape with dies on massive 200-ton presses. Twisted cranks are forged in one piece and the throws are twisted into place all at once on huge auto­mated machines. Twisted forgings are less expensive to manufacture, but the initial tooling investment is much higher even though overall maintenance costs less.

A non-twisted crank is forged with the throws in the correct position all at once and the tooling lasts longer because there is less overall displace­ment of metal. Depending on which manufacturer you talk to, the dif­ference is negligible if the process is performed correctly. Debate over the relative merits of each type remains ongoing, but many manufacturers recommended a non-twisted struc­ture as superior for competition use.

Most OEM performance cranks are twisted forgings and they are plenty strong for most applications. Note that the interruption of grain structure within the metal is greater in a twisted forging than a non-twisted forging. In a non-twisted forg­ing, grain displacement is limited to roughly half the stroke length versus a similar displacement in a twisted forging plus a severe twist that fur­ther disrupts the internal grain struc­ture. Hence, in theory, non-twisted forgings may be somewhat tougher and more able to resist crankshaft deflection under severe load.

Drilled crankpins are the most effec¬tive way of reducing rotational inertia in a racing crankshaft without affect¬ing the overall strength of individual crankpins. (Courtesy Scat Enterprises)

Drilled crankpins are the most effective way of reducing rotational inertia in a racing crankshaft without affecting the overall strength of individual crankpins. Courtesy Scat Enterprises



Forged cranks are manufactured in a broad range of materials with varying degrees of strength depend­ing on the alloy and the heat treat­ment involved. Factory-forged cranks are made from plain carbon steel, typically 1053 alloy and some­times 1045. The tensile strength of these alloys is about 110,000 psi. While this doesn’t seem that much better than a good cast crankshaft, the ductility of the forged crank is much better with nearly 25-percent-greater elongation factor prior to fail­ure. The next strongest alloy is 5140 chromium steel, which is graded at 115,000-psi tensile strength. Beyond that, chromium and higher-carbon-content alloys such as 4140 are used for most high-performance forged crankshafts. These alloys are rated up to 125,000 psi. Top-of-the-line forged cranks for racing applications use 4340, a stronger and tougher alloy rated at 140,000 psi.

Forged cranks are more expen­sive than cast cranks and less expen­sive than billet cranks, but with the exception of absolute high-end applications requiring uncompro­mising strength (4340 billets), 4140 forged cranks are the primary choice for most sportsman racing applica­tions. Additionally, most manufac­turers today have cleverly designed their forgings to allow a range of dif­ferent stroke lengths to be machined from the same basic forging. In many cases they can grind a particular stroke from an existing forging if it is within a specified range.

Other advantages of purchasing a forged crankshaft almost always include a generous radius on the journal fillets, which adds strength and resists cracking. This is no small matter as most crank manufacturers consider the design and machining of a proper fillet radius one of the most important features of a racing crankshaft. In particular they have begun to favor a non-circular con­tour or non-constant radius that is thought to impart greater strength to the transition from journal to throw. Most aftermarket forged racing cranks also have knife edging or bull nosing on the crank throws, a feature designed to help the crank throw cut through the swirling oil mass with reduced aerodynamic drag.

Billet Cranks

Billet cranks are CNC machined from a solid chunk of high-strength billet steel alloy. They are typically the strongest and stiffest units avail­able and are the overwhelming choice for unlimited applications such as Top Fuel dragsters, Funny Cars, Pro/ Stock and Pro/Modified, drag racers, Sprint Cup engines, and any form of unlimited competition that requires maximum strength and durability.

Among the major advantages of billet cranks is the ability to machine any desired combination of stroke and journal size while maintaining maximum strength. In contrast to a forging where the grain structure of the metal is stretched and deformed at high temperature, the grain structure of a billet crank remains unchanged with no residual stress from the forg­ing process. The machining process interrupts the grain in both types, but it is seen as less intrusive in a billet. Further, billets permit very precise shaping and location of the crank throws to ensure maximum strength and precise balancing. Bil­lets are typically manufactured from 4340 alloy or better.

Note the generous fillet radius on this Scat rod journal. The radius spreads the load and removes a common point of stress fractures.

Note the generous fillet radius on this Scat rod journal. The radius spreads the load and removes a common point of stress fractures.


Gun-drilled mains are also used to lighten a racing crankshaft.

Gun-drilled mains are also used to lighten a racing crankshaft.

If you subscribe to the notion that a piston, rod, and crankshaft all serve as shock absorbers it becomes a moot point. An unyielding crankshaft may be more prone to hammering bear­ings even as it provides more precise piston positioning. This is especially true under conditions of detonation where piston rattling and rocking may be severe. Since variable crank deflection can’t be measured practi­cally, all we can do is try to predict its consequence and act accordingly. It’s a sound reason to tightly control pis­ton ring side clearance, radial depth, and tension to very close tolerances. This ensures the best possible seal at the ring face and the ring land to minimize ring movement in the ring grooves when subjected to the ham­mering effects of detonation or high-speed flutter. Consider what happens to ring seal when the ring is tempo­rarily unloaded and how hard it is to regain that seal at elevated engine speeds. This is one reason piston rings have a slightly rounded face. They must maintain the seal even while the piston is rocking.

Strength Treatments

Various treatments are employed to toughen crankshafts for their severe-duty environment. They include heat treatments, hardening techniques, and severe-cold cryogenic treatments to stabilize the metal.

Heat Treatment

Interestingly, cast crankshafts generally don’t require heat treat­ment because the manufacturing process hardens the metal during the machining process. That means cast cranks can be easily reground without loss of surface hardening. The same applies to OEM-style forge cranks if they have not been tuftrided or nitrided. In such cases the crank requires another heat treatment to restore the initial journal surface hardness. Forged cranks are actually softer initially and require specific heat treatments to make them suit­able for competition use. Heat treat­ing is applied to OEM forged cranks to improve their durability and resis­tance to wear.

Induction Hardening

Factory cranks are induction hardened a process that generates rapid heat in the crank surface via a high-frequency alternating magnetic field. This procedure is quick and inexpensive and typically provides hardening penetration to a depth as much as .080 inch. Unfortunately it introduces stress to various parts of the crank because the heating and cooling process is restricted to the surface material and does not affect the rest of the crank material equally. The process is economical and use­ful for OEM applications, which generally never see the severe duty imposed by racing conditions.


Tuftriding is a process developed specifically to avoid the uneven stresses caused by induction harden­ing. It is performed by immersing the crank in hot cyanide compounds, which creates a hard surface that is resistant to wear and fatigue. Tuftrid­ing typically applies only a thin layer of hardening and it often introduces warping, which has to be dealt with in the final machining process.


The most common and favored heat treatment is nitriding a chem­ical process in which the crank is heated in a special furnace and sub­jected to ammonia and nitrogen gas, which react with the carbon on the surface of the crank, hardening it to a depth specific to the length of time the crank is exposed to the hot gases.

Nitriding does not usually pen­etrate as deep as tuftriding, generally only about .010 to .013 inch or so, but it does not impart the localized stress caused by tuftriding. Its pri­mary benefit is the improvement in impact and resistance to wear. Nitrid­ing is a lower-temperature process that, unlike induction hardening, does not affect the core strength of the crank. According to Scat, it treats the crank evenly and imparts a surface tension that increases fatigue life by 18 to 20 percent. The process is more expensive and the cranks require re-nitriding if they are ever reground.

Many manufacturers still prefer to induction harden their cranks to control costs and it’s not that these cranks are bad, the engine builder just needs to know this and contem­plate the consequences if crank work is required later. Race engine builders weigh the relative merits of both and make their choice based on operating and endurance requirements and, in many cases, the allotted budget for any given engine project.


Cryogenic treatment is another procedure used to improve strength and fatigue resistance. The cryo­genic process chills the crank to 300 degrees F for a specified length of time, typically 24 to 36 hours. The crank is then set aside to slowly return to room temperature. This deep-freezing process relieves resid­ual stresses in the metal, making the crank more resistant to breaking. Considered snake oil in the not too distant past, cryogenic treatments have come into favor and are now used by many top engine builders for maximum-effort engines.


There is wide belief among crank­shaft manufacturers that standard two-plane V-8 cranks offer power and durability gains with the use of additional counterweights around the center main journal, particularly in long-stroke applications that have more flex due to less overlap between the throws and the mains. Center counterweights have previously not been used because they increase the difficulty and expense of manufac­turing the crankshaft and because traditional six-counterweight cranks have a lower mass moment of inertia (MMOI). More builders are recogniz­ing the advantages of eight counter­weights, particularly in long-stroke, higher-RPM applications.

Center counterweights reduce bending moment deflection at the center main bearing, which helps preserve overall crank stiffness and resistance to deflection along the crank axis. In this regard some short-stroke applications favor them because they make balancing more precise. Adding counterweights at the center throws also helps lessen the bending loads and reduces the overall size of the counterweights. This is frequently applied to shorter-stroke cranks intended for very high RPM operation.

From a durability and power standpoint center counterweights may be more desirable than a low MMOI except perhaps in a drag rac­ing engine with short-event dura­tion and a requirement for very rapid transient acceleration. Designers also concern themselves with the hard­ness and ductility of cranks. The movement is shifting toward harder and stiffer cranks to maintain dimen­sional integrity for improved power production, but a crank must still be able to give a little without cracking. This is called ductility. Cranks need some measure of ductility to absorb extreme loads without failure, but they also need high tensile strength to provide consistent bearing loads. This is currently an area of ongo­ing investigation among crankshaft designers.

Shaped counterweights use various strategies to combat crankcase windage. This Scat crankshaft incorporates Aero-Wing counterweights, which are custom shaped to cut cleanly through the oil mass while the pendulum cut reduces weight and concentrates it in the most desirable location for optimum balance. (Courtesy Scat Enterprises)

Shaped counterweights use various strategies to combat crankcase windage. This Scat crankshaft incorporates Aero-Wing counterweights, which are custom shaped to cut cleanly through the oil mass while the pendulum cut reduces weight and concentrates it in the most desirable location for optimum balance. Courtesy Scat Enterprises


A billet crank begins with a solid billet of material weighing several hundred pounds. The setup and programming time for this procedure is lengthy and expensive. (Courtesy Scat Enterprises)

A billet crank begins with a solid billet of material weighing several hundred pounds. The setup and programming time for this procedure is lengthy and expensive. Courtesy Scat Enterprises


The machining process for a billet crank is long and precise, but the result is a superior crankshaft with uninterrupted grain structure and maximum strength. (Courtesy Scat Enterprises)

The machining process for a billet crank is long and precise, but the result is a superior crankshaft with uninterrupted grain structure and maximum strength. Courtesy Scat Enterprises

Counterweights are necessary to offset the forces of the reciprocating components as the crankshaft spins. They are a necessary and effective balancing component, but they also present various problems with regard to crankcase windage. Crankshaft manufacturers all have their own preferred methods of shaping coun­terweights to reduce parasitic drag. The most common approach is to either radius or knife-edge the lead­ing and trailing edges of the counter­weights so they cut through oil more easily. This practice is generally more effective when accompanied by new oil-shedding coatings that enhance the overall effect.

Counterweights can also be undercut and/or narrowed on one or both sides to reduce mass and frontal area as presented to the swirl­ing oil mass. Material is removed  from the sides and the leading and trailing edges closest to the center of the crank, leaving a concentration of mass toward the outer radius of the counterweight for optimum bal­ancing effect. Heavy metal or slugs of tungsten/nickel/copper alloy are often used to provide additional mass in cases where it is required to achieve proper balance. They are also used to reduce the frontal area of a counter­weight, allowing the removal of con­siderable material from the leading and trailing edge while maintaining adequate mass with the heavy metal inserts. Heavy metal inserts must always be installed perpendicular to the direction of rotation by pressing them into appropriately bored holes in the counterweights.

A popular feature of racing crankshafts is gun drilling the mains, which effectively lightens the crank, but has little effect on the rotating inertia of the crankshaft because material removed from the center of the crank represents almost no leverage or moment arm. The rear flange can also be machined in a star pattern to eliminate material and further reduce weight without com­promising the crankshaft-to-flywheel interface.

Operating Conditions

Like all objects, crankshafts are subject to the physical effects of mass and inertia. Crankshafts are constantly accelerating and deceler­ating. We are mostly concerned with the accelerating side and attending factors that contribute to deflection, inertia, resistance to transient torque, and the engine’s ability to accelerate through a given RPM range. These factors include crankshaft weight, stroke length, and the distribution of mass within the crank and com­panion components such as rods and pistons.

For a fixed mass (rod, piston, and attending components), a heavy crankshaft absorbs more torque and accelerates less rapidly. More torque is required to overcome its inertia or resistance to acceleration. Lighter cranks are desirable for this reason, but they may trade away durability and performance if reduced structure results in greater crank deflection, reduced stability, and the resulting effects on durability, ring seal, and in some cases, event timing. The prob­lem intensifies with stroke length; hence efforts are made to concen­trate crankshaft mass closer to the crank axis to reduce the moment of inertia or resistance to change in acceleration.

Savvy machinists keep balance weight as close to the axis as possible. When removing weight for balanc­ing purposes, they try to take weight from as far out as possible without actually drilling into the outer face of the crank throws, which can exac­erbate windage problems at high engine speeds. Many cranks have the crankpins drilled. This is generally the most effective way to lighten a crankshaft unless you get into expen­sive counterweight machining. Drill­ing the crank pins removes weight at the farthest possible point from the center of the crank, thus reducing the rotating inertia.

Crankshaft Oiling

Pressurized oil is delivered to the main bearings via the main oil gal­leries in the cylinder block. Once oil enters the main journals it flows through drilled passages to the rod journals to lubricate the connect­ing rod bearings. This oiling system is used in virtually every production automobile because it is the most efficient way to lubricate the rotating assembly.

Cross Drilling

Many years ago racers decided that cross-drilling an additional hole straight through the mains provided better lubrication to the bearings, likely a result of trying to fix an oil­ing problem or some other problem caused by improper clearances and bearings that were not originally designed for the stress of a racing environment. In some cases the through holes were drilled in both the mains and the rod journals. This did no real harm to engines that never ran elevated engine speeds. But racers soon learned that at very high engine speeds, a cross-drilled crank centrifuged the oil out of the main journal, preventing it from flowing freely to the rod journals.

All top-of-the-line racing crankshafts utilize “straight-shot oiling” in which the rods are oiled via a straight, uninterrupted passage from the mains.

All top-of-the-line racing crankshafts utilize “straight-shot oiling” in which the rods are oiled via a straight, uninterrupted passage from the mains.

Smaller-Diameter Rod Bearings

In any properly designed oiling system, cross-drilling is unneces­sary, particularly now that most race blocks feature priority main oiling. You won’t see any high-end racing teams in NASCAR or professional drag racing using cross-drilled crank­shafts. One important consideration regarding cross-drilling is rod journal size. Many of today’s race engines have reduced bearing diameter and width. While this does good things for bearing speed and friction reduc­tion, it increases bearing loading and makes adequate rod journal lubrica­tion more important than ever.

Conventional practice incorporates oiling holes in the rod journals that are chamfered to ease the flow of oil onto the bearing.

Conventional practice incorporates oiling holes in the rod journals that are chamfered to ease the flow of oil onto the bearing.

Cup teams have developed this to a high degree and they know what they can and can’t do. They run a 1.889-inch-diameter Honda rod bearing, which requires a specially designed crankshaft to ensure proper oiling. The smaller journals reduce bearing surface area and friction, but unit loading is increased. This tends to raise oil temperatures and pro­mote bearing fatigue, but it becomes an acceptable trade-off because the engines only have to last about 600 to 700 miles before a rebuild and the teams have tweaked oiling sys­tem efficiency to support the use of smaller journals. One area that they do not compromise on is the fillet radius between the bearing journal and the crank throws.

Smaller-Diameter Main Bearings

While there is power to be gained from these procedures, mistakes are unforgiving. Main journal diameter is a primary factor of crankshaft tor­sional stiffness and thus influences a crank’s resistance to bending and deflection under load. In other types of racing (like sprint cars and late-model dirt racers), builders prefer larger mains for greater durability because these engines make a lot of torque and they are expected to last for 1,000 laps or more without requir­ing a rebuild. Smaller mains have been tried mostly without success because they permit unacceptable crankshaft deflection under high loading.

Smaller mains reduce bearing speed and friction, but the stabil­ity and durability penalties are fre­quently unacceptable except in cases with exceptionally short strokes and minimal reciprocating mass—typi­cally short-deck, small-displacement drag racing engines that turn a lot of RPM with lightweight reciprocating components and short event dura­tion. Builders contemplating these types of engines often seek engi­neering expertise from crankshaft manufacturers to ensure the most compatible package.

Rod Journal Oiling

Of particular note are recent changes in oiling strategy for the delivery of oil to the rod journals. Engineers have long recognized the critical function of the hydrody­namic wedge of oil that prevents metal-to-metal contact in a fluid bearing design. They also recognize that the hydrodynamic wedge offers its greatest load-carrying ability at the center of the bearing journal interface as the wedge functionally tapers toward the sides of the bearing due to oil leakage. This is one reason appropriate rod side clearance is so important.

More recently, crank designers have turned their attention to the methodology of oiling the rod jour­nals, particularly as it applies to the common engine builder practice of grinding chamfers on the oil holes to help distribute the oil to the bearing. This practice has always seemed logi­cal, but there are several caveats.

The common rod journal oil­ing strategy is called “straight-shot oiling.” Each main journal feeds an adjacent rod journal via a straight passage from the main at a position of minimum load (upper) to the rod journal at a position of maximum compressive loading. Virtually every performance crank is drilled this way because it minimizes oil starva­tion caused by centrifugal force at high engine speeds and it provides optimum oil feed to support the hydrodynamic wedge at the point of highest load on the journal.

Still, two potential problems remain. First, the angled passage from the main naturally creates an ellipti­cal opening at the rod journal. This opening is often further enlarged by the chamfering many engine build­ers perform.

Second, the chamfer is ground in the direction of rotation to help feed the oil onto the journal, but some designers now see this as an interruption or potential point ofhydrodynamic wedge collapse directly in the center of the bearing where the wedge carries the greatest load. Also, with the reduced bear­ing width favored by most builders there is further potential to weaken the wedge at the point of highest loading. This line of thought was first introduced by Cosworth in the 1960s and its application remains controversial.

Some engineers propose drill­ing vertical journal holes centered in the path of each rod bearing so the resulting opening is perfectly circu­lar. Each rod bearing on the journal is then fed via an angled passage from its adjacent main bearing. The smaller (cross-sectional opening) ver­tical holes concentrate oil delivery in the center of each rod bearing where the wedge forms its highest pressure and the smaller circular oiling hole minimizes disruption to the wedge. A second method of accomplishing this uses drilled horizontal passages from the main journal through the crank pin overlap. The horizontal passages are connected by vertical passages and ultimately lead to the perpendic­ular oil holes in the journal.

While complicated and more expensive, it is seen as advantageous because each pair of rod journals can be fed from only one main bearing. Thus the rod journals in a V-8 can be oiled from mains 1, 3, and 5 while mains 2 and 4 are left to enjoy the full benefit of priority oiling without hav­ing to also feed a pair of rod journals. According to some research, mains 2 and 4 in Chevy V-8s are more highly loaded because they have the thrust bearing at the rear instead of on the center main. Thus the application of direct oil pressure and reduced leak­age ensures optimum lubrication of the most highly loaded mains.


These are some things to con­sider if you are experiencing or antic­ipating bearing issues. Consult your crank manufacturer for specifics and consider yourself in good company if they are willing to discuss the rela­tive merits of these emerging views. By all means, provide the crankshaft manufacturer with every possible detail about your application and its operational requirements. The more details they have, the more closely they can match a crankshaft to your specific requirements.

Engine Balancing

Engine balancing is often thought of as a “black art” practiced by wily machine shop wizards, but it’s not really all that mysterious. Thousands of highly competent engine shops do it every day and rarely experience balance-related engine problems. Bal­ancing has become even more precise with today’s modern computer-con­trolled equipment. Balancing com­ponents within 2 grams used to be commonplace in performance cir­cles, but not anymore. Many balance shops claim to balance within 1/2 gram or less for maximum precision and engine smoothness, but this is largely sales hype. While everything in modern performance engines is lighter and potentially more fragile, particularly in a high-speed envi­ronment, balancing components to within 2 grams is still perfectly acceptable even on the stoutest rac­ing engines.

Calculating Balance Weight

The primary difficulty with engine balancing is that some of the parts go round and round while oth­ers go up and down. Getting them to do it harmoniously requires pre­cision balancing to within 2 grams or less. Adjustable bob weights are used to simulate the weight of the parts during balancing. Rotating weight includes the big end of the connect­ing rod, rod bolts, and rod bearings plus a small amount (2 to 3 grams) to simulate the oil between the crank journals and bearings. Reciprocat­ing weight includes the small end of the rod, the piston, piston pins, piston rings and retainers if they are used, and a few grams for the oil that clings to the various moving parts. Once all of the component weights are equalized, the bob weights are calculated. A normal bob weight includes 100 percent of the rotating weight and 50 percent of the recip­rocating weight. The crankshaft is electronically balanced with the bob weights attached and normal bal­ance is easily achieved.

This forged Scat crank displays aero-shaped counterweights, heavy metal inserts for balancing, radiused journal fillets, and chamfered rod journal oiling holes.

This forged Scat crank displays aero-shaped counterweights, heavy metal inserts for balancing, radiused journal fillets, and chamfered rod journal oiling holes.


High-RPM engines are fre­quently overbalanced to improve high-speed balance with less regard to low-speed smoothness. The intent is to further smooth the engine’s state of balance in its intended operating range. Crank manufactur­ers view this with skepticism and most of them recommend the stan­dard balance percentages. In theory, when an assembly is overbalanced, the trick is to balance it so that any critical imbalance falls outside of the intended operating range (either above or below it). To accomplish this, the bob weights are adjusted from the calculated norm. Instead of adding 50 percent of the recip­rocating weight, the percentage is often increased to something in the 52- to 54-percent range.

If any of this is truly a black art it may be in actually determining the correct percentage of overbalance. Many builders claim to know from experience, but new combinations often require an educated guess and most builders don’t seem inclined to reveal their preferred overbal­ance percentages or the strategy they employ to determine them. The most common approach attempts to err on the conservative side, say 51 to 52 percent. If the engine’s performance and smoothness improves within its primary operating range, builders may overbalance it a bit more on the next go around.

  • Normal Balance = 100-percent rotating weight plus 50-percent reciprocating weight
  • Overbalance = 100-percent rotat­ing weight plus desired percent­age of increase in reciprocating weight (52 percent)

The overbalance percentage may cause dramatic vibrations outside of the engine’s normal operating range, but it is considered a minor concern since you don’t run it there for any length of time. Opinions vary regard­ing these balancing techniques. Many engine builders swear by the traditional 100-percent rotating and 50-percent reciprocating while some even prefer a small degree of under­ balance, say 48 to 49 percent of recip­rocating weight while others believe that an overbalance in the 52- to 53-percent range is highly advanta­geous for power and durability.

Overbalancing is a competition engine practice and not something normally done to a street or street/ strip engine that operates over a broader RPM range. For race engines it is thought to have the potential to save parts and improve performance by reducing vibrations that might be harmful to ring seal, valve train dynamics, and other factors that affect power within a specific power­ band. Also note that not all engine builders and crank manufacturers believe that it is necessary. If you feel the need to consider it, consult your crankshaft manufacturer first to get a recommendation.

Crankshaft Dampening

Another area considered a black art by many is the strange world of dampeners or, more correctly, tor­sional absorbers. The previously men­tioned crankshaft deflections and bending moments cause vibration in the crankshaft. A crankshaft may be viewed in much the same manner as a torsion bar in that it has a specific mass and a spring rate that resonates at some particular frequency as deter­mined by the engine’s continuous firing sequence and influenced by crankshaft material, torsional stiff­ness, length, stroke, reciprocating mass, and contributing moments of inertia from flywheels, clutches, and even rotating assemblies driven off the crank such as water pumps, alter­nators, and dry sump pumps.



Many crank manufacturers recom¬mend the smallest and lightest dampener available. This BHJ S.F.I. 18.1 unit for a big-block Chevy fits the description. Elastomer-type dampen¬ers are preferred because of their durability and superior high-frequency damping qualities. And because they are capable of damping all types of vibrations: torsional, axial, and radial.

Many crank manufacturers recommend the smallest and lightest dampener available. This BHJ S.F.I. 18.1 unit for a big-block Chevy fits the description. Elastomer-type dampeners are preferred because of their durability and superior high-frequency damping qualities. And because they are capable of damping all types of vibrations: torsional, axial, and radial.

A resonate frequency is recog­nized as the frequency change with engine speed and the amplitude or degree of excitation varies accord­ingly. Hence the engine may run very smoothly at some speeds and shake at others depending on the influence of the various contribu­tors. The frequency is the number of vibration cycles per second, as in 600 cycles per second, or 600 hertz. When a frequency is multi­plied by an “order,” it incorporates the number of times the excitation is produced (e.g., four power strokes per revolution in a V-8 engine). This represents a fourth-order excitation from which the frequency can be calculated. In a drag racing engine running at 9,000 rpm, the frequency of this fourth-order excitation calcu­lates as follows.


Order x RPM /60

4 x 9,000/60 = 600 hertz


The process of controlling this vibration is called attenuation and refers in this case to the torsional absorber. Absorber is the correct nomenclature since an absorber is designed to cancel out a specific frequency or order by oscillating in opposition to the vibration. For the purpose of this discussion I stick to convention and call it a dampener.

All racing engines need a damp­ener to control vibration. One exception is a sprint car engine, which operates without a flywheel. A flywheel is normally part of the absorption stack and operates in conjunction with the dampener to control vibrations. The lack of one in a sprint car changes the frequen­cies of the crankshaft and (in con­junction with the damping effect of the crank-driven water pump) allows them to operate safely, even in very high horsepower with frequent throttling under very high loading.

Written by John Baechtel and Posted with Permission of CarTechBooks

This Tech Tip is From the Full Book, COMPETITION ENGINE BUILDING. For a comprehensive guide on this entire subject you can visit this link: