Converting cylinder pressure into crankshaft torque, particularly since these pressures produce intermittent loads, involves a complex set of problems. Whether you build your own engines, pay for this service or provide it for customers, a fundamental understanding of the crankshaft’s function can be of value. Too often, it seems we become laced into concerns for crankshaft weight, durability and balance. Think of this component as a “flexible” part. As cylinder pressure creates intermittent torsional loads in a crankshaft, there is evidence of continual elasticity within the part—steel begins to act like rubber.

Even though pistons and connecting rods tend to become shock absorbers to these loads, radial deflection (both positive and negative) leads to “oscillations” created about the crank’s axis of rotation. This will be discussed in more detail later in the story. For now, it’s important to recognize that crankshafts, as a function of torque produced, tend to become “rubber” in the manner by which reciprocating motion (pistons) is converted into rotary motion (torque). As has been previously pointed out, engines are “parts packages” and should be treated as such where no single part operates completely independently. Accordingly, crankshafts can have influence on such variables as valve timing, intake manifold selection and header sizing. But first, we should discuss some of the more common issues.

What is Torque?

We’ve been through this before, but to refresh your memory, torque is the result of a force being applied on an object at some distance from its axis of rotation. If we allow torque to act during a period of time, we observe horsepower. Crankshafts make torque. Cylinder pressure causes torque to be made. In the case of engines, for the most part, how much torque is made depends upon cylinder pressure and the distance from crankshaft axis of rotation to the point of applied force—cylinder pressure acting on a piston/rod assembly. Crankshaft “stroke” plays a part in this factor. In other words, the longer the stroke, the greater the distance from the axis of crank rotation, and the more the torque produced. Long-stroke engines make more torque at lower rpm than shorter stroke ones, right?

Now, while this all may appear oversimplified, it’s important to understand how a crankshaft reacts to torque and the influence such reaction can have on other engine parts and functions. This leads us to a brief discussion about crankshaft materials and how these parts are constructed. Material selection and crankshaft design can affect a given shaft’s reaction to and delivery of torque.

Crank Materials and Construction

Essentially, the range of crankshaft materials runs as follows: billet steel, steel forgings, cast steel, nodular iron, malleable steel or (in some cases) cast iron. If we were to produce one crankshaft design and reproduce it in all these materials, the order of strength would approximately follow this same list. While cast cranks are typically less expensive than forgings, they can be produced in shapes not available with forgings. But dollar for dollar, forged cranks tend to be the better method of manufacture, certainly with respect to high output durability.

Often a subject of discussion and frequently believed to be critical in the design, modification and service life of a crankshaft, is how fillet radii are configured. If we were to perform a stress analysis test that included all other design features and conditions of a given crankshaft, fillet radii could be considered the most critical factor in overall design and/or modification procedure. There is belief among crankshaft manufacturers that the use of fillets of non-constant radius—sometimes called “non-circular” contours—is preferred over those of constant radius. Worst case, this is an area worth discussing with your engine builder or crankshaft manufacturer of choice.

Lightweight vs. Heavyweight Rotating Mass

Let’s talk about “transient” torque. For purposes of this discussion, transient torque is a measure of how quickly an engine can accelerate (including under load) through its useful rpm range. Stated another way, under sudden conditions of WOT, how fast it will span from low to high rpm. From a measurement standpoint, this is torque as measured on an inertia dyno: not a so-called “accel” test as performed on an engine dyno whereby there is a controlled unloading of the power absorption unit. This is “real-to-the-track” torque and it relates to an engine’s ability to overcome its internal resistance (inertia) to gaining rpm.

Based on these considerations, it is fair to say crankshafts don’t normally operate at constant rpm. They’re either accelerating or decelerating. Their resistance, in either case, includes static weight and dimensional landscape (stroke length, location and distribution of mass, etc.). Technically speaking, in a dynamic environment, crankshafts are continually changing potential energy into kinetic energy. So what, you say? Well, these are all factors that go right to the issue of how much torque is available at the output end of a crankshaft—and need to be considered for power optimization.

From a practical standpoint, acceleration of a “heavy” crankshaft absorbs more torque than one of less weight, thereby reducing the amount of net torque available to accelerate the car. But there are trade-offs in terms of durability, flexibility and potential longevity that should be considered when trimming crankshaft weight. Furthermore, it’s not all about weight. Placement of weight, relative to a crank’s axis of rotation, is also important. For example, the moment of inertia (resistance to a change in state of rotation or acceleration/deceleration) increases as weight is moved away from the axis of rotation. Even between two crankshafts of the same total static weight, the one with more weight near its axis of rotation will exhibit less resistance to a change in rotational speed; it has a lower moment of inertia. Keep this in mind when adding “heavy metal” to crankshaft counterweights during the process of dynamic balancing.

Finally, where total crankshaft weight may relate to overall flexibility, it’s best to err on the side of stiffness, if this can be accomplished by selecting a crank that trends toward stiffness in combination with durability and lightweight. Involved in making these type of choices, torsional vibration or deflection becomes another important issue.

Torsional Vibration and Damping

Visualize a single-cylinder engine, running in very slow motion. Upon a rapid increase in cylinder pressure, as during normal combustion, quick application of force is applied to the crankshaft. The “throw” connected to the piston/rod assembly is “deflected” in the direction of crank rotation. Stated another way, it gets a “kick in the butt” that amounts to an increase in crankshaft rotational acceleration. In so doing, the crankshaft’s throw acts somewhat like a “spring” in that once its maximum deflection is reached, elasticity of the crankshaft material causes the throw to act back against the load that caused its acceleration increase.

Until the next firing cycle and subsequent load on the crankshaft throw, this action and reaction condition continues as an “oscillation” of the crankshaft throw, even though the crank is in rotational motion. Note the accompanying illustration. If you were to view this example crankshaft along its axis of rotation, the oscillation described would appear as clockwise and counter-clockwise movement of the crank throw, all during normal rotation of the shaft.

Now, transfer this image to a multi-cylinder engine. Since the delivery of each firing impulse causes its respective crankshaft throw to experience torsional oscillation (or vibration), imagine the interaction of these pulses along the length of a crank and you begin to realize the complexity of the overall system. Moreover, since all the pistons are “tied” to this oscillating crankshaft, interruptions in “smooth” piston movement result. Plus, if the engine has a carburetor (and we know how piston motion affects a carburetor’s fuel metering signals), we have now linked a torsionally vibrating crankshaft to fuel delivery, and the corresponding effect this can have on carburetor calibration and power.

Further, if piston motion has been interrupted and is no longer directly coordinated with specific crankshaft angularity position, is it not reasonable to assume the relationship between valve timing and piston position as been upset? Well, it can be. Notwithstanding the fact that camshaft timing chains or gear sets become “shock absorbers” to the torsional oscillation of crankshafts, it’s possible to address this problem by the proper selection and use of torsional oscillation (vibration) dampers. So, we’ve come full circle, so to speak.

While it is not the purpose of this article to identify and justify specific brands or types of torsional vibration dampers, it is our intent to point out and substantiate the absolute importance of their use and the influence they can have on peripheral engine parts and functions. The bottom-line to this particular issue is that crankshafts are going to exhibit torsional deflections and oscillations.

Tuning Crankshafts

What happens to net torque when a crankshaft deflects against its normal direction of rotation? Power is lost, plain and simple. How much power and where in the rpm range does this occur? Well, there are ample variables involved and the rules of thumb recognize that the amplitude and rpm at which maximum and minimum deflections (and power losses) occur varies, and includes: the level of power produced, range of rpm, dynamic properties of the engine’s rotating and reciprocating assembly, static weight of the moving parts and other factors.

Based on orders of critical engine speed, that is, first-order, second-order, etc., there will be certain rpm at which “negative torque” (counter-rotational crankshaft deflection) is greater than at other rpm. For example, a negative torque measured at 3,500 rpm may be less at 4,000 and then more at 4,500. While oversimplified, this projects the notion that torque losses occurring from negative deflection of the crankshaft are cyclical in nature, varying in magnitude and at engine speeds according to a crank’s modes of vibration. Specifically, in an engine producing 540 shaft horsepower, I’ve seen negative torque losses approaching 35-40 lb-ft of torque, in an otherwise torsionally-damped engine. Other examples have been greater and less. The point is that such losses can be neither insignificant nor unworthy of elimination or reduction.

What all this implies is that because the engine speed and extent of power loss are variable, cannot crankshafts and dampers be “tuned” to these rpm (or frequencies)? And the answer is, yes. From a practical standpoint, damper tuning is often more easily accomplished than crank tuning—unless, of course, you’re an “accomplished” engine balancer. Know the range of rpm where the engine will spend most of its on-track time and then target the damper selection accordingly. If you have difficulty obtaining this type information from your choice of damper manufacturer, be persistent. You will likely come away from the experience with: 1) the correct part, 2) an urging that this is unimportant information, 3) it can’t be done, or 4) don’t split horsepower hairs to this extent. If you’re trying to increase parts life and make more power, opt to accomplish point #1. (For additional information on parasitic power losses, refer to the July ’02 issue of Circle Track .)

Crankshafts and Windage

Even the use of dry-sump oiling systems does not prevent crankshafts from being influenced by the migration of engine oil from the crankcase to pan. It’s a given that oil will become attached to a spinning crankshaft, adding to its “dynamic” weight and inertial resistance. In fact, high speed photography reveals that the oil can look like toffee rope wrapped around the spinning crankshaft. By the use of various oil pan devices, attached oil can be physically removed from crankshaft surfaces. And since tapered edges tend to release/shed oil more easily that “squared off” portions, the shaping of counterweights is another common method of addressing the problem. Special coatings, of the type that reduce surface tension, applied to crank throws and counterweights can also be used. But in any event, assume that oil passing by the crankshaft and pulses of air created by rapidly descending pistons and the volume of air entrapped up inside them will combine to increase the importance of reducing the amount of oil that becomes a “passenger” at high crank rpm.

Crank’s Influence on Parts

For example, let’s say you decide to increase crankshaft stroke. This implies an increase in low- and mid-rpm torque is desirable. Therefore, so would the selection of an intake manifold, camshaft and header system that favors torque output in the same range of engine speed. Similar to events from lengthening connecting rods (all else being equal), a stroke increase changes both the rate at which intake flow velocities are created (vs. crank angle). It also affects piston dwell around TDC and BDC. This suggests a re-think/adjustment of sparking timing (at least initial spark) when comparing engine applications of a stroked and un-stroked crank.

Maybe you’ve decided to decrease, by whatever means, the inertial resistance of the crank you’re using. That suggests an engine that will accelerate quicker and, likely, one that would benefit from an increase in compression ratio. Short of performing single-cylinder dynamic pressure analysis as a function of crank angle (perhaps with Engine Cycle Analysis or comparable in-cylinder measurements), it’s possible to re-coup some power lost to “retarded” rear cylinders by juggling rocker ratios that serve them. Consider what you’d do to improve higher rpm power with valve timing that is acting retarded—maybe a shade higher ratio, for an initial test.

It’s safe to assume, although you may not know the exact amount of crankshaft deflection (front to rear), that an engine’s rear cylinders are “running behind” those in the front. Depending upon how much deflection is occurring, rearward cylinders may be running in a “retarded” position, relative to those in front. Some adjustment to spark timing to these cylinders can be helpful. Of course, camshafts also deflect, so this condition of “retard” can result from a stacking of events from deflection in both cam and crank. Again, to help minimize this problem, reducing the amount (frequency and amplitude) of crankshaft deflection by the sensible use of a torsional vibration damper is virtually mandatory to extending parts life and optimizing power.