Last month we had a close look at the composition of pistons, and what this means in the real world for your performance engine. Obviously pistons are only a small piece of the puzzle when it comes to selecting parts to build a strong and reliable engine that will meet all your performance criteria. This month we will take an in-depth look at crankshafts and what you need to know to choose a suitable part.
Cast Versus Forged
Much like the pistons we discussed last month, there are two typical manufacturing methods for producing OEM crankshafts. For many years, the only offering available in road cars was a cast crankshaft. In this manufacturing technique, molten iron is poured into a crankshaft-shaped mould and allowed to cool. The process is cheap and the tooling used has a long service life, making it ideal for mass production. The cast item is also very close to the finished shape of the crankshaft, which reduces machining time. The downside is that the cast material has a low density and no real grain structure, which results in a weaker product. A cast crankshaft is also quite brittle, which is not a desirable trait for a performance engine.
Forged crankshafts, on the other hand, are formed by placing a hot piece of steel between two forging dies and forcing it into the shape of a crankshaft under immense pressure. This results in a denser material with a superior grain structure. That makes a forged crankshaft stiffer and stronger than its cast counterpart, while it is also less brittle, which improves its reliability.
One more consideration with a forged crankshaft is the way the crank pins are positioned. The cheapest method is to use a single-plane forging die, which forms the crankshaft with all the crank pins in a single plane. After forging, the crank pins are twisted to index them at the required angle, which imparts significant stress into the crankshaft. A better technique is to forge the crankshaft with the crank pins already indexed in place. This requires much more complex tooling, which adds to the expense, but the finished crankshaft is superior.
There are a number of ways to tell if you are looking at a forged crankshaft or a cast item, with one of the easiest being to inspect the parting line down the throws of the crankshaft. A cast unit will have a sharp, thin parting line from the two halves of the mould, while a forged item has a wider (15-20mm) and smoother parting line.
A billet crankshaft represents the high end of the performance crankshaft market. This process begins with a solid bar of high-quality steel, which is machined in a CNC lathe to remove everything that isn’t part of the crankshaft. The biggest advantage of a billet crankshaft is that the material is typically superior to a forging, and can be much more closely controlled during selection and manufacture.
While the forging process results in a uniform grain structure, which is an advantage, the process still imparts stress into the crankshaft. A billet crankshaft is typically stiffer, more resilient than its forged counterpart and significantly lighter, but the time-consuming and complex machining makes this the most expensive option.
Unsurprisingly, a variety of materials is used in the manufacture of crankshafts. A cast crankshaft will typically use a ductile iron, which may have a tensile strength in the range of 6205 bar (90,000psi), but this material is quite brittle with an elongation rate of three to five per cent (elongation rate refers to the material’s ductility). An OEM forged crankshaft is usually made from plain carbon steel, which has a tensile strength of around 7580 bar. While the tensile strength isn’t a huge increase, this material also has an elongation rate of around 20 per cent, which makes the finished crankshaft far stronger than a cast item.
Aftermarket forged crankshafts are typically manufactured from 4130, with a tensile strength of around 8274 bar, or 4340 with a tensile strength of around 9653 bar. Billet cranks typically use a material such as 6415 billet steel, which has a tensile strength of more than 11,032 bar. As you would expect, the stronger alloys of steel are more expensive.
A newly manufactured crankshaft is too soft to give a useable service life without being heat treated to provide a hardened surface finish to the journals. The hardening process also leaves residual stresses on the surface of the crankshaft, which helps resist cracking and improves the crankshaft’s fatigue life. There are a variety of options available to achieve these aims, but induction hardening is a favourite because it is quick and cheap.
Induction hardening uses electromagnetic induction to heat the surface of the crankshaft, after which it is quenched. This results in a very hard surface that may be 1.5 to 2mm deep. However, induction hardening causes uneven heating and cooling, which results in stresses being introduced to the crankshaft. A further disadvantage with this method is that the fillet area of the crankshaft is not usually hardened, hence there is no benefit to the most stressed part of the crankshaft. While it is a quick and cost-effective process, it is not ideal for a performance crankshaft.
Nitriding is a chemical hardening process in which the crankshaft is heated in a vacuum and exposed to a nitrogen-rich gas such as ammonia. Nitriding does not require as much heat as induction hardening, and hence doesn’t result in stresses in the crankshaft, which makes it a better choice for a performance crankshaft.
The surface hardness of a nitrided crankshaft may only be around 0.25 to 0.5mm deep, so if the crankshaft is ground it will need to be re-nitrided. Nitriding also dramatically improves the steel’s fatigue properties, so it is the best option for a performance crankshaft.
It seems reasonable to expect that if a crankshaft passes a crack test during a routine rebuild then it is good as new, but this isn’t always the case. All metals have a fatigue life, which means the part can only withstand being loaded and unloaded a certain number of times before it will fail, even if the load never actually exceeds the ultimate tensile strength of the component. The number of load cycles the part can withstand is related to the amount of load applied to it. A relatively unstressed crankshaft may last almost indefinitely, while a similar unit that has been run near to its tensile strength limit may be close to failing. This is an important concept to keep in mind when considering reusing parts in a highly stressed engine.
Most crankshaft failures occur at the fillet, where the con rod or main journals meet the throws of the crankshaft. The fillets are the most stressed part of the crankshaft, and are prime candidates for cracking. The finish of the fillets can have a big impact on the stress concentration in these areas, and hence the strength and reliability of the crankshaft. Most OEM crankshafts have an undercut radius at which the corner of the journal is machined away, leaving a groove. To improve strength, performance crankshafts will normally employ a fillet radius that means extra material is left and the journal diameter is smoothly rounded into the throws of the crankshaft. The downside with a full radius fillet is that OEM bearing shells will often not have sufficient chamfer on their edges to clear the fillet radius, requiring either special bearings or modifications to the stock part.
Choosing the right parts for a performance engine can be confusing, and the world of aftermarket parts is daunting. While selecting a crankshaft for your engine needs to be done in consultation with your engine builder, you should now understand the pros and cons of what is available and be able to match them to your application and budget.
Words: Andre Simon
This article is from NZV8 Magazine issue 64. Click here to check it out.