Words: Richard Holdener

Given the interrelated dynamics of the internal combustion engine, it might be hard to select one component as THE most important, but rest assured, no engine will run without the all-important crankshaft.

The truth is that the top-side trio, namely heads, cam, and intake, get all the love when talk turns to performance, but no less important is that hefty chunk of spinning metal buried deep within.

The mere fact that we have come to expect a crankshaft weighing 40, 50, or 60 pounds (or more) to rotate 100 times per second means the component at least deserves our attention (if not outright applause). So much is expected of a crankshaft that we thought it might be a good idea to take a closer look at the differences between the two most popular types.

We also asked Mike McLaughlin, tech consultant at Lunati, to give us a few recommendations of how he determines what type of crank a given engine needs. Everyone knows that a forged-steel crank is stronger than a cast crank, but there is a great deal more information than this simplistic view might suggest.

Most enthusiasts seem content in their knowledge that a forged crank is superior to its cast counterpart, but did you know that many forged cranks differ in their forging process, material, heat treatment, and finishing process?

The term forged crank has become all but generic, but the differences can be as significant as those between a casting and forging itself. For the uninitiated, the two major differences between cast and forged cranks include the material and forming process.

As the name suggests, cast cranks are created by pouring molten material (typically cast iron) into a mold to create a raw casting. The benefits of this process casting include lower cost, inexpensive tooling, and minimal machining required to produce a finished product. Given the list of benefits, it is not surprising that the vast majority of OEM cranks are created using this process.

By contrast, forged cranks are created by placing a hot chunk of rolled steel between two forming dies. Pressure supplied by a forging press is used to force the rolled steel into the basic shape of a crankshaft.

When combined with the proper material, the forging process obviously produces a stronger product, but neither the forging process nor the materials involved are universal. The majority of forged cranks are created by forging dies that produce cranks with all the throws in one plane. To finish the cranks and index the throws at the required 90 degrees, the raw forgings are actually rotated (twisted). It is also possible to produce stronger, non-twist forged cranks, but the tooling and final product are both more complex and expensive.

The final, strongest (and most expensive) process actually starts with a round chunk of bar stock (usually 4340) and the crankshaft is then CNC machined from the solid billet. Like the forging process itself, the material chosen for the crankshaft ultimately affects the strength. Factory forged cranks generally rely on plain carbon steel, but material upgrades include 5140 chromium, 4130 molybdenum, and finally 4340 nickel chromium alloy.

The one downside to forged cranks is the need for heat treating. Where the journals on cast-iron cranks becomes work hardened during the machining process, forged steel cranks are not so lucky, and must instead be heat treated after machining.

Common heat treating methods for forged cranks include induction hardening, tuftriding, and nitriding. Induction hardening relies on a high-frequency magnetic field to quickly heat the surface before quenching. Employed by most OEMs for their forged cranks, induction hardening offers the benefits of cost, speed, and deep surface penetration (meaning it can be re-machined without the need to re-heat treat). Both tuftriding and nitriding feature surface hardening though chemical compounds. Tuftriding involves dipping the crank in a heated cyanide compound while the nitriding process features the introduction of a chemical gas in a heated furnace.

Once heat treated, the forgings offer both superior tensil strength and improvements in elongation (essentially the amount of deflection before breakage). In terms of tensil strength, cast cranks start near 60,000 psi, but forgings can more than double this rating to (as high as) 125,000 psi.

The elongation rating of factory cast cranks is an indication of just how brittle they are. The rating of 3 percent can be nearly doubled with the use of nodular iron, but even these pale in comparison to the 20+-percent rating offered by high-end, forged cranks.

The final piece of the puzzle to consider is the finishing process. We won’t delve too deeply into this because there are so many factors involved, but McLaughlin gave us a few things to consider.

“The finishing process is kind of like fine tuning,” he notes. “Even if the materials and forging were the same, there are so many different factors that make a crank distinct to a given company. Things like quality of manufacturing, accepted tolerances, the machine used, and even the employee doing the work. But some general things to look at would be the different ways to shave weight, reduce windage, and maintain balance.”

While technical talk is all well and good, what does it all mean to the average consumer? With the Internet abuzz with the ability to exceed 1,000 hp with factory cast cranks, why on earth would anyone need to spend the money on a forging?

Crank strength is much more than a function of what it will survive on a single hero run. Factory cranks were designed by the OEMs to last 100,000 miles or more (literally millions of cycles) and that life span is a function of more than just tensil strength or elongation. It is possible for even the wimpiest of cast cranks to survive indefinitely if run at lower engine speeds and power levels.

If you have a hard time grasping this concept, think about how far you could walk at a slow pace versus run at full speed.

Run at full speed and load, the cycle life of a crankshaft quickly diminishes, which is why a 400-horsepower small block run in a daily driven street Camaro might work well with a cast crank, but the same motor powering a pleasure boat would require forged components. The time spent at high load and rpm determine the need for a forged crank every bit as much as a peak power level.

So if you have a performance engine, you now know that a forged crank is the way you should go for longevity, but which one?

“Lunati has two lines of cranks: Voodoo and Signature Series,” McLaughlin says. “Both cranks are made from 4340 non-twist steel forgings, so there isn’t a huge difference there. The difference comes in the heat treat.

“The Voodoo line is induction hardened, which will save you some money and is great for most applications under 1,000 horsepower. If you are running a normally aspirated engine, or something with mild boost (10 psi or less), or a very light shot of nitrous, then your engine will be happy with the lower priced Voodoo crank.

“The signature series on the other hand, gets a nitride heat treatment. It can handle the higher horsepower and rigors associated with high-boost and heavy nitrous applications. There is some more work on the finishing process for the Signature Series as well, like wing cutting the counterweight to reduce windage, and on big-block Chevy cranks we center counterweight them to maintain the balance.”

No discussion on crankshafts would be complete without covering stroker cranks. Stroker cranks are simply cranks equipped with increased stroke length (the distance the pistons moves from the bottom to the top of the bore). McLaughlin says that the majority of cranks that come out of Lunati are stroked.

The benefit of increasing the stroke length is increased displacement, which aides in power production. It is easier to make power (and especially torque) with increased displacement. The difference between a 350 small block and a stroker 383 can be 50–60 lb.-ft. of torque, and best of all, that additional power comes at every rpm. Having an extra 40–50 horsepower at the redline is good, but having and extra 50–60 lb.-ft. of torque down at 3,000 rpm is even more useful and something that can be enjoyed on a daily basis without fear of invoking the wrath of the boys in blue.

Best of all, if you are in the market for a crank, the extra stroke (and attending power increase) doesn’t cost a cent, as whether cast or forged, a 350 crank generally costs the same as one for a 383 (Ford equivalent would be 302 vs 347).

Source

Lunati
lunatipower.com