'The performance world is full of cool stroker engine combinations these days that fill more pages in crankshaft catalogs than ever before. We're talkin' about long-arm 460 Fords, wild Cleveland combos, oddball inline-six cylinders, and stroke bumps for the new generation of GM Gen III engines. All of this is in the service of the "mine is bigger than yours" approach to horsepower heroism. But all this stroker hype can also cause confusion.
Car crafters are famous for buying parts through nontraditional means such as swap meets, shop closeouts, and clandestine good-guy deals. While the price is usually right, this can also lead to hidden costs when it comes time to balance the rotating assembly. Balancing sounds simple, but as we found, there are many shops out there still drilling holes the traditional way when applying a few simple tricks may make life easier and less expensive. We ran across a typical weight issue when it came time to balance a small-block 331ci stroker Ford. The gruff old guy at the local balance shop said, "I'm not gonna balance this. I'd have to turn it into a piece of Swiss cheese. Take it someplace else. . . . " So we did and learned a little about balancing engines in the process.
 A properly balanced high-performance...  A properly balanced high-performance crankshaft and rotating assembly should come in at less than 1 gram to ensure the engine does not suffer bearing or crankshaft durability problems. |  Crankshaft counterweights...  Crankshaft counterweights are positioned to offset the weight of the combined upper portion of the two connecting rods including the pistons, wristpins, and rings. Heavier pistons require more counterweight. |  This is a stock, elastomeric...  This is a stock, elastomeric big-block Chevy balancer that has been subjected to multiple 6,000-plus-rpm dyno tests. The black streaks indicate the extent that the outer balance ring moves relative to the inner hub. Smaller, lighter balancers are much easier on a crankshaft than heavy, large-diameter balancers, imparting less twist into the nose of the crank. |
A Question of Balance
Before digging into the custom stuff, we figured we'd better brush up on exactly how OE engines are balanced and look into the internal/external-balance issue. To start us on our journey, we decided to talk to Scat Crankshaft's Tom Lieb, who has a strong background in the area of balancing. Lieb is very opinionated about this issue because he has seen just about every imaginable variation on crankshaft failure. Most of these are not due to poor quality, as many think. Usually, the engine has either been abused with excessive rpm, balanced improperly, or often suffered some parts-abusing combination of the two.
Let's start with a short lesson on crankshaft design. Crankshaft counterweights are designed to offset (or balance, if you will) the inertia effect of a relatively heavy piston and connecting rod moving in both a rotational and reciprocating (up-and-down) fashion at speed. The weight of the piston-and-rod combination affects the size and placement of the counterweight. A longer stroke combined with a heavy piston, pin, and ring package requires a larger counterweight (more mass) to balance the greater reciprocating weight. Most V-8 engines use large counterweights toward the front and rear of the crankshaft, leaving the center portion without counterweights. That splits the engine into front and rear halves. The positions of the counterweights on all V-8 90-degree crankshafts are the same. The height of the counterweight as measured outward from the crankshaft centerline is limited by both the cylinder block and by the placement of the bottom of the cylinders. A counterweight placed farther away from the crank centerline has more balance effect, but it is limited by the width of the block crankcase. Weights placed toward both ends of the crank also have a greater effect and therefore don't need to be as large to effectively balance the engine. This makes the overall crank lighter.