32-40 Cranks 6/5/14 11:39 AM Page 40
Rotating Assemblies bobweight of the crank is to the actual balance the crank. parts, the less time and drilling it will Most production V8 crankshafts take to balance the crank. have six counterweights to reduce Determining the bobweight requires weight and cost. This works well weighing all of the parts. Rod weights enough for most applications, tend to vary more than piston weights including racing. But for high revving due to heavier mass. endurance engines and longer stroke With lightweight cranks or those engines, adding two additional where the outside diameter of the counterweights for the center pistons counterweights have been turned down to reduce weight, there may not be enough metal in the counterweights to completely offset the pistons and rods. This will require heavy metal tungsten (Mallory) plugs in the counterweights to balance the crank The journals on a high performance crank should be perfectly round and polished to specifications. and possibly Photo courtesy of Scat Enterprises Inc. externally balance the engine with additional weight on the flywheel and allows better balance and reduces harmonic balancer. Heavy metal adds crank flexing that can lead to fatigue cost as well as extra time and labor to and crank failure.
Circle 40 for more information 40 June 2014 | EngineBuilder
Some cranks have holes drilled lengthwise through the main journals. This has no effect on balance and only reduces the overall weight of the crank maybe 2 or 3 lbs. The main purpose of these holes is to allow air to move back and forth in the crankcase as the pistons move up and down. On Chevy LS engines, this type of breathing is essential because the crankcase is very tight and restricts airflow between cylinders. If you are installing a windage tray on a Chevy LS, the tray needs to be positioned far enough away from the crank so that it doesn't inhibit this back and forth airflow within the crankcase. The tray needs to be above the oil level in the pan so oil is whipped up by the spinning crank, but at least an inch or more away from the crank to allow good airflow. A dry sump oil system that has enough suction to pull vacuum in the crankcase will solve such breathing problems by pulling out most of the air along with the oil. â–