Cast Vs Forged crankshafts

After Wednesdays post about cast vs forged pistons, I thought I would write about forged vs cast crankshafts today.  Unlike with pistons, there are more materials to work with when talking about cranks.

Starting with cast cranks, there are three options in increasing order of strength:

Cast iron

Nodular cast iron

Cast steel

The difference between cast iron and nodular cast iron is the shape of the graphite particles in the iron.  Cast iron is not just iron, pure iron is too soft to be used as a crankshaft.  In regular cast iron, the graphite particles are flakes.  In nodular cast iron, they are spherical nodules.  This gives the cast iron more strength and flexibility.

Cast steel is stronger due to the fact that steel is stronger than iron.  To make a cast crankshaft, they heat up iron or steel until it is melted then pour it into a mold, the same as with cast pistons.  Like cast pistons, there is less finish machining to do so they cost less.

With forged steel cranks, a metal bar is heated until it is soft then pounded into the rough shape of the crankshaft.  From there it is machined into the final shape.  There is more machine work involved, along with the cost of the forging equipment it makes for a higher cost of the final piece.  There are different alloys of steel used, that give a crank different levels of strength.

There is one more option for manufacturing a crankshaft, and that is a billet steel crankshaft.  To make them, a much larger bar of steel is forged into a cylinder shape as large as the total diameter of the finished crank, then it is machined to the final shape.  As you can imagine that is a lot more machining and the final cost reflects this.

So what kind do you need?  As it often does, it depends.  A cast iron or nodular cast iron crank is probably enough for most mild street motors.  Higher horsepower and higher sustained RPM cause the stress on a crank to increase.  Most aftermarket crankshaft manufacturers will have an approximate horsepower level that they rate their cranks at.

One thing to think about is that depending on the alloy used, a cast steel crank can be almost as strong as a forged crank.

Just check with the crank manufacturer to see what horsepower level their crank will handle and make sure that is the same or more than what your engine will produce.  you need to remember that if you want to add more power later with boost or nitrous, you need to factor that in when you build your engine.

As far as forged versus billet, it depends on who you believe on which one is stronger, but given the price of a billet crank, there should be some advantage to it.  There is a theory that while forging aligns the crystals of the steel, all the bending that is needed to make the final shape breaks some of those.  Versus the billet, that while it doesn’t use as much pressure as forging and doesn’t produce as tight of a grain, it isn’t disrupted by bending.  Instead it is machined to the final shape.

If you are looking at forged cranks, then you are probably working with someone who has a preference based on their experience and it would probably be good to follow their advice.


Comments

Cast Vs Forged crankshafts — 2 Comments

    • I have heard that it is good up to 8,000 RPM, but I am not sure how reliable the source is. I think that the pistons and rods would give out before a steel crank, but if you are in doubt, an aftermarket crank may be cheap insurance versus joining the “I just drove over my crankshaft club”.

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