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Crankshafts and Bearings for Street Performance and Racing
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Then there’s the question of what type of crankshaft to use in a street performance or racing engine. The cast iron crankshafts that come in most stock engines are engineered to handle stock torque outputs. There’s a certain amount of extra capacity designed into every crankshaft, but there’s also a limit as to how much additional torque a stock crank can reliably handle when the engine has been heavily modified.
We’ve all read articles in car buff magazines that describe how to build a 600 horsepower budget small block motor, or an 800 horsepower budget big block motor using the stock cast iron crankshaft. Yes, it can be done -- if you start with a good crank, magnaflux it to make sure there are no hairline cracks or flaws, then shotpeen it to harden the surface. But the question is, how long will that stock crank hold up?
The crankshaft in a big block motor has larger journals and a lot more metal than a small block motor. Consequently, it can safely handle higher torque loads when the engine has been modified. But any stock crank has its limits, especially when the modifications go far beyond simple bolt-ons such as a hotter cam, intake manifold, larger carburetor and exhaust headers.
When you add aftermarket heads that can make serious horsepower, or use a power booster such as nitrous oxide, a turbocharger or supercharger, the crankshaft better be able to handle the higher torque loads -- or you are going to have one unhappy customer.
Most of the crankshaft suppliers we interviewed for this article said that when the power output of a small block motor exceeds 400 to 450 horsepower, it’s time to upgrade to a forged steel crank. For a big block motor, that number is around 500 to 550 horsepower.
Forged crankshafts are usually made of 4340 steel. The ultimate strength of the crank depends on the quality of the steel. Not all cranks that are claimed to be 4340 actually meet ASTM specifications. Strength also depends on the size of the journals (larger is stronger), the radius of the journal fillets (larger is stronger), the location, number and size of the oil holes, the heat treatment and case hardening method used (such a nitriding), and cryogenic treatment (if used).
Because of these differences, you can’t judge the quality of an aftermarket crankshaft by appearance alone. What may appear to be identical crankshafts from different suppliers may in fact have significant differences in strength and durability. In short, you usually get what you pay for. If there’s a price difference of several hundred dollars or more between “similar” crankshafts, there is usually a reason why.
Even so, there is a lot of demand for budget priced cranks, especially stroker cranks for street performance engines. Many suppliers have a line of budget cranks for street performance applications and entry level racers who can’t afford a high end crank. But for serious racing, you shouldn’t use anything less than a top quality, purpose-built racing crank from a reputable supplier.
Other differences you need to consider when choosing a crankshaft include weight, the shape and mass of the counterweights, and special design features and/or coatings that may be available. A lighter crankshaft improves throttle response, but the rotating mass of the crank is more important than its total overall weight.
Removing weight near the center of the crankshaft has much less effect on its rotating momentum than weight removed from the outer edges of the counterweights. Consequently, two different cranks with the same overall weight may have entirely different inertia. One revs quicker than the other because its mass is concentrated closer to its axis of rotation.
For some types of racing, the inertia of a spinning crankshaft (or lack thereof) can actually have a negative impact on acceleration, deceleration and handling. In drag racing, you want a lot of inertia to help launch the car off the line and maintain engine rpm with each gear shift. By comparison, in circle track racing you usually want minimal inertia because the driver is always on and off the throttle.
But on a high speed NASCAR track, if the engine rpm drops too quickly when the driver lets up on the throttle, the sudden deceleration may unload the chassis too much, causing the rear wheels to break loose and skid. Losing control when entering a turn at 200 mph can ruin a driver’s day, so for this reason some NASCAR teams run a heavier crank at super speedways.
The weight of the crank also has to be matched to the weight of the pistons and connecting rods. There’s not much of an advantage to using a light crank if the engine has relatively heavy pistons and rods. One crank manufacturer said if the pistons and rods weigh 1750 grams or less, you’re good to go with a light crank. Otherwise, there’s not much to be gained.
The profile of the counterweights on the crankshaft can also affect windage and drag inside the crankcase. In a low rpm torque motor or street motor, the shape of the counterweights doesn’t make much difference so the effect on windage and drag is negligible. But in a high revving engine, counterweights that have been profiled to reduce drag can have a measurable effect on power.
Some cranks are available with holes drilled though the rod journals. This helps lighten the rod throws to reduce inertia for faster acceleration. Holes drilled in the main journals reduce the overall weight of the crank, but have minimal effect on inertia. But holes drilled in the crankshaft main journals can help equalize pressure inside the crankcase for better piston ring sealing, reduced blowby and more power.
Billet cranks are another alternative to consider. A billet crank can be CNC machined from a solid chunk of steel to almost any stroke or dimensions. This allows a crank manufacturer to custom fabricate a crankshaft from scratch for an application where no forging is available, or to custom-make a crank to your exact specifications. Billet cranks don’t have the grain structure and flow of a forging, but are nearly as strong (some say stronger). The only drawback is that billet cranks are expensive: typically $1,800 to $3,000 or more.
Some forged and billet racing cranks have a larger than stock journal fillet radius to improve strength, and require a narrower racing bearing. As for bearing compatibility, tri-metal or aluminum bearings can be used with any type of crank, be it cast iron, forged or billet.
Various types of coatings may also be applied to a crankshaft. The most commonly used coatings are ones that shed oil to reduce drag at high rpm. Such coatings can provide benefits in high revving engines but don’t do much in low rpm torque motors.
Crankshafts can also be cryogenically treated to improve strength and fatigue resistance. The cryogenic process involves chilling the crank down to minus 300 degrees below zero for a certain period of time (typically 24 to 36 hours or longer), then allowing it to slowly warm back up to room temperature. The freezing process relieves residual stresses in the metal and alters its microstructure, making the crank more resistant to breaking. A cryo treatment typically costs a couple hundred dollars, but provides added insurance against a crank failure.
Visit our online Engine Builders Buyers Guides for contact information of crankshaft and engine bearing manufacturers.
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