It may come as a surprise, but your choice of crankshaft can include concerns about valve timing, header sizing, and (in some cases) intake manifold choices. In the process of making such a selection, it may be helpful to first consider the functional environment in which cranks operate.
For example, the flow of cylinder pressure as applied to a crankshaft is neither linear nor constant, in terms of loading amplitude. And while such pressure is applied in a pulsating fashion, as a function of rpm, it nonetheless amounts to a series of loads applied to an object that tends to deflect in both positive (direction of load) and negative (reaction to loads) directions. Given sufficient time to react, this oscillating of positive and negative deflection of a crank tries to become oscillatory about the part's axis of rotation. It's during this conversion of reciprocating motion (pistons) into rotary motion (torque) that crankshafts must absorb the punishment.
Now let's get the subject of torque out of the way. Torque may be defined as a load imposed on an object at some distance from its centerline or axis of rotation. If the load is measured in pounds and distance in feet, the product of the two is defined in pound-feet. Whether motion is created or not, the load applied amounts to torque. Note there is no reference to time. Torque is applied but there is no element of time. However, when torque is applied, causing rotary motion to occur, time enters the equation and the result is horsepower (torque measured over a period of time).
What does this have to do with a crankshaft? Well, every firing impulse loads a crank at some point along its length. The load initially creates rotary motion, but even as it begins to rotate, that part of the crank (crank pin and arm) begins to oscillate; first in the direction of rotation and then opposite to that direction... even as the crank continues to rotate in its primary direction. Imagine the complex system of firing impulses created throughout a given rpm range, turning a crankshaft into a maze of elastic loadings that are essentially unavoidable. It's little wonder why crankshafts are a vital and critical part of any engine intended for racing?
Now let's talk about crankshaft mass (weight), particularly as an engine is accelerated through its operational range of rpm. Suppose we link this to another term; "transient torque." Especially under load, this is a condition that contemplates how quickly rpm can be achieved. From a practical standpoint, we're talking about how rapidly an engine can accelerate under load at sudden WOT Interestingly, if we were tasked to measure an engine's transient torque characteristics, it would require the rotational acceleration against a mass (such as a flywheel) that approximates the weight of the car in which it will be installed. This type of test protocol shouldn't be confused with so-called "acceleration" tests that simply involve the controlled unloading of a dyno's power absorption unit at X-rpm/second. But we digress.
It's fair to say that racing engines are typically required to accelerate and decelerate on a fairly consistent basis, Superspeedway Cup engines leaning toward an exception to this notion. As a result, crankshaft mass can obviously affect how quickly an engine accelerates, or decelerates (brakes), for that matter. Of course, there are factors other than mass that affect these changes in crankshaft energy. Included among them are the type of crank material, stroke length, distribution of mass, throw design, and ability to shed oil (particularly in wet-sump engines). Keep also in mind that as a crankshaft is distorted or "flexes" during normal operation, energy is lost to the system that tends to reduce net useable torque at the flywheel.
Not to be overlooked is another factor involving crankshaft weight placement. Let's say you have two cranks of virtually equal weight but with different mass location. The terms that are applicable here address differences in moments of inertia. For our purposes concerning crankshafts, we'll define moment of inertia as a crank's resistance to a change in rotation; e.g., during crankshaft acceleration or deceleration. In this instance, a crankshaft's moment of inertia increases as mass is located further from the axis of rotation. And, as you'd expect, the MOI decreases as weight is moved toward the rotational axis. Essentially, as a rule of thumb (especially for longer life), it's wise to consider crankshaft stiffness in lieu of a lighter weight version that will absorb more energy (torque) from a lack of stiffness.
We've used the following analogy in prior discussions, but it's particularly appropriate for describing the damping of torsional vibrations in a crankshaft. So, if you will, visualize a single-cylinder, four-stroke-cycle engine. At each firing cycle, the crankshaft receives a positive force of acceleration, in the primary direction of rotation. At the same time, the crank throw will flex, also in the same direction. Because of its elasticity, the crank throw will then flex back in a direction opposite to its rotation. Between this point and the next firing cycle, the crankshaft will continue oscillating back and forth, all during its primary direction of rotation.
Now, visualize a multi-cylinder engine in which this series of events is taking place throughout the firing order. It's not a stretch to correlate the unilateral influences an oscillating crankshaft has among all the cylinders of such an engine and how it might be compared to reversion pulses ("cross-talk") among cylinders connected through a single-plane intake manifold. Moreover, each crank throw oscillation impacts piston motion, simply because of the mechanical connection to the crankshaft. Add to this mix the fact camshafts experience torsional wrap-up (from an engine's front to back) which can sufficiently change valve events (relative to piston motion), creating a potential need for cams with slightly advanced valve timing for cylinders toward the rear of an engine. But again, we digressed.
Let's button this up by reviewing a few additional basics and include some crankshaft tips for both choice and maintenance.
- Don't take the issue of crankshaft oscillation lightly. These perturbations can manifest themselves in premature wear of timing chains and gears, disruption of carburetor fuel metering (pulsating flow in the inlet stream) and an incorrect relationship between valve motion and piston position.
- The use of a good torsional vibration damper is a wise investment. Just make certain you survey all the choices, consult with experienced engine builders and make the best selection possible. A few extra dollars for a good damper can pay dividends from longer parts life and increased power. I have seen damper tests that revealed torque losses on the order of 30-40 pound-feet (corrected) in a race engine making roughly 500 hp. Such losses will vary throughout a given rpm range, simply because these are harmonics that increase and decrease with changes in rpm.
- It's been said that the most important considerations in crankshaft selection are quality of the material used, "clocking" of oil holes, position of the counterweights and distance of these weights from the crank's centerline of rotation.
- When replacing bearings in a previously run engine, make certain the block is align-bored first. Putting new bearings in used bearing housings can lead to spun bearings. This is because during normal operation of an engine, as bearings are wearing, the housings will distort to an out-of-round concentricity. Unless the bores are realigned, chances are good the new bearing will contact the crank bearing surface where it should not... .and you know the rest.
Finally, it's safe to assume crankshafts do more that convert vertical motion into rotational motion. They experience inordinately high mechanical loadings, are required to maintain some degree of elasticity, absorb a complicated array of forces that can cause damaging distortion and not become so flexible as to reduce net power at the flywheel. How much more complex could the task be for a single engine component?
Just make certain you survey all the choices, consult with experienced engine builders and make the best selection possible