The best places to remove weight are on the outside edges of the crank. In order to keep the entire crank/connecting rod/piston assembly from vibrating itself-and the engine-apart, the crankshaft has counterweights designed to balance out the moving mass of the rods and pistons. Cutting the combined weight of the piston, rod, wristpin, and rings (commonly referred to as the bob weight) means you can also make a corresponding reduction in the size of the crank's counterweights. Interestingly, not only is the amount of metal in the counterweight important, but its location in terms of distance from the crank centerline is also critical to provide proper balance. Basically, a longer connecting rod moves the center of balance of the rod/piston combination farther away from the crank centerline, so the center of mass of the counterweight should be moved outboard, too.
"In a perfect world, you would want to have the maximum amount of material as far out as you could get it so that the crank would still fit in the engine block," explains Peter Lieb, the manufacturing manager for Scat Crankshafts. "That's generally the best-case scenario for balancing the crank. Now, what happens when you go with a shorter rod? You have to cut the crankshaft so that it takes material off of the bottom of the counterweight. You have to move the weight toward the pin (the rod end of the crankshaft), which makes it not balance as well."
The advantage is that a long rod is a good match for a pendulum (or undercut) counterweight. On a pendulum counterweight, the majority of the mass is hung on the outside edge of the counterweight, allowing an overall lighter crank that will still balance against the inertial forces of the moving rod and piston.
King recommends using a pendulum-cut crank on applications up to about 600 horsepower. "After that," he says, "you start to get a little too much flexing in there because of the power."
Small Journals Like weight, the journal sizes originally set by the manufacturer might not be the best choice for racing. For a Chevrolet 350, the traditional rod journal size is 2.1 inches. Engine builders have found there are numerous advantages of a smaller journal size, the most significant being that it slows the bearing speed.
For example, imagine a fixed point on the outside circumference of the crankshaft's rod journal. While the crank spins, that point on the rod journal will turn while the rod bearing surrounding it essentially remains still. This means that during one full revolution of the crank, that spot on the rod journal will make a complete revolution around the inside of the bearing. Even if the rpm level of the engine is the same, if the journal size is larger, the speed of the movement between the journal and the bearing is greater. Conversely, smaller equals slower. Even though a thin film of lubricant separates the journal and the bearing, a greater bearing speed still equals increased friction.
By reducing the size of the rod journal, you can essentially reduce the friction created. That is why many racing cranks for the Chevy 350 use the 2-inch 327 journal size. This is possible because there are still plenty of rod options available. Nextel Cup teams use a 1.888-inch journal (commonly called a Honda journal) for an even greater reduction in friction. The problem is the availability of connecting rods. Honda bearings are available for the size, but racing rods must be specially made. While the Honda journal is becoming more common in racing, the rods are still hard to come by.