Internal versus External Balance
Offset weights on a damper or flexplate are required for externally balanced engines. Thes
Additional stroke also means more weight swinging in a wider arc. But often, there is not room inside the crankcase for longer or heavier counterweights on the crankshaft. One solution is external balancing, where additional counterweight is placed at the damper and at the flexplate/flywheel. By placing this offset weight at each end of the crank, less is needed. However, at higher engine speeds, this additional offset weight can contribute to harmonic problems. This is why most forged cranks are designed for internally balanced assemblies where the weight is located on the crank counterweights themselves and not on offset weights on the damper and flywheel.
While on the subject of dampers, we spoke with Scat owner, Tom Lieb, who is very vocal about the effect of heavy dampers on crankshafts. While these components are designed to dampen the oscillations of the crank, it's also important to remember that under maximum acceleration, you must accelerate the damper with the engine. A heavy damper tends to resist this effort, creating a twisting motion of its own. A lighter damper creates less stress on the crank snout and is easier to accelerate. It's best to discuss your application with your crankshaft manufacturer for damper recommendations, but smaller in diameter and lighter is generally better than big and heavy if there is high rpm in your engine's future.
The chart below lists deck heights for the most popular production V-8 engines. Deck height is defined as the distance from the crankshaft centerline to the height of the head-sealing surface. Aftermarket blocks often come with taller deck heights to allow longer rods to be used with longer strokes.
|ENGINE ||DECK HEIGHT (inches) |
|AMC 401 || 9.208 |
|Buick 455 ||10.57 |
|Cadillac 425/472/500 ||10.815 |
|Chrysler small-blocks || 9.585 |
|Chrysler 383 "B" || 9.980 |
|Chrysler 440 "RB" ||10.725 |
|Chevy 283-400 || 9.025 |
|GM Gen III LS1 || 9.240 |
|Chevy 454 pass. || 9.800 |
|Chevy big-block truck block ||10.20 |
|Ford 289-302 || 8.206 |
|Ford 351W || 9.480 9.503 '71-'72 |
|Ford 390-428 ||10.17 |
|Ford 429/460 ||10.300-10.322 |
|Olds 455 ||10.620 |
|Pontiac 389-455 ||10.24 |
Adding stroke will automatically increase compression even if no other changes are made to
Piston deck clearance is determined by adding half the stroke to the rod length and the piston compression height. Often, the block will be slightly taller, allowing for additional piston-to-head clearance. As an example, a small-block Chevy has a production block deck height of 9.025 inches, though blocks are often cut for less deck height. Taking half a 3.750-inch stroke, plus a 5.700-inch rod, plus a JE piston compression height of 1.425 gives you 9.000 inches. When the engine is mocked up and the piston is found to be 0.010 inch below the deck surface, the actual block deck height would be 9.010 inches.
Decking and Static Compression
Below is an example of how compression ratio would change on a Buick 455 with different block deck heights and a fixed piston compression height. These aren't all the variables, but it gives you an idea.
Stock block deck height: 10.570
Piston compression height: 1.985
Rod length: 6.6
|Deck Height ||Piston Deck Clearance ||Compression Ratio ||10.570 ||0.035 ||9.54:1 |
|10.565 ||0.030 ||9.63 |
|10.560 ||0.025 ||9.73 |
|10.555 ||0.020 ||9.82 |
|10.550 ||0.015 ||9.93 |
|10.545 ||0.010 ||10.03 |
|10.540 ||0.005 ||10.13 |