01-06-2014, 05:06 AM
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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.
Internal vs. External Balance
Packaging is also an important issue. During the design of the 400ci small-block, a major engineering hurdle was insufficient real estate inside the small-block crankcase for the larger counterweights demanded by the 400's supersized 4.125-inch piston. This was especially difficult in the rear of the engine because the rear crankcase area on a small-block Chevy is restricted by the placement of the oil filter. The solution called for external balance weights placed on the harmonic balancer and flywheel/flexplate. One advantage to external weights is that they are generally lighter because they are positioned at the extreme ends of the crankshaft. The disadvantage is that these offset weights impart their own twisting forces back into the crankshaft, which is not good. This same situation occurs with the 454ci big-block Chevy, which is also the only production big-block that uses external weights to balance the engine.
Complete article with photos:
http://www.carcraft.com/howto/ccrp_0803_...ewall.html
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.
Internal vs. External Balance
Packaging is also an important issue. During the design of the 400ci small-block, a major engineering hurdle was insufficient real estate inside the small-block crankcase for the larger counterweights demanded by the 400's supersized 4.125-inch piston. This was especially difficult in the rear of the engine because the rear crankcase area on a small-block Chevy is restricted by the placement of the oil filter. The solution called for external balance weights placed on the harmonic balancer and flywheel/flexplate. One advantage to external weights is that they are generally lighter because they are positioned at the extreme ends of the crankshaft. The disadvantage is that these offset weights impart their own twisting forces back into the crankshaft, which is not good. This same situation occurs with the 454ci big-block Chevy, which is also the only production big-block that uses external weights to balance the engine.
Complete article with photos:
http://www.carcraft.com/howto/ccrp_0803_...ewall.html