Crankshafts Part 2

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In our last issue we started our look at racing and high performance cranks. If you spin the browser back, you’ll see we examined cast cranks and forged cranks. Tom Molnar is our guiding light for this series, and that’s good news, because he’s a true crankshaft and connecting rod guru! This issue, we’ll examine billet crankshafts along with tech on journals and counterweights for all crankshafts (billet and otherwise). Check it out:

Billet Steel Crankshafts: Every billet steel crankshaft begins as a large blank of high strength steel alloy. That large blank of steel is called a “billet.” It’s thought by many that a billet crank is the strongest possible of all crankshafts, but that isn’t necessarily true. As Tom Molnar points out, it is easier to get special alloys in a billet than it is to get them in a forging; however, there are some areas where a forging may actually prove stronger because of the manufacturing process. With a forging, the grain structure is interwoven. That isn’t the case with a billet (but keep in mind Tom’s earlier comments noting that after heat treating, the grain flow is gone and even under a scanning electron microscope it is difficult to detect if the part was forged or made from billet). In the case of a billet crankshaft, there are no stress risers because the metal grain structure runs parallel to the length of the crankshaft. Another advantage of a billet crank is that with large stroke applications, a billet might make it easier for some manufacturers to obtain healthy journal overlap (this isn’t so in all cranks, however). Molnar states that every crankshaft needs journal overlap, which is simply how much the main and rod journals of the crankshaft overlap each other. When the crankshaft stroke is increased, the rod journals are moved away from the main journals. This eventually has an effect of reducing the ultimate strength of the crankshaft.

The cost of a billet also proves to be a big disadvantage. This is due to the considerable amount of machining necessary. The bottom line here is, a billet crank can cost upwards of several thousand dollars - even higher, depending upon the application and the options the purchaser selects.

An important part of any crankshaft (forged, billet or cast) is the counterweights. Here’s what to consider:

Counterweights: When you consider the weight of the rod journals, the connecting rods, pistons, wrist pins, locks and rings, that weight in the reciprocating assembly must be counterbalanced by something. If it isn’t, the engine will shake violently during operation. The task of counterbalancing is handled by the counterweight on the crankshaft. A crankshaft found in something like a common Chevy V8 engine usually features a half-dozen counterweights. That's a common practice with most domestic V8 engines.

The problem with having six counterweights is that there are actually eight cylinders in a typical V8 (!). When a V8 crankshaft is fitted with eight counterweights, it is “fully counterweighted.” Fully counterweighted cranks are certainly heavier than their six-counterweight contemporaries, but with a fully counterweighted crank such as this there tends to be less bending (but not twisting) on the center mains under load. With a long stroke crank, full counterweights have an advantage because they’re easier to balance (for one thing, they require less heavy metal). Another advantage is that the actual counterweights can become smaller when there are eight of them.

Tom tells us this important info, which many tend to overlook: “Counterweights have two jobs on a crank. One, of course, is to counter the out-of-balance forces produced by the mass on the rod pin side of the centerline. The other is to reduce the bending forces. You can have a perfectly balanced crankshaft that runs smooth, but due to improperly placed counterweights, the crank could be seeing large bending forces that will break the crank due to fatigue. An example of this is on a 4-cylinder crank. A 4-cylinder crank can be balanced without any counterweights, but at high RPM with the rod pins, rods etc. pulling in one direction and the center rod pins pulling in another, it induces huge bending forces that over time with break the crank.”

Once a crank is turning inside the engine, it pretty much functions just like the blades of a kitchen blender. It chops and whips crankcase air, oil and exhaust blow-by, which in turn creates windage. Windage inside a race engine can cost as much as 40-50 horsepower at wide-open throttle. In effort to slice through the windage you’ll sometimes find high performance crankshafts with specially profiled counterweights. They’re often rounded or heavily profiled with a “knife edge” on the leading edge of the counterweight. Does it work? There’s not a lot of consensus – however, several well-respected engine builders interviewed tend to lean toward the rounded counterweight profile instead of a sharp knife-edge layout.

Journals: The cubic inch displacement of an engine is determined by the number of cylinders, the bore diameter and the length of the crankshaft stroke. Stroke is the distance from Top Dead Center (TDC) to Bottom Dead Center (BDC). The formula for calculating displacement is simple: Bore X Bore X Stroke X 0.7854 X Number of Cylinders. Here’s an example: 4.5 X 4.5 X 4.25 X 0.7854 X 8 = 540.7 or more commonly, 540 cubic inches.

Crank journals can (obviously) be of different sizes. In a race engine, it’s not uncommon to reduce connecting rod journal diameter in order to reduce bearing speed (this reduces internal friction). That’s where, in some racing venues (NASCAR for example), it’s common to see Honda-sized rod journals used in relatively high RPM engines. These journals measure 1.88 inches (with the minimum size of 1.850-inch allowed in NASCAR). By comparison, a small journal small block Chevy has a rod journal size of 2.00 inches, while a big journal small block Chevy crank has a dimension of 2.10 inches.

Molnar also tells us: “By reducing the rod journals you end up with less overlap, as stated earlier. With longer strokes, small rod pins and small mains, due to less overlap, the crank starts to become a piece of spaghetti. If energy produced by the burning fuel is used to twist and bend the crank, it is energy that is not getting to the tires, and a portion of it is wasted.”

That’s a wrap for this issue, but we’re far from done. Next time around, Tom Molnar will dig deeper into journals. We’ll also get into cross drilling, polishing and chamfering oil holes. Watch for it.

In this photo, if you examine the width of this crankshaft, you’ll note it is a fully counterweighted example. That means there are counterweights for all of the throws. More in the next photo:

With a typical North American V8, the two center counterweights are often left out. A fully counterweighted crank such as this offers a number of benefits. The text offers more info.

The counterweights on this crank have been lightly profiled. They are certainly not knife edged, however. Knife edging is a shape that is machined into the counterweight, with the theory being it can cut through windage. Not all pro engine builders buy into knife edging.

It should be no secret that the main and connecting rod journals of the crank must be precise in size and shape. This Molnar Technologies crank arrived completely finished, indexed (for correct stroke) and ready to go.