Information About Crankshafts - CHP How It Works

Crankshaft Experts Talk Metallurgy, Race Balancing, Counterweight Profiling, Heat Treating, Production Techniques, And Much More.

Stephen Kim May 28, 2010 0 Comment(s)
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All of a sudden, the ubiquitous 383 is the wee-man on the block. With 400-plus cubic-inch small-blocks becoming more the rule than the exception, yesteryear's stroker combos have developed a bad case of Napoleon syndrome. The 540ci Rat is the new 496, and if you want to build a 632, tracking down the short-block components necessary to fulfill your displacement fantasies-and finding enough cylinder head to feed it-is only a mail order catalog away. What makes it all possible is the massive influx of affordable stroker crankshafts. Gone are the days of offset grinding factory cranks for miniscule displacement gains, and scouring salvage yards for that elusive 400 crank for your small-block build. Today's aftermarket cast-steel cranks can take a serious beating, and modern 4340 alloys can handle as much abuse as you can realistically throw at them.

1007chp_02_o Information_about_crankshafts Crank 2/8

Even so, there are some very basic questions that need to be addressed when shopping for a new crankshaft. Everyone knows that forgings are stronger than castings, but how much stronger are they, and how much power can they take? Where do billet cranks fall into the hierarchy? What differentiates the various grades of alloys? What are the different heat treating procedures, and how effective are they? How advantageous are lightweight cranks? To find some answers, we contacted the foremost authorities in the crankshaft business including Alan Davis of Eagle, Tom Lieb of Scat, Dwayne Boes of Callies, Tim Langley of Lunati, and Shawn Mendenhall of Coast High Performance. Here's what they had to say.

Cast vs. Forged vs. Billet
Alan Davis: "Understanding the hierarchy of crankshafts can get confusing due to nomenclature. Take a 4340 forged steel crank, for instance. The '4340' refers to the type of metal alloy a crank is made from, and the 'forged' refers to how that steel is manufactured. You could make a cast 4340 crank, but that would make no sense because you'd be putting a very expensive metal alloy through a very basic manufacturing process that doesn't yield the strongest final product. As its name implies, a cast crank starts out as liquid iron or steel, and is poured into a mold that closely resembles the final shape of a crank. The benefit of the casting procedure is that it reduces the finishing machine work required to get the crank from a raw state to a finished state, which cuts down on costs. Likewise, the equipment used to make a cast crank is relatively inexpensive. This explains why most production engines use cast cranks, and why quality aftermarket cast-steel units can be had for as little as $170.

1007chp_03_o Information_about_crankshafts Crankshaft 3/8

"On the other hand, manufacturing a forged crank is a much more involved process. With a forging, you start with a large ingot of steel alloy, and then pound it into shape with 200-ton press and dies. The heavy-duty presses needed to forge a crank cost at least $100,000, so you have to commit to building a ton of cranks before you can even recoup your investment in equipment. Additionally, the forging process isn't as precise as the casting process as far as the shape of the crank is concerned, so it requires more extensive machine work. That's why forged cranks cost three to four times more than cast cranks, but the trade-off is a dramatic increase in strength. A typical small-block cast crank is good up to 500 hp, while a 4340 forging can take 1,500 hp.

"Billet cranks are similar to forged cranks in that they also start out as a big ingot of steel. The difference is that the billet blanks are already forged, and they're machined into shape instead of being pounded into a die by a hydraulic press. However, the benefit of billet cranks is that they're highly adaptable to short production runs since you don't have to invest in expensive presses and dies. It might cost $3,000, but you can order up a billet crank with any length stroke and any journal diameter you want. You don't have that flexibility with a forged crank. Even if you manufactured a batch of 1,000 cranks, you still wouldn't be able to recover the costs of the dies and presses needed to make it."